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1 \documentclass[twocolumn,a4paper]{article}
2 \usepackage[dvips]{graphicx}
3 \usepackage{color} % Need the color package
4 \usepackage{listings}
5 \usepackage{url}
6 %\usepackage{epsfig}
7 \title{\Huge A Guided Tour of CLIM, \\ Common Lisp Interface Manager}
8 \author{
9 2006 Update \\
10 Clemens Fruhwirth \texttt{<clemens@endorphin.org>} \\ The McCLIM Project
11 \bigskip \\
12 \begin{tabular}{ll}
13 \multicolumn{2}{c}{Original article\footnote{Published in Lisp Pointers 1991}} \\
14 Ramana Rao \texttt{<rao@xerox.com>} & Xerox Palo Alto Research Center \\
15 William M. York \texttt{<york@ila.com>} & International Lisp Associates, Inc. \\
16 Dennis Doughty \texttt{<doughty@ila.com>} & International Lisp Associates, Inc.
17 \end{tabular}
18 }
19
20 \def\VRfont{\sffamily}
21
22 \lstset{%
23 basicstyle=\small\sffamily,
24 keywordstyle=\small\sffamily,
25 stringstyle=\rmfamily,
26 commentstyle=\rmfamily\itshape,
27 frame=lines,
28 language=Lisp }
29
30 \lstdefinestyle{framestyle}
31 {frame=lines}
32 \lstdefinestyle{inlinestyle}
33 {frame=none}
34
35
36 \makeatletter
37 \newcommand {\concept} [1] {{\sl #1}\index{#1}}
38 \newcommand {\term} [1] {{\sl #1}}
39 \newcommand {\code}[1]{{\sffamily #1}}
40 \newcommand {\CLIM}{\textsc{clim}}
41 \newcommand {\CLOS}{\textsc{clos}}
42 \newcommand {\mcclim}{\textsc{McCLIM}}
43 \let\class\code
44 \let\method\code
45 \let\constant\code
46 \let\variable\code
47 \let\macro\code
48 \let\keyword\code
49 \makeatother
50
51 \begin{document}
52 \maketitle
53 \begin{abstract}
54 \noindent The Common Lisp Interface Manager (\CLIM{}) provides a
55 layered set of facilities for building user interfaces. These
56 facilities include a portable layers for basic windowing, input,
57 output services, and mechanisms for constructing window types and user
58 interface components; stream-oriented input and output facilities
59 extended with presentations and context sensitive
60 input;\footnote{Similar to the work pioneered in the Genera UI system}
61 and a gadget-oriented toolkit similar to those found in the X world
62 extended with support for look and feel adaptiveness. In this article,
63 we present an overview of \CLIM{}'s broad range of functionality and
64 present a series of examples that illustrates \CLIM{}'s power. The
65 article originally appeared in Lisp Pointers in 1991 and was updated
66 in 2006 by Clemens Fruhwirth.\footnote{The CLIM 2 specification
67 changed significant parts of \CLIM{} rendering legacy code
68 unusable. Clemens Fruhwirth has rewritten all examples and the
69 corresponding text sections for the \CLIM{} 2 specification. In
70 addition, he has restructured the whole article, adding sections
71 to provide additional insights into \CLIM{} concepts.} All examples
72 in this article have been run with \mcclim{}\cite{mcclim}, a free
73 \CLIM{} implementation, as of January 2006.
74 \end{abstract}
75
76 \section*{Introduction}
77 Common Lisp is a language standard that has provided a broad range of
78 functionality, and that has, to a large degree, successfully enabled
79 the writing of truly portable Lisp programs. The emergence of \CLOS{} and
80 the cleanup efforts of ANSI X3J13 have further enhanced the utility
81 and portability of Common Lisp. However, one major stumbling block
82 remains in the path of those endeavoring to write large portable
83 applications. The Common Lisp community has not yet provided a
84 standard interface for implementing user interfaces beyond the most
85 basic operations based on stream reading and printing.\footnote{Notice
86 that this sentence was written in 1991; it is still
87 true 15 years later.}
88
89 The Common Lisp Interface Manager addresses this problem by specifying
90 an interface to a broad range of services necessary or useful for
91 developing graphical user interfaces. These services include low level
92 facilities such as geometry, graphics, event-oriented input, and
93 windowing; intermediate level facilities such as support for Common Lisp
94 stream operations, output recording, and advanced output formatting;
95 and high level facilities such as context sensitive input, an adaptive
96 toolkit, and an application building framework.
97
98 \CLIM{} implementations will eventually support a large number of window environments
99 including X Windows, Mac OS X and Microsoft Windows. \CLIM{} is
100 designed to exploit the functionality provided by the host environment
101 to the degree that it makes sense. For example, \CLIM{} top level
102 windows are typically mapped onto host windows, and input and output
103 operations are ultimately performed by host window system
104 code. \CLIM{} supports the incorporation of
105 toolkits written in other languages. A uniform interface provided by
106 \CLIM{} allows Lisp application programmers to deal only with Lisp
107 objects and functions regardless of the operating platform.
108
109 An important goal that has guided the design of \CLIM{} was to
110 layer the specification into a number of distinct
111 facilities. Furthermore, the specification does not distinguish the
112 use of a facility by higher level \CLIM{} facilities from its use by
113 \CLIM{} users. For example, the geometry substrate, which includes
114 transformations and regions, is designed for efficient use by the
115 graphics and windowing substrates as well as by \CLIM{} programmers. This
116 means that, in general, a \CLIM{} programmer can reimplement higher level
117 \CLIM{} facilities using the interfaces provided by lower level
118 facilities.
119
120 This modular, layered design has a number of benefits. The \CLIM{}
121 architecture balances the goal of ease of use on one hand, and the
122 goal of versatility on the other. High level facilities allow
123 programmers to build portable user interfaces quickly, whereas lower
124 level facilities provide a useful platform for building toolkits or
125 frameworks that better support the specific needs or requirements of a
126 particular application.
127
128 For example, \CLIM{}'s application framework and adaptive toolkit
129 allow programmers to develop applications that automatically adopt the look
130 and feel of the host's environment. We often call this
131 ``adaptiveness,'' ``look and feel independence,'' or occasionally more
132 picturesquely, ``chameleon look and feel''. However, many users may
133 need or want to define a particular look and feel that is constant
134 across all host environments (we call this ``portable look and
135 feel''). Such users can circumvent the look and feel adaptiveness
136 provided by \CLIM{}, while still using most of the application
137 framework facility and other high level \CLIM{} facilities like
138 context sensitive input. Furthermore, using the lower level facilities
139 of \CLIM{}, they can develop portable toolkit libraries that define
140 and implement their own particular look and feel. For instance, the
141 \CLIM{} programmer can implement new gadget types on top of the drawing
142 primitives and treat them equally to the built-in gadget types.
143
144 We will use the term \concept{CLIM implementor} for the party
145 implementing low-level and high-level parts according to the \CLIM{}
146 specification, \concept{CLIM programmer} for the party that will use
147 the facilities provided by the implementor, and \concept{CLIM user}
148 for the party that will use the programs provided by the programmer.
149
150 The next section presents an overview of the functionality provided by
151 \CLIM{} facilities.
152
153 \section{Overview of Functionality}
154
155 Figure \ref{clim-facilities} shows the various aspects of a host
156 environment in which \CLIM{} lives as well as the various elements
157 provided by \CLIM{}. Below we briefly describe a number of \CLIM{}'s
158 areas of functionality. Later sections will illustrate many of these
159 components.
160
161 \paragraph*{Geometry} \CLIM{} provides points, rectangles, and
162 transformations; and functions for manipulating these object types.
163
164 \begin{figure*}
165 \begin{center}
166 \input{techno-dep.pstex_t}
167 \end{center}
168 \caption{An Overview of \CLIM{} facilities}\label{clim-facilities}
169 \end{figure*}
170
171 \paragraph*{Graphics substrate} \CLIM{} provides a portable interface
172 to a broad set of graphics functions for drawing complex geometric
173 shapes.
174
175 \paragraph*{Windowing substrate} \CLIM{} provides a portable layer for
176 implementing sheets (windows and other window-like objects).
177
178 \paragraph*{Extended Streams} \CLIM{} integrates the Common Lisp
179 Stream I/O functionality with the \CLIM{} graphics, windowing, and
180 panes facilities.
181 % I believe that this was what was intended [2006/03/14:rpg]
182 In addition to ordinary text, the programmer can send a
183 button, a picture or any other arbitrary widget to a \CLIM{} output
184 stream and \CLIM{} will display the widget in the sheet associated
185 with the output stream.
186
187 \paragraph*{Output Recording} \CLIM{} provides output recording for
188 capturing all output done to an extended stream and automatically
189 repainting it when necessary.
190
191 \paragraph*{Formatted Output} \CLIM{} provides a set of high-level
192 macros that enable programs to produce neatly formatted tabular and
193 graphical displays easily.\footnote{This also includes Graph
194 Formatting. Graph formatting is only partly implemented in McCLIM
195 at this date (March 2006).}
196
197 \paragraph*{Presentations} \CLIM{} provides the ability to associate
198 semantics with output, such that Lisp objects may be retrieved later
199 via user gestures (e.g.{} mouse clicks) on their displayed
200 representation. This context sensitive input is modularly layered on
201 top of the output recording facility and is integrated with the Common
202 Lisp type system.
203 % I understand this, but I suspect it's not going to be obvious to the
204 % ordinary reader why type coercion provides the basis for a user
205 % interface... [2006/03/14:rpg]
206 A mechanism for type coercion is also included,
207 providing the basis for powerful user interfaces.
208
209 \paragraph*{Panes} \CLIM{} provides \concept{panes} that are analogous
210 to the gadgets or widgets of toolkits like the X toolkit, GTK or Mac
211 OS X's toolkit.
212
213 Supported pane types include layout panes for arranging other panes,
214 gadget panes for presenting users with feedback information or
215 mechanisms for invoking application behavior, and application panes
216 for displaying and allowing users to interact with application data.
217
218 \paragraph*{Look and Feel Adaptiveness} \CLIM{} supports look and feel
219 independence by specifying a set of abstract gadget pane
220 protocols. These protocols define a gadget in terms of its function
221 and not in terms of the details of its appearance or
222 operation. Applications that use these gadget types and related
223 facilities will automatically adapt to use whatever toolkit is
224 available on and appropriate for the host environment. In addition,
225 portable Lisp-based implementations of the abstract gadget pane
226 protocols are provided.\footnote{\mcclim{} does not support look and
227 feel adaptiveness at the moment except for the experimental beagle
228 backend for Mac OS X's Cocoa platform, or the Gtkairo backend using
229 GTK. Hence, \mcclim{} mostly uses this portable Lisp-based
230 implementation.}
231
232 \paragraph*{Application Building} \CLIM{} provides a set of tools for
233 defining application frames. These tools allow the programmer to
234 specify all aspects of an application's user interface, including pane
235 layout, interaction style, look and feel, and command menus and/or
236 menu bars.
237
238 \paragraph*{Commands} \CLIM{} supports the separation of command
239 execution and command invocation. A \CLIM{} user can invoke commands
240 either via typing it into an interactor, clicking on a menu entry or
241 implicitly invoking a presentation-translator by clicking on a
242 presentation. Commands can also be invoked explicitly by the
243 programmer.
244
245 % added ``Redisplay'' below so that the paragraph header harmonizes
246 % with the jargon used in the paragraph.
247 \paragraph*{Dialogs and Incremental Update/Redisplay} Incremental Redisplay goes
248 a bit further than Output Recording. With Incremental Redisplay, an
249 output record can not only reproduce content that was written to a
250 stream, the \CLIM{} programmer can also attach the code that generated
251 the content to the content itself. Whenever necessary, the application
252 programmer can ask an output stream to update itself. \CLIM{} will
253 query all drawn elements for obsolescence, and if necessary, rerun the
254 code to produce fresh output.
255
256 \noindent\hrulefill
257
258 \noindent This is a large number of facilities to explore. The most
259 systematic way -- progressing from the lowest-level to the highest
260 -- would also be the lengthiest. Therefore, we start
261 with showing several facilities in action with the most fundamental
262 example in the realm of programming: Hello World.
263
264 \section{Our first application}
265
266 We will start with a few lines of code for the trivial Hello World example
267 to give the reader a test case to verify his \CLIM{} setup. It also
268 serves as a point of reference from where the reader can start his
269 explorations of more challenging \CLIM{} facilities. We do not try to
270 elaborate the \CLIM{} concepts in detail here, but simply use them
271 with a brief discussion. The confused reader may hope for a more
272 in-depth explanation in the following section. Please regard
273 \concept{pane}, \concept{application frame}, \concept{sheet},
274 \concept{sheet hierarchy}, \concept{graft} and \concept{top-level
275 loop} as terms we will discuss later.
276
277 We provide extensive \CLIM{} specification references in
278 footnotes. The motivation for this is to show that all the relevant
279 information can be found in the \CLIM{} 2
280 specification\cite{clim-spec}. Before a good \CLIM{} programmer can
281 master any \CLIM{} concept, he has to get used to the style of writing
282 of the specification, as this is the most relevant work for
283 \CLIM{}. The best we can do in this short paper is provide pointers and
284 references and hope that the interested reader starts to explore the
285 surrounding text sections on his own.
286
287 After loading a \CLIM{} implementation, the package
288 \keyword{:clim-user} is available to absorb user code. This package is
289 a good start for experimentation and first steps. When proper
290 packaging is required, simply include the packages \keyword{:clim} and
291 \keyword{:clim-lisp} in your new package's \keyword{:use} list.
292
293 The central element of \CLIM{} application programming is the
294 \concept{application-frame}. An application frame is defined via
295 \code{define-application-frame}.\footnote{See Section 28.2 ``Defining
296 and Creating Application Frames'' in \cite{clim-spec}.} Here is
297 the application frame definition for Hello World:
298 \lstset{style=inlinestyle}
299 \lstinputlisting{hello-world-def-app}
300
301 \begin{figure*}
302 \lstset{style=framestyle}
303 \lstinputlisting{hello-world-handle-repaint}
304 \caption{\method{handle-repaint} for \class{hello-world-pane}}\label{hello-world-repaint}
305 \end{figure*}
306 \code{define-application-frame}'s basic syntax is similar to \code{defclass} because
307 \code{define-application-frame} also generates classes. In this case,
308 it creates a frame class \class{hello-world} that has no superclass
309 except \class{frame} (which is added automatically).
310
311 With \code{:pane}, we define a \concept{top-level-pane} that becomes
312 the content of a fresh window that belongs to an application
313 frame. Although the usual case is for an application frame to
314 correspond to a top level window, sometimes an application frame is swallowed by another
315 application and only space in another existing window is
316 reserved. For instance, a web site management tool might swallow a
317 text editor, so that the user has the option to edit web sites without
318 switching to another application.
319
320 % \footnote{The graft is the root of a sheet hierarchy and on most
321 % windowing system, all children of a graft are mapped to windows.}
322 % With elaborating on details, this means that the content of the
323 % top-level-pane becomes the content of the application
324 % window.\footnote{This is not 100\% correct. A frame might decorate
325 % the top-level-pane with a menu-bar or other refinements. The pane
326 % adopted by the graft is then a composite pane containing a
327 % menu-bar and the original top-level-pane.}
328
329 The list given after the \keyword{:pane} option is a form which is
330 evaluated when an instance of the \class{hello-world} class is
331 created. We use \method{make-pane} to construct a pane as the
332 top-level-pane for frame instances. \method{make-pane} is a
333 constructor for panes.\footnote{See Section 29.2 ``Basic Pane
334 Construction'' in \cite{clim-spec}.} We can treat it as an analog to
335 \code{make-instance} especially made for pane classes. Let us have a
336 look at the definition of \class{hello-world-pane}.
337
338 \lstset{style=inlinestyle} \lstinputlisting{hello-world-defclass}
339
340 The one and only superclass of \class{hello-world-pane} is
341 \class{clim-stream-pane}.\footnote{See Section 29.4 ``CLIM Stream
342 Panes'' in \cite{clim-spec}.} As there are no additional slots, an
343 experienced \CLOS{} programmer might guess that we will use
344 \class{hello-world-pane} solely for method specialization. Before doing so,
345 let us have a look what we have actually
346 inherited from \class{clim-stream-pane}:\footnote{Internal classes
347 removed from listing.}
348
349 \lstset{style=inlinestyle}
350 \begin{lstlisting}
351 CLIM-USER> (describe (find-class
352 'clim-stream-pane))
353 DIRECT-SUPERCLASSES:
354 PERMANENT-MEDIUM-SHEET-OUTPUT-MIXIN
355 STANDARD-REPAINTING-MIXIN
356 STANDARD-EXTENDED-INPUT-STREAM
357 STANDARD-EXTENDED-OUTPUT-STREAM
358 STANDARD-OUTPUT-RECORDING-STREAM
359 SHEET-MULTIPLE-CHILD-MIXIN
360 BASIC-PANE
361 \end{lstlisting}
362
363 % would it be appropriate to define the phrase ``protocol class''
364 % here? I'm not sufficiently confident in my CLIM fu to provide a
365 % definition myself. [2006/03/14:rpg]
366
367 \class{basic-pane} is the foundation of all pane classes. It provides
368 reasonable defaults for all protocol methods and inherits from the
369 protocol class \class{pane}. In turn, \class{pane} inherits from
370 \class{basic-sheet}. Hence, all panes we construct via
371 \class{basic-pane} automatically adhere to the sheet protocol.
372
373 % The standard extended-input provides \class{clim-stream-pane} with a
374 % concept called \concept{command-inversion}. This concept allows the
375 % treatment of events as if they were coming from a stream with
376 % blocking reads.
377
378 % The permanent-sheet-output-mixin class delivers all methods for the
379 % \concept{output protocol} \footnote{See Section 8.3 ``Output
380 % Protocol'' in \cite{clim-spec}}. This protocol used for delivering
381 % output to the user's screen. The special feature of this mixin is
382 % that the output medium is always present, it's permanent. That
383 % means, there is always a screen on which graphic operations can be
384 % carried out. In contrast, there are also output classes, that does
385 % not guarentee an output medium, but more on that later.
386
387 Our \class{hello-world-pane} needs some methods to be useful. With
388 \method{handle-repaint} in \figurename~\ref{hello-world-repaint}, we
389 participate in the \concept{repaint protocol}.\footnote{See Section
390 8.4 ``Repaint Protocol'' in \cite{clim-spec}} This method has two
391 tasks: paint the pane with the pane's background and draw the greeting
392 of \code{*application-frame*} in the center of the pane. The
393 \code{hello-world} frame instance is automatically available in the
394 context of \method{handle-repaint} via the dynamically scoped variable
395 \code{*application-frame*} and so it is possible to use the accessor
396 \code{greeting} to obtain the instance's slot content.
397
398 Two functions are needed to see this code in action:
399 \code{make-application-frame} and \code{run-frame-top-level}. The
400 former is a constructor for instances of frame classes, the latter for
401 running the \concept{top-level loop} of the application frame. The
402 top-level loop is used for command reading, execution and displaying
403 results. But all we need to know for the moment is that it makes the
404 frame visible before entering the loop.
405 \lstset{style=inlinestyle}
406 \begin{lstlisting}
407 (run-frame-top-level
408 (make-application-frame 'hello-world))
409 \end{lstlisting}
410 This completes the Hello World example. In the next section, we catch
411 up to the details we skipped in this example.
412
413 \section{Basic Facilities}
414
415 \subsection{Geometry}
416
417 % The footnote describing ``protocol'' below seems to give a critical
418 % insight into the style and functioning of CLIM. It should certainly
419 % be promoted out of footnote and into body text. I'm inclined to
420 % think it should be promoted to the introduction. The notion of
421 % Protocol is alluded to there, but not clearly described.
422 % [2006/03/14:rpg]
423
424 To \CLIM{}, geometry means \concept{regions}. A region is either bound
425 or unbound and has a dimensionality of either zero, one or two. That
426 corresponds to a point, a path or an area respectively. Regions can be
427 compared (region predicate protocol\footnote{\CLIM{} relies heavily on
428 \CLOS{}. In \CLOS{}, the term \concept{protocol} means a set of generic
429 functions that together form a coherent interface. A protocol
430 specification not only includes the syntactic details of the
431 function names and the number of function arguments, but also the
432 functions' purpose and the semantics of the return values (and/or side
433 effects) must be given in a textual specification.}) and composed
434 (region composition protocol).
435
436 Every bounded region has a \concept{bounding rectangle}. It is the
437 smallest rectangle that contains every point in the region. The
438 bounding rectangle of every bounded region can be accessed via the
439 \concept{bounding rectangle protocol}.
440
441 \CLIM{} supports \concept{affine transformations} of regions. Such
442 transformations can move, stretch and rotate a region. A
443 transformation is affine when every straight line remains straight
444 after transformation. Transformations can be composed arbitrarily. The
445 programmer can attach transformations to mediums and panes. In layout
446 panes, \CLIM{} uses transformations to map the coordinates of child
447 panes to the coordinate system of their parents.
448
449 % This was attached to the previous paragraph, but doesn't seem to
450 % have anything to do with its topic. I'm inclined to think that this
451 % could use some further expansions (have we adequately explained what
452 % a drawing setting is?) [2006/03/14:rpg]
453 All drawing settings can be changed either permanently, or temporarily
454 in the context of the \macro{with-drawing-options} macro.
455
456
457 \subsection{The Windowing Substrate}
458
459 \CLIM{} does not directly talk to the window system. Instead, \CLIM{}
460 is layered on top of a windowing substrate.\footnote{formerly known as Silica.}
461 This substrate is a middleware between \CLIM{} on one side and the
462 host window system on the other. This middleware layer provides a
463 portable windowing model that insulates higher levels of \CLIM{} and
464 \CLIM{} programmers from the details of the host window system. From
465 the perspective of a \CLIM{} programmer, talking to the window system
466 is equal to talking to this abstraction layer. The implementation
467 details are hidden in \concept{backends}, like in \mcclim{} the CLX
468 backend, which hides X11 details, or the beagle backend, which hides
469 details for Mac OS X's toolkit. These backends will use the services
470 of a host window system to provide efficient windowing, input and
471 output facilities. Thanks to this middleware, \CLIM{} is portable
472 across different host windowing systems.
473
474 This framework allows uniform treatment of the following GUI building
475 blocks:
476 \begin{itemize}
477 \item Windows like those in X, Mac OS X and Microsoft Windows.
478 \item Gadgets typical of toolkit layers, such as Gtk+, QT or Mac OS X's
479 toolkit. The backend provides a frame manager which takes care of
480 mapping the abstract gadget types found in \CLIM{} to appropriate
481 gadget with a native look and feel.
482 \item Structured graphics like output records and an application's
483 presentation objects
484 \end{itemize}
485
486 The central abstraction specified by \CLIM{} is the \concept{sheet}. A
487 sheet is a surface that can be painted on and to which input gestures
488 can be directed, and that lives in a hierarchy of other such objects.
489 To get used to the notation of sheets, you can think of them as
490 swallowable windows.
491
492 The fundamental notion in \CLIM{} is the nesting of sheets within
493 another sheet called a windowing relationship. In a windowing
494 relationship, a parent sheet provides space to or groups a number of
495 other children sheets. The \concept{sheet protocols} specify
496 functionality for constructing, using, and managing hierarchies of
497 sheets.
498
499 Sheets have the following properties:
500 \begin{description}
501 \item[parent/children:] a sheet is part of a windowing hierarchy and
502 maintains links to its parent and its children.
503 \item[coordinate system:] a sheet has its own coordinate system that
504 might have a different scaling or orientation than its parent.
505 \item[transformation:] via a sheet transformation these differing
506 sheet coordinate systems are mapped into coordinate system of their
507 parents.
508 \item[clipping region:] defines an area within a sheet's coordinate
509 system that indicates the area of interest.
510 \end{description}
511
512 \paragraph*{Window hierarchies} Sheets participate in a number of
513 protocols. The windowing protocols describes the relationship between
514 sheets. Every sheet has a parent, a sheet might have one or more
515 children. A sheet can be adopted by a parent sheet and disowned later.
516 A sheet is grafted when it is connected to a \concept{graft} either
517 directly or through it's ancestors. A graft is a special kind of sheet
518 that stands in for a host window, typically a root window (i.e.{} screen
519 level). A sheet is attached to a particular host window system by
520 making it a child of an associated graft. A host window will be
521 allocated for that sheet; the sheet will then appear to be a child of
522 the window associated with the graft.
523
524 Sheet hierarchies are displayed and manipulated on particular host
525 window systems by establishing a connection to that window system and
526 attaching them to an appropriate place in that window system's window
527 hierarchy. Ports and grafts provide the functionality for managing
528 this process. A port is a connection to a display service that is
529 responsible for managing host window system resources and for
530 processing input events received from the host window system.
531
532 \paragraph*{Window input} The input architecture of the windowing
533 substrate allows sheets to receive any window event from the host
534 windowing system. The event class hierarchy descends from the class
535 \code{event} to \code{device-event} (including all keyboard and mouse
536 actions), \code{window-event} (window-size-changed and window-repaint
537 events), \code{window-manager-events} (window-deleted) to artificial
538 \code{timer-events}. See 8.2 ``Standard Device Events'' in
539 \cite{clim-spec}.
540
541 The function pair \code{dispatch-event} and \code{handle-event} is the
542 center of all event handling. \code{dispatch-event} is intended to be
543 called when an event is to be dispatched to a client either
544 immediately or queued for later processing in an event queue. On the
545 other side, \code{handle-event} is intended for further
546 specialization, so the application developer can implement special
547 policies for selected events. For instance, when a sheet notices
548 through a \code{window-configuration-event} that the sheet's size
549 has changed, it might redo its layout for its children.
550
551 % There are two mixins that specialize on the
552 % \code{window-repaint-event} class as event argument to
553 % dispatch-event. The mixins are named standard-repaint-mixin and
554 % immediate-repainting-mixin. The first queues the repaint events when
555 % they arive via dispatch-event and redirect calls to handle-event
556 % with repaint events to the method handle-repaint. The latter mixin
557 % also redirects handle-event to handle-repaint for repaint events,
558 % but in contrast to the former it also immediately calls
559 % handle-repaint for any events ariving via dispatch-event. See Figure
560 % \ref{fig1}.
561
562 % \paragraph*{The repaint protocol} By demonstrating the use of
563 % handle-event with the repaint mixins, we have actually shown smaller
564 % protocol at work: the \concept{repaint protocol}. It's usually
565 % triggered by events generated from the host window system via the
566 % handle-event mechanism, but the \CLIM{} programmer can also trigger
567 % a repaint manually via the \code{repaint-sheet} function. The
568 % programmer should specialize on handle-repaint for his own classes.
569 % We have done that in the Hello World example.
570
571 \paragraph*{Window output}
572
573 All drawing operation happen on a \concept{medium}. This object
574 captures the state of the drawing like foreground, background, line
575 style, and the transformation which is applied to the coordinates
576 before drawing. Every medium is associated with a sheet. In turn, a
577 sheet must be associated with a medium whenever drawing operation
578 should be executed. Many \CLIM{} sheets are associated with a medium
579 permanently. \code{sheet-medium} obtains the medium associated with a
580 sheet.
581 % \footnote{Notice that the \CLIM{} specification \cite{clim-spec}
582 % specifies draw-text, draw-line, draw-rectangle, etc. as functions
583 % that operate on mediums. \mcclim{} names these functions
584 % medium-draw-text, medium-draw-line, medium-draw-rectangle, and so
585 % on. The functions that are available in \mcclim{} without the
586 % ``medium-'' prefix operate on sheets. These are trampoline
587 % functions that call the appropriate medium-prefixed function on
588 % the sheet's medium. According to the \mcclim{} authors, this is done
589 % for optimization.}
590
591 % in the topic sentence here, should it read ``of sheets'' or ``of
592 % mediums'' (media?). I'm not sure, but if ``sheets'' is meant, we
593 % should probably have some transition wording here to explain how we
594 % got from discussing mediums above to discussing sheets here.
595 % [2006/03/14:rpg]
596
597 The graphic output capabilities of sheets range from simple line style
598 and text style customization over rendering various geometrical
599 shapes, a color model capable of doing alpha blending, composable
600 affine transformations to pattern, stencil and tiling filling, and
601 pixmaps usage. The features of the \concept{output protocol} are
602 specified briefly in Section 8.3 ``Output Protocol''and more precisely
603 in Chapters 10-14 of \cite{clim-spec}.
604
605 \CLIM{} lives in an idealized world in terms of graphics operations. A
606 \CLIM{} programmer can use an infinitely long and wide drawing pane
607 with arbitrarily precise resolution and continuously variable
608 opacity. As rendering devices with these properties are rare,
609 we need to render the idealized graphic description to a device
610 with finite size and a fixed drawing precision. The rendering rules
611 are specified in Section 12.4 ``Rendering Conventions for Geometric
612 Shapes''of \cite{clim-spec}.
613
614 \section{Building Applications}
615
616 In this section, we explain a number of the necessary ingredients for
617 building applications in \CLIM{}. We will illustrate frames, panes,
618 and simple commands with two examples: a color editor and a simple
619 line and text drawing program.
620
621 \subsection{Application Frames}
622
623 Frames are the central abstraction defined by the \CLIM{} interface
624 for presenting an application's user interface. Many of the high level
625 features and facilities for application building provided by \CLIM{}
626 can be conveniently accessed through the frame facility.
627
628 Frames are typically displayed as top level windows on a desktop.
629 \concept{Frame managers} provide the machinery for realizing frames on
630 particular host window systems. A frame manager acts as an mediator
631 between the frame and what is typically called a desktop manager, or
632 in X terminology, a window manager. The \concept{frame manager} is
633 responsible for attaching the pane hierarchy of a frame to an
634 appropriate place in a sheet hierarchy (and therefore to a host window
635 system window hierarchy) when the frame is adopted.
636
637 To build a user interface, an application programmer defines one or
638 more frame classes. These frame classes define a number of frame
639 properties including application specific state and a hierarchy of
640 panes (i.e.{} user interface gadgets and regions, for interacting with
641 the users). Frame classes also provide hooks for customizing
642 application behavior during various portions of the frame protocol.
643 For example, an \keyword{:after} method on generic functions in the
644 frame protocol can allow applications to manage application resources
645 when the frame is made visible on some display server.
646
647 \CLIM{} is able to show dialog windows, but the code that brings them
648 up is usually quite different from the code that is used to generate
649 the content of application frames. This is unusual for a windowing
650 toolkit as most of them unify the generation of dialog content and
651 content of other window types.
652
653 \CLIM{} generates a dialog with the appropriate input gadget as
654 consequence of a series of input requests. Thanks to the stream
655 facility, the programmer can actually request input synchronously with
656 a blocking read request. He does not have to take care of
657 asynchronously handling confirmation or cancel button clicks. For
658 instance, the programmer requests a string from the user and the user
659 is presented with a prompt, an editable text field, and two buttons
660 for confirmation and canceling.
661 The string requesting function returns only after the user hits the
662 confirmation button. The
663 programmer can directly use the function's return value which is the
664 string provided by the user.
665 % Is the following rewrite correct? Seems like in the actual code an
666 % abort-gesture is signaled, but it is handled by invoking an abort,
667 % if no abort-gesture handler is found. [2006/03/14:rpg]
668 % OLD:
669 % Clicking the cancel button is dealt with by throwing to an
670 % \code{abort} tag.
671 Clicking the cancel button is dealt with by signaling an abort-gesture
672 condition.
673
674 From the caller's perspective, an attempt to separate application
675 frames and dialogs could be: a dialog window itself is side-effect
676 free with respect to the application state and therefore the whole
677 sense of calling a dialog creation routine must arise from the values
678 it returns. For example, the code that modifies the state of the text
679 editor in a text-replace operation does not rest with the callback
680 code of the Ok button in the dialog. This task rests with the code
681 that is executed after the dialog returns its values, namely the code
682 of the Search/Replace command.
683
684 % \footnote{This isn't a sharp distinction. We can also reinterprete
685 % an application frame as something resulting from a command in
686 % progress at a higher level. For instance, assume the application
687 % frame to be the intermediate dialog resulting from the click on
688 % your text editor icon of the desktop's launch menu/button, or to
689 % be the result from typing the application name in your shell. A
690 % more nifty example would be a text editor that allows embedded
691 % graphics which can be edited in-place by graphics editor. The text
692 % editor's application frame would then use another the graphic
693 % editor's application frame intermediately returning the modified
694 % graphic object.}
695
696 An intermediate dialog is something that is brought up to collect
697 additional information for (or before) an operation. When the user
698 selects the ``Search'' command, he is queried for a search string in
699 an additional dialog window; probably offering other search options
700 like case-insensitive search or backwards search. This is done in a
701 synchronous manner, blocking until the requested input is made
702 available by the user.
703
704 % The notation of sheets already capture the idea of windows.
705 % Remember, sheets are drawable entities that can receive user input.
706 % In fact, the entire screen area is also represented as sheet. The
707 % sheet representing the whole screen real-estate is named
708 % \concept{graft} in \CLIM{}\footnote{In X terminology, a graft would
709 % be called root window.}. To create a new window, we have to order
710 % a graft to adopt a new sheet as its children. The new sheet is
711 % inserted into the sheet hierarchy below the graft (the sheet is said
712 % to be \concept{grafted}). But actually, we rarely do it that way.
713
714 % \CLIM{} provides a higher level abstraction of windows and what
715 % their purpose are. In \CLIM{}, we either have application frames or
716 % intermediate dialogs.
717
718 An application frame is an interface that provides the user with a
719 variety of commands to choose as his next step. For instance, the user
720 may choose from commands like Open, Save, Search, or Quit. The frame
721 is a long-living GUI object compared to dialogs, and there is no linear
722 execution path as there is in after a dialog as the user is free to
723 select any commands he likes as his next action.
724
725 Hence, the synchronous programming pattern for dialogs is more
726 convenient because after dialog confirmations there is a predetermined
727 path of execution, while an application frame has to be prepared to
728 handle an arbitrary sequence of commands.
729
730 % In contrast to dialogs, application frames tend to life longer than
731 % dialogs. Hence, application frames are usually required to maintain
732 % some state of some form. This is also the reason why
733 % \method{define-application-frame} was implemented on top of
734 % \macro{defclass}. A dialog does not need to maintain state as they
735 % are disposed too quickly.\footnote{They do have a state but this
736 % state is not important to the programmer as it mostly consist of
737 % state of the input gadgets, like the content of a text input
738 % field. The amount of information contained in there is usually
739 % neglectible, so we do not speak of state management here.}
740
741 % \paragraph*{Frame Managers} One important function provided by a
742 % frame manager is to adapt a frame to the look and feel of the host
743 % window manager. Before the top-level loop of a frame is entered, the
744 % frame is adopted by a frame manager provided by a \CLIM{} backend.
745
746
747 \subsection{Panes}
748
749 An application frame constructs its pane tree by specifying a
750 top-level pane in \macro{define-application-frame}. This pane is
751 usually a layout pane that contains more gadget and/or layout panes as
752 its children. With the help of layout panes, a pane hierarchy can be
753 constructed. The top-level pane (and the whole hierarchy when it is a
754 layout pane) is created when the application frame is adopted by a
755 frame manager and made visible to the user. The programmer can compose
756 an interface consisting of pre-defined gadget panes, layout panes, or
757 application-specific panes. \CLIM{} panes are rectangular sheets that
758 are analogous to the gadgets or widgets of other toolkits.
759
760 Panes and sheets as defined by the windowing substrate have in common
761 that they are associated with a region on screen, a parent, and
762 optional children. They differ in their usage of the input and output
763 capabilities. A sheet is passive and intended to be used by other,
764 active components,
765 while a pane already contains this active part.
766 For this reason,
767 panes are implemented as subclasses of \class{basic-sheet}
768 augmenting the class with an active part. For instance, a button-pane
769 actively draws its own button representation on its allotted screen
770 area and a click on the correct button area triggers a callback for
771 the button. A composite pane lays out its child elements and
772 requests them to draw themselves onto specific screen regions.
773
774 \CLIM{} comes with a set of predefined gadget panes. They consist of
775 push-button, toggle-button, slider, radio-box, text-field, text-editor
776 panes ready for use by the \CLIM{} application programmer. These
777 gadgets might be remapped to native system gadgets by the frame
778 manager, so a native look and feel is possible.\footnote{Only possible
779 in \mcclim{} with the experimental beagle backend for Mac OS X.}
780
781 Each gadget pane class is associated with a set of generic functions
782 that act as callbacks do in traditional toolkits. For example, a pushbutton
783 has an ``activate'' callback method which is invoked when its button
784 is pressed. For this particular callback, a method named
785 \method{activate-callback} is invoked by default, and a \CLIM{}
786 programmer can provide a specialized method to implement
787 application-specific behavior for a subclassed button-pane. But except
788 in the case where the programmer needs a lot of buttons with related
789 behavior, creating a subclass for changing a single specific callback
790 is not economical. Hence upon gadget creation, the programmer can
791 specify an alternative callback method for any callback available. For
792 example, by
793 providing the \keyword{:activate-callback} initarg, the programmer can
794 change the callback to any regular or generic function. By convention,
795 any callback can be changed by providing an initarg keyword equal to
796 the callback's name. See Chapter 30 in \cite{clim-spec} for a listing
797 and description of available callbacks.
798
799 \CLIM{} also provides composite and layout panes. These pane types are
800 used for aggregating several child panes into a bigger single pane
801 that has a layout according to the requested directives. For example,
802 \CLIM{} provides two pane classes, \class{hbox-pane} and
803 \class{vbox-pane}, that lay out their children in horizontal rows or
804 vertical columns respectively. The parent/child relations are managed
805 via the sheet's windowing protocol. If the user interface does not
806 change in ways unpredictable in advance (as in a user interface
807 builder for instance), the program does not have to do hierarchy
808 management via the windowing protocol. He is provided with a set of
809 convenience macros that allows elegant interfaces composed simply by
810 wrapping the respective pane construction code into the convenience
811 macros. % could we name the convenience macros here? [2006/03/14:rpg]
812
813 Application pane classes can be used for subclassing. They can be used
814 to present application specific data -- for instance by specializing
815 \method{handle-repaint} -- and to manage user interactions -- for
816 instance by specializing \method{handle-event}.
817
818 \subsection{Commands}
819
820 Most applications have a set of operations that can be invoked by the
821 user. In \CLIM{}, the command facility is used to define these
822 operations. Commands support the goal of separating an application's
823 user interface from its underlying functionality. In particular,
824 commands separate the notion of an operation from the details of how
825 the operation is invoked by the user.
826
827 Application programmers define a command for each operation that they
828 choose to export as an explicit user entry point. A command is defined
829 to have a name and a set of zero or more operands, or arguments. These
830 commands can then be invoked using a variety of interaction
831 techniques. For example, commands can be invoked from menus, keyboard
832 accelerators, direct typein, mouse clicks on application data, or
833 gadgets.
834
835 The commands are processed in a REPL-like loop. Every application
836 frame has its own running top-level loop specified via
837 \keyword{:top-level} in \method{define-application-frame}. For a
838 \CLIM{} application, it is rarely necessary to change the default top
839 level loop.
840
841 The top-level loop becomes visible when an interactor-pane is added to
842 the user interface. Here the \CLIM{} user gains direct access to the
843 command loop. The loop steps are similar to read-eval-print:
844
845 \begin{enumerate}
846 \item Read a command.
847 \item Execute the command.
848 \item Run the \concept{display function} for each pane in the frame
849 associated with the top-level loop as necessary.
850 \end{enumerate}
851
852 Whenever a command is invoked other than by typing, an appropriate
853 command-invoking text appears after the command prompt
854 nonetheless. Here, the user can directly see how his commands are
855 synthesized from other invocation methods like pointer clicks or menu
856 item selections.
857
858 \section{Simple applications}
859
860 \subsection{Color Editor} In this next example, we define a frame for
861 color selection by manipulating their red, green, and blue components
862 separately. This example illustrates the use of gadget panes provided
863 by \CLIM{}. In the code in Figure \ref{fig-color-editor}, we define an
864 application frame using \method{define-application-frame}. As said
865 before, the syntax of this macro is similar to that of \class{defclass}. It
866 defines a new frame class which automatically inherits from the class
867 \class{frame} which provides most of the functionality for handling
868 frames.
869
870 One requirement must be fulfilled by all frame definitions: code to
871 generate a pane hierarchy must be supplied. The \keyword{:pane} option
872 is the simplest way to supply this code. The \class{hello-world} frame
873 constructs a hierarchy with only one application pane via
874 \method{make-pane}. The pane hierarchy of color editor is more
875 interesting.
876
877 Every pane can request space via its initarg. A request for space
878 allocation is specified as a preferred size (\keyword{:height},
879 \keyword{:width}), a maximum size (\keyword{:max-height},
880 \keyword{:max-width}), and a minimum size (\keyword{:min-width},
881 \keyword{:max-width}). The special constant \constant{+fill+} can be
882 used to indicate the tolerance for any width or height.
883
884 The color editor frame uses three application slots for storing the
885 current RGB values as well as two panes for showing the currently
886 selected colors. The variable \variable{*application-frame*} is
887 dynamically scoped and is defined in any event handler as well as for
888 all code that is evaluated in the context of the \keyword{:pane(s)}
889 frame option.
890
891 \begin{figure*} \lstset{style=framestyle}
892 \lstinputlisting[firstline=7]{color-editor.lisp}
893 \caption{Color Editor}\label{fig-color-editor}
894 \end{figure*}
895
896 The code provided in the \keyword{:pane} option in Figure
897 \ref{fig-color-editor} uses all three kinds of panes provided by
898 \CLIM{}. It uses two application-panes to display colors, two layout
899 panes (\class{vbox-pane} and \class{hbox-pane}) and three gadget panes
900 (\class{slider-pane}).
901
902 In contrast to \class{hello-world}, we do not need a specialized pane
903 class here. We can use an application-pane for displaying chosen
904 colors. An application pane supports graphics operations and invokes
905 generic functions on the pane when input events are received. For the
906 color editor, we only need its ability to refresh its background
907 color.
908
909 The \macro{vertically} and \macro{horizontally} convenience macros
910 provide an interface to the \class{hbox-pane} and \class{vbox-pane}
911 classes. Most \CLIM{} layout panes provide similar convenience
912 macros. The vertical box pane arranges its children in a stack from
913 top to bottom in the order they are listed at creation in the
914 vertically form. This pane type also supports inter-element space and
915 ``pieces of glue'' at arbitrary points in the children sequence. In
916 the color editor frame, the \constant{+fill+} ``glue'' is used to
917 absorb all extra space when too much vertical space is allocated to
918 the vertical box. \CLIM{} also provides a horizontal box which does
919 the same thing except in the horizontal direction.
920
921 In the \macro{horizontally} macro in
922 \figurename~\ref{fig-color-editor}, we do not supply forms to be
923 evaluated directly. Instead, \method{horizontally} processes a form
924 wrapped up in a list. The first list element is a rational number
925 which denotes the amount of space the pane generated by the following
926 form is allowed to occupy in the resulting horizontal layout. In our
927 code in \figurename~\ref{fig-color-editor}, \class{hbox-pane}
928 generated from the \macro{horizontally} macro has a space requirement
929 on its own. The whole composite pane is forced to have a size 200
930 pixels high.
931
932 Now we are ready to turn to the gadget panes. The color editor uses
933 three sliders one for each component of RGB.
934 \method{make-color-slider} creates all three sliders that differ by
935 \variable{id}, \variable{initval} and \variable{label}. The first two
936 variables are handed to \method{make-pane} to construct the slider
937 panes. The remaining initargs for \method{make-pane} are shared across
938 all three entities. To decorate each slider-pane with a proper label,
939 each of them is wrapped up in a \class{label-pane} via the
940 \macro{labelling} convenience macro.
941
942 The slider gadget protocol defines two callback functions:
943 \method{drag-callback} is repeatedly invoked while the slider is being
944 dragged by the user, and \method{value-change-callback} is invoked
945 when the slider is released in a new location. Notice that this
946 specification is sufficiently abstract to allow a variety of different
947 look and feels for a slider. For example, no guarantee is made as to
948 whether the mouse button is held down during dragging, or whether the
949 mouse button is pressed once to start and again to stop dragging.
950
951 We use the gadget ID to distinguish between the red, green and blue
952 slider in the callback code. We could use three different callback
953 functions here, but such callbacks would have much more similarities
954 than differences, thus we do not do that here. Instead, we distinguish
955 the gadget by their \variable{id}.
956
957 The \method{drag-callback} method resets the background of
958 \variable{drag-feedback-pane} and delegates its redrawing to
959 \method{redisplay-frame-pane}, while \method{value-change-callback}
960 does the same for the frame's current color pane. These methods use
961 the \variable{id} argument of the gadget to determine which color
962 component was changed.
963
964 The color editor frame uses the \keyword{:menu-bar} option to indicate
965 that a menu-bar should be added to the application frame. Frame
966 commands defined with \code{:menu t} are accessible from the
967 menu-bar. In this example, we define only one command named
968 \code{com-quit} which is presented as ``Quit'' to the
969 user. \code{com-quit} is defined via
970 \macro{define-color-editor-command}. This macro is generated
971 automatically along \macro{define-application-frame} from the
972 application frame's name. \code{com-quit} simply closes the
973 application frame causing the termination of the frame's
974 top-level-loop. In the next example, commands will be explored in
975 greater detail.
976
977 \begin{figure*}[t] \lstset{style=framestyle}
978 \lstinputlisting{draw-frame-def-app}
979 \caption{define-application-frame for
980 \class{draw-frame}}\label{fig-draw-defapp}\label{fig-draw-handlerepaint}
981 \end{figure*}
982
983 We can invoke the color-editor with the regular
984 \method{run-frame-top-level}/\method{make-application-frame}
985 combination.
986 \lstset{style=inlinestyle}
987 \begin{lstlisting}
988 (run-frame-top-level
989 (make-application-frame 'color-editor))
990 \end{lstlisting}
991
992
993 \subsection{A simple drawing application}
994
995 We move on to a simple application that draws
996 lines and inserts text interactively. Our simple drawing program
997 defines commands for various drawing operations and binds specific
998 input events to these commands.
999
1000 The application frame is defined in
1001 \figurename~\ref{fig-draw-defapp}. A different approach is used to
1002 generate the pane hierarchy: instead of mixing the layout and pane
1003 information, we use the \keyword{:panes} keyword to list all panes
1004 that should be available in frame layouts. The \keyword{:layouts}
1005 keyword combines them into several layouts. It is possible to define
1006 multiple layouts with this option. We define two simple layouts,
1007 \code{default-layout} and \code{alternative}.
1008
1009 % I was confused by the discussion of layouts here. Looking at Figure
1010 % 4, I do not see an alternative layout, and the default-default in
1011 % the listing looks odd to me. Is this the remainder of a discussion
1012 % that was deleted or an extension yet to be written? [2006/03/19:rpg]
1013
1014 \begin{figure*}[t]
1015 \lstset{style=framestyle}
1016 \lstinputlisting{draw-frame-commands}
1017 \caption{Commands for \class{draw-frame}}\label{fig-draw-commands}
1018 \end{figure*}
1019
1020 \begin{figure*}
1021 \lstset{style=framestyle}
1022 \lstinputlisting{draw-frame-interfacing}
1023 \caption{User Interfaces}\label{fig-draw-interfacing}
1024 \end{figure*}
1025
1026 There are two options to \macro{define-application-frame} that we have
1027 not seen before, \keyword{:command-definer} and \keyword{:top-level}
1028 (both with there defaults). \keyword{:command-definer} is used to
1029 specify a name for a command-defining macro for this frame
1030 class. Passing \constant{t} to this option (its default) generates a
1031 command definer named
1032 \macro{define-\texttt{<}frame-name\texttt{>}-command}. We can use the
1033 command-defining macro as a convenience macro for
1034 \method{define-command} which is used to define frame commands, and
1035 will see it in action before long. \keyword{:top-level} specifies a
1036 special form that is used as top level command loop. The top level is
1037 responsible for dequeuing and executing commands that have been
1038 invoked by the user. This loop is to a application frame what the REPL
1039 is to a terminal.
1040
1041 This is the first example that does not use \class{clim-stream-pane}
1042 (or one of its subclasses) as a pane class.\footnote{When using \mcclim{},
1043 we have to do this as there are bugs in the behavior of
1044 \class{clim-stream-pane} that have not been fixed yet.} Instead, we
1045 compose our own drawing pane using
1046 \class{standard-extended-input-stream}, \class{basic-pane} and
1047 \class{permanent-medium-sheet-output-mixin}. The first class is used
1048 to provide the application programmer with stream properties for the
1049 draw-pane that are required for the convenience macro
1050 \macro{tracking-pointer}. \class{basic-pane} is responsible for
1051 handling all requests belonging to the sheet and pane protocols in a
1052 standard manner. The last class is a mixin to tag the pane as
1053 permanently visible on screen during its instances' lifetime. Most
1054 static user interfaces use this mixin. Note that \mcclim{} is sensitive
1055 to the order of the superclasses.\footnote{All stream classes like
1056 \class{standard-extended-input-stream} must be listed before
1057 \class{basic-pane}. Otherwise, no stream handling facilities
1058 will be available.}
1059
1060 % The above footnote refers to what looks like a very big potential
1061 % ``gotcha'' for the beginning McCLIM programmer. Suggestion: promote
1062 % this out of the footnote into body text as a main body paragraph
1063 % here. That paragraph should explain (1) why is it that putting
1064 % basic-pane before standard-extended-input-stream will cause stream
1065 % handling facilities to be crushed and (2) what exactly are the
1066 % implications of not having stream handling facilities. The phrase
1067 % ``stream handling'' hasn't been defined, and it's not obvious what
1068 % these facilities are. [2006/03/19:rpg]
1069
1070 For \class{draw-pane}, the \method{handle-repaint} method shown in
1071 \figurename~\ref{fig-draw-handlerepaint} is straightforward. It
1072 delegates background filling to the next less specific method and then
1073 iterates through the lines and strings painting them. Here, we
1074 implicitly defined the format of the \code{lines} slot and the
1075 \code{strings} slot of the application frame class. The elements of
1076 the list stored in \code{lines} are pairs of points, namely the start
1077 and end point for a line. In \code{strings}, we store the text's
1078 position as the \code{car} of a cons and the text as its \code{cdr}.
1079
1080 % \class{draw-frame} uses two options in
1081 % \macro{define-application-frame} that we have not encountered
1082 % before. The \keyword{:command-definer} option is used to specify a
1083 % name for a command-defining macro for this frame class. Passing
1084 % \constant{t} to this option, as in the example, indicates that a
1085 % name of the form
1086 % \macro{define-\texttt{<}frame-name\texttt{>}-command} should be
1087 % used. The command-defining macro can then be used to define commands
1088 % that are specialized to the defined frame class.
1089
1090
1091 Figure \ref{fig-draw-commands} shows the command definitions. We have
1092 commands for adding a text string, for adding a line and resetting the
1093 draw pane. After every command, we update the drawing pane via the
1094 auxiliary method \method{update-draw-pane}.\footnote{An experienced
1095 \CLIM{} programmer would define a display-function for
1096 \class{draw-pane}. This function is run after a command is
1097 executed, and causes the display pane to be updated with any changes. We
1098 will save this technique for later examples.}
1099
1100 At this point, we can actually use the application; although not very
1101 conveniently. The interactor pane can be used to invoke one of three
1102 commands, either by typing the complete command including all its
1103 parameters or by typing only the command name, then a dialog queries
1104 the user for the missing command argument(s). Clicking on the menu bar
1105 entries is another way to invoke commands. When commands are invoked
1106 through the menu bar, the user will be queried for the missing
1107 argument(s) in the same way as if a command had been typed into the
1108 interactor pane.
1109 % also possible. Also in this case, the user is queried for
1110 % the missing arguments.
1111
1112 But drawing by typing coordinates is not convenient. Therefore, we
1113 attach these commands to other user interactions. Figure
1114 \ref{fig-draw-interfacing} defines input methods (methods for
1115 \code{handle-event}) for pointer button
1116 presses as well as key presses on the draw pane.
1117 Each handler invokes a tracking function (\code{track-line-drawing} and
1118 \code{track-text-drawing}) that uses \macro{tracking-pointer} to
1119 bypass the regular input distribution channels and to dispatch events
1120 to user defined handlers.
1121 % old text: [2006/03/19:rpg]
1122 % Both handlers invoke
1123 % the respective tracking function that uses \macro{tracking-pointer} to
1124 % bypass the regular input distribution channels and to dispatch events
1125 % to user defined handlers.
1126
1127 For \class{pointer-button-press-event}, which is used to draw lines,
1128 the input loop manages a
1129 ``rubber-banding'' line. %
1130 % what is pointer-motion here? Is it an event? And it's not really
1131 % the event that's invoked, to be absolutely clear. The event is
1132 % signaled, and the tracking-pointer macro provides a handler for that
1133 % event. So it is the code associated with :pointer-motion that is
1134 % invoked... I wasn't confident enough in my understanding of the
1135 % precise organization of the (Mc)CLIM architecture to venture a
1136 % rewrite, but there should be a clarification here. [2006/03/19:rpg]
1137 The \keyword{:pointer-motion} is invoked
1138 whenever the mouse pointer is moved by the user. The code attached to
1139 \keyword{:pointer-motion} clears the previously-drawn line and draws a new
1140 line with the new pointer position. It can easily undraw the old line
1141 by the using the special ink \constant{+flipping-ink+}. When the user
1142 confirms the line by releasing the pointer button, a command to the
1143 application is synthesized via \method{execute-frame-command}
1144 supplying all required parameters.
1145
1146 We provide a similar input facility for text input. Whenever
1147 the user hits a key in the draw-pane, the respective
1148 \method{handle-event} calls \method{track-text-drawing} which attaches
1149 the character entered to the mouse pointer. As with the
1150 rubber-banding line, the user can move the displayed string around while
1151 he is free to append additional string characters by additional key
1152 presses. He can confirm the text position with a mouse click causing a
1153 command to be dispatched to the application frame that will add the text to
1154 the application frame permanently.
1155
1156 As each of these methods invoke \code{execute-frame-command} passing
1157 in a special command invocation form, this naturally leads to a
1158 separation between the code that specifies how a command is invoked (menu-bar click, click on
1159 draw-pane or typing in the interactor pane) and the code for command
1160 execution (code bodies of \method{define-command}).
1161
1162 % \section{Shaping concepts}
1163 %
1164 % \subsection{The Extended Input Stream vs. The Event-Based Input Protocol of Sheets}
1165 %
1166 % \hrulefill
1167 %
1168 % Usually, UI toolkits are event driven from design as most windowing
1169 % systems use an event-driven design on their own. \CLIM{} supports a
1170 % different style of programming. The difference command-inversion
1171 % makes becomes mostly visible from the coding style of input
1172 % processing (performing output is rarely an asynchronus operation).
1173 %
1174 % The \CLIM{} programmer can treat the input as if they are coming
1175 % from an stream. The extendend-input-stream even provides blocking
1176 % read methods. The regular stream classes focus on character based
1177 % interaction such as \method{stream-read-char} or
1178 % \method{stream-read-line}, while the extended stream input class
1179 % provides reading of mouse clicks and keyboard presses of the pane
1180 % stream.
1181 %
1182 % Later, when we talk about presentation-type, we will present a way
1183 % to request objects with a special semantic from the user. For
1184 % instance, we require the user to provide a file pathname, or
1185 % clicking on a weekday item in a calendar. But more of
1186 % command-inversion and presentations later.
1187 %
1188 % \hrulefill
1189 %
1190 % Even when you are familar with event-based GUI programming, this
1191 % section is likely to be new to you. As said earlier, a \CLIM{}
1192 % programmer can persue a different programming style with the
1193 % extended input stream facilities. Any event-handler or command code
1194 % can query an input stream for information.
1195 %
1196 % \method{handle-event} from accept-demo shows an event handler for
1197 % pointer release events. The handler calls two stream reading
1198 % function \method{stream-read-line} and \method{stream-read-accept}.
1199 % Notice that both functions are blocking. The argument to
1200 % \method{stream-read-accept} signals that the predefined presentation
1201 % type 'pathname' is requested.
1202 %
1203 % \lstinputlisting{accept-demo-handle-event} For the second pathname
1204 % prompt, the user can either type a character-based representation of
1205 % the pathname into the query window or he can select a pathname that
1206 % is already present on screen with the pointer. accept-demo doesn't
1207 % show any pathnames, so that is only a theoretical option, but we
1208 % will see more about this option later in the presentation type demo.
1209 %
1210 % The evaluation of both format forms is block by the evaluation of
1211 % one of its argument, the stream-reading functions. Only after
1212 % further interaction with the user, the accept call returns. In many
1213 % other GUI toolkits, such a programming style is not possible as
1214 % event handlers would not be allowed to block and therefore stall the
1215 % event-handling thread.
1216 %
1217
1218 \section{High Level Facilities}
1219
1220 In this section, we explain a number of higher level facilities
1221 provided by \CLIM{}, including output recording, formatted output,
1222 presentations, context sensitive input, and command
1223 processors.\footnote{Many of these facilities are derived from work
1224 done at Symbolics on the Dynamic Windows (DW) project for
1225 Genera\cite{prog-ref-manual}. See \cite{presentation-manager} for
1226 more detailed information on the motivations and design details
1227 behind DW. Many of the original contributers to DW have participated
1228 in the redesign of these facilities for \CLIM{}.} We illustrate
1229 these facilities in two examples: a directory lister and a simple
1230 schedule browser.
1231
1232 \paragraph*{Output Recording} Many of the higher level facilities in
1233 \CLIM{} are based on the concept of output recording. The \CLIM{}
1234 output recording facility is simply a mechanism wherein a window
1235 remembers all of the output that has been performed on it. This output
1236 history (stored basically as a display list) can be used by \CLIM{}
1237 for several purposes. For example, the output history can be used to
1238 automatically support window contents refreshing (or ``damage
1239 repaint'' events). The application programmer has considerable
1240 control over the output history. Output recording can be enabled or
1241 suspended, and the history itself can be cleared or rearranged.
1242
1243 Output records can be nested, thereby forming their own hierarchy. The
1244 leaves of this tree are typically records that represent a piece of
1245 output, say the result of a call to \method{draw-rectangle} or
1246 \method{write-string}. The intermediate nodes typically provide
1247 additional semantics to the tree, such as marking a subtree of nodes
1248 as resultant output of one particular phase of an application, for
1249 instance row and column formatting information. Chapter 16 in
1250 \cite{clim-spec} has all the details.
1251
1252 \paragraph*{Output Formatting} \CLIM{} provides a convenient table and
1253 graph formatting facility, which is built on top of the output
1254 recording facility. The key to these formatting tools (as opposed to,
1255 say, \code{format}'s \code{~T} directive) is that they dynamically compute the
1256 formatting parameters based on the actual size of the
1257 application-generated output.
1258
1259 The application programmer uses these tools by wrapping any piece of
1260 output-producing code with advisory macros that help the system
1261 determine the structure of the output.
1262
1263 For example, start with a simple output function that shows some
1264 information about the packages in the Lisp environment:
1265 \lstset{style=inlinestyle}
1266 \begin{lstlisting}
1267 (defun show-package-info (stream)
1268 (dolist (package (list-all-packages))
1269 (write-string (package-name package)
1270 stream)
1271 (write-string " " stream)
1272 (format stream "~D"
1273 (count-package-symbols
1274 package))
1275 (terpri stream)))
1276 \end{lstlisting}
1277 Any attempt to fix this function to produce tabular output by building
1278 in a certain fixed spacing between the package name and symbol count
1279 will either get caught by an unexpectedly long package name, or will
1280 have to reserve way too much space for the typical case. In \CLIM{}, we
1281 can use the code in Figure \ref{fig-formatting} to produce a neatly
1282 formatted table for any set of package names.
1283 \begin{figure*}
1284 \lstset{style=framestyle}
1285 \begin{lstlisting}
1286 (defun show-packages (stream)
1287 (formatting-table (stream)
1288 (dolist (package (list-all-packages))
1289 (formatting-row (stream)
1290 ;; The first column contains the package name
1291 (formatting-cell (stream)
1292 (write-string (package-name package) stream))
1293
1294 ;; The second color contains the symbol count, aligned with the right
1295 ;; edge of the column
1296 (formatting-cell (stream :align-x ':right)
1297 (format stream "~D" (count-package-symbols-package)))))))
1298 \end{lstlisting}
1299 \caption{An output function that uses table formatting.}\label{fig-formatting}
1300 \end{figure*}
1301
1302 % \paragraph*{Updating output} FIXME think of a GOOD example for
1303 % updating output.
1304
1305 \paragraph*{Presentations} The next step up from preserving the mere
1306 physical appearance of output done to a window is to preserve its
1307 semantics. For example, when an application displays a Lisp pathname
1308 on the screen via \code{(format t "\~{}A" path)}, the string
1309 ``/clim/demo/cad-demo.lisp'' may appear. To the user this string has
1310 obvious semantic meaning; it is a pathname. However, to Lisp and the
1311 underlying system it is just a text string. Fortunately, in many cases
1312 the semantics can be recovered from the string. Thus the power of the
1313 various textual cut-and-paste mechanisms supported by contemporary
1314 computer systems. However, it is possible to improve upon the utility
1315 of this lowest common denominator facility (i.e.{} squeezing everything
1316 through its printed representation) by remembering the semantics of
1317 the output as well as its appearance. This is the idea behind
1318 presentations.
1319
1320 A presentation is a special kind of output record that maintains the
1321 link between screen output and the Lisp data structure that it
1322 represents. A presentation remembers three things: the displayed
1323 output by capturing a subtree of output records, the Lisp object
1324 associated with the output, and the presentation type of the output.
1325 By maintaining this back pointer to the underlying Lisp data
1326 structure, the presentation facility allows output to be reused at a
1327 higher semantic level.
1328
1329 An application can produce semantically tagged output by calling the
1330 \CLIM{} function \method{present}. For example, to display the
1331 pathname referred to above as a presentation, the application would
1332 execute:
1333 \begin{lstlisting}
1334 (present path 'pathname)
1335 \end{lstlisting}
1336 \method{present} captures the resulting output and the pathname object
1337 in a presentation of type \code{'pathname}.
1338
1339 \paragraph*{Presentation Types} \CLIM{} defines a set of presentation
1340 types, which are arranged in a super-type/subtype lattice like the CL
1341 types. In fact, the presentation type hierarchy is an extension of the
1342 CL type hierarchy. The reason that this extended type system is needed
1343 is that the CL type system is insufficient from the UI perspective. For
1344 example, the integer 72 might represent a heart rate in one
1345 application and a Fahrenheit temperature in another, but it will
1346 always be just an integer to Lisp.
1347
1348 The application programmer can define the UI entities of the
1349 application by defining presentation types, thus extending the
1350 presentation type library. By defining a presentation type, the
1351 programmer can centralize all of the UI aspects of the new type in one
1352 place, including output appearance and input syntax. As an example,
1353 \CLIM{} defines a pathname presentation type that defines how a
1354 pathname is displayed and how one is input. The pathname input side
1355 provides pathname completion and display of possibilities. By
1356 defining this behavior in one place and using it in all applications
1357 that need to display or read pathnames, \CLIM{} helps build consistent
1358 user interfaces.
1359
1360 Note that in the pathname output example given above \method{present}
1361 invokes the standard pathname displayer defined by the presentation
1362 type. However, since the presentation facility is simply based on the
1363 output recording facility, presentation semantics can be given to any
1364 output. The following example shows how the pathname object could be
1365 associated with some graphics that were displayed on the screen.
1366
1367 \begin{lstlisting}
1368 (with-output-as-presentation
1369 (:object path
1370 :type 'pathname
1371 :stream s)
1372 (draw-rectangle* s 0 0 30 30))
1373 \end{lstlisting}
1374
1375 \paragraph*{Context-Dependent Input} Once output is
1376 semantically-tagged, it can be reused as semantically-meaningful
1377 input. To achieve this, the application does not only have to tag its
1378 output but it also has to provide additional semantic information when
1379 doing input operations. For instance, whenever a pathname is required,
1380 the program has to use a special method in contrast to reading a
1381 string of characters.
1382
1383 The counterpart to \method{present} is \method{accept}. It is used to
1384 establish an \concept{input context}. The input context is a
1385 presentation type that is appropriate for the current input point. For
1386 example, if the application requires the user to input a pathname, it
1387 can trigger an appropriate prompt, input parser and input context
1388 with:
1389 \begin{lstlisting}
1390 (accept 'pathname :stream s)
1391 \end{lstlisting}
1392 Typically, this invokes the input reader (or parser) that was defined
1393 for the pathname type and establishes an input context that indicates
1394 that it is waiting for a pathname.
1395
1396 Once the input context is established, \CLIM{} automatically makes any
1397 appropriate existing output available to the user via mouse gestures.
1398 After calling \method{accept} as shown above, the user can move the
1399 mouse over any presentation that is of type pathname (or is a subtype
1400 of pathname), click on it, and the pathname object underlying the
1401 presentation is returned as the value of the call to \code{accept}.
1402
1403 \paragraph*{Command Processors} \CLIM{} promotes the separation of
1404 command execution code and command invocation code. The command
1405 facility aids this task. Every application frame can define
1406 application commands that are accessible via various input methods.
1407 The most common are clicking on the command's entry in the menu bar or
1408 a context menu, typing the command in the interactor pane, or the
1409 application dispatches a command to itself (maybe triggered by other
1410 ways of user input or network interactions, etc.). The latter case is
1411 seen in \code{track-line-drawing} and \code{track-text-drawing} of the
1412 \class{draw-frame} example. These methods generate ``Add Line'' and
1413 ``Add String'' commands as the result of event-handling on the
1414 draw-pane.
1415
1416 A command has a name specified as string and a set arguments specified
1417 as presentation types. Looking back at the command ``Add String'' of
1418 draw-frame, we see that this command takes a string and two integer as
1419 arguments. This type information is useful for partial command
1420 parsing. Assume the user clicks on a menu entry or types only the
1421 command name in the interactor. \CLIM{} notices that there are
1422 arguments missing in the command and requests the missing ones in a
1423 dialog via calls to \code{accept}. Mentioned early, \code{accept}
1424 establishes an input context and so the user is able to fill the
1425 missing argument with clicks on appropriate visible output objects.
1426 Also keyboard users will find semantically-tagged input methods
1427 convenient as implementations of presentation types can provide the
1428 user with a completion facility.
1429
1430 In addition, the command processor is extensible by application
1431 programmers. For example, the command processor can be extended to
1432 support a ``noun then verb'' interaction style, where the user can
1433 first click on a displayed presentation and then invoke a command that
1434 is defined to take an argument of the selected presentation type.
1435
1436 \subsection{A Directory Browser}
1437 \begin{figure*}
1438 \lstset{style=framestyle}
1439 \lstinputlisting{file-browser-all}
1440 \caption{File Browser}\label{fig-file-browser}
1441 \end{figure*}
1442
1443 The \class{dirlist-frame} application is a very simple file
1444 system browser using presentation types. It defines two panes, an
1445 output pane to display directory contents and an input pane to handle
1446 user typein. For the output pane, the application defines a display
1447 function. \code{dirlist-display-files} works in conjunction with the
1448 top level command loop. By default, the command loop calls all
1449 display functions after a command was processed to make all changes in
1450 the application's data structures visible that the command execution
1451 might have caused. When all display functions have updated their
1452 panes, the command loop displays a prompt in the
1453 interactor\footnote{Omitted when there is no interactor pane.}
1454 and waits for the next command.
1455
1456 The \method{dirlist-display-files} display function iterates over the
1457 contents of the current directory displaying the files one by one.
1458 Each output line is produced by a call to \method{present}.
1459 \method{present} creates the association between the text lines on the
1460 screen and the Lisp pathname objects.
1461
1462 \class{draw-frame} has a single command ``Edit directory''. The
1463 command's body interprets the pathname that it receives as a
1464 directory, obtaining a list of the files contained therein. The
1465 command simply updates the application variable
1466 \variable{active-files} with the new list.
1467
1468 The \CLIM{} presentation substrate supports a general concept of
1469 presentation type translation. This translation mechanism can be used
1470 to map objects of one type into a different presentation type, if
1471 appropriate. For example, it might be possible to satisfy an input
1472 request for a pathname by selecting a computer user's login ID and
1473 returning the pathname of the user's home directory. This would be
1474 accomplished by defining a translator from a user-id presentation type
1475 to the pathname type. The translator would consult the system's user
1476 database to retrieve the home directory information to achieve its
1477 conversion task.
1478
1479 The command loop of an application frame is regularly requesting
1480 commands for processing and it uses \method{accept} for its request.
1481 Hence, an input context for reading an object with the presentation
1482 type \class{command} is established and all objects that can be used
1483 as commands will become clickable on screen. Here, we can define a
1484 presentation-to-command translator that translates a pathname into the
1485 edit directory command.
1486
1487 The presentation-to-command translator in
1488 \figurename~\ref{fig-file-browser} is very simple. It has the name
1489 \class{pathname-to-edit-command} and converts the presentation type
1490 \code{pathname} to the command \code{com-edit-directory} for the
1491 command table of \class{file-browser}. The gesture option specifies
1492 that it works with the select gesture, while the string supplied to
1493 \keyword{:documentation} is used in the context menu.
1494
1495 This command translator has little work to do. The body of the
1496 translator has to return a list of arguments that are handed to the
1497 command \code{com-edit-directory}. But we do not need to any
1498 conversion of the supplied object, as it is already a pathname. Thus,
1499 the object is wrapped in a list and returned to the caller
1500 which will apply \code{com-edit-directory} to the list.
1501
1502 A simple trick is used to dispatch an initial command to the
1503 application frame. An after-method is provided for
1504 \method{adopt-frame} which runs after the frame manager has adopted
1505 the frame for displaying. This is different for an after-method on
1506 \method{initialize-instance} as an after-method on
1507 \method{adopt-frame} runs later, when the application frame instance
1508 has already a command queue associated with it.
1509
1510 Since this application was defined with an interactor pane for user
1511 input, the user can invoke the sole command by typing its name, ``Edit
1512 Directory.'' Here, \CLIM{} supports for automatic command completion
1513 becomes visible. Only the first letter has to be typed followed by the
1514 complete action (usually Tab) and the command completion facility will
1515 complete the input to ``Edit Directory''. At this point, the \CLIM{}
1516 command loop will begin reading the arguments for that command, and
1517 will automatically enter a pathname input context. Thus, the user can
1518 fill in the required argument either by typing a pathname, or by
1519 clicking on one of the pathnames visible in the display pane.
1520
1521 \subsection{Schedule Example}
1522
1523 In this example, we build a simple appointment browser. Since the user
1524 of this application will frequently be dealing with the days of the
1525 week, we start by defining a new presentation type weekday. This
1526 simple presentation type, shown in Figure \ref{fig-scheduler1},
1527 represents a day of the week as a number from 0 to 6. Each day number
1528 is associated with an abbreviated day name, ``Mon,'' ``Tue,'' etc.
1529
1530 \begin{figure*}
1531 \lstset{style=framestyle}
1532 \lstinputlisting{scheduler-part1}
1533 \caption{Scheduler: application frame, presentation type and commands}\label{fig-scheduler1}
1534 \end{figure*}
1535 \begin{figure*}
1536 \lstset{style=framestyle}
1537 \lstinputlisting{scheduler-part2}
1538 \caption{Scheduler: display functions}\label{fig-scheduler2}
1539 \end{figure*}
1540
1541 The weekday presentation type defines two basic pieces of behavior:
1542 how a weekday is displayed, and how it is read as input. The macro
1543 \macro{define-presentation-method} is intended for specializing
1544 methods on presentation types. Via this macro, we define a printer
1545 and a parser function for the type \code{weekday}. As is the case with
1546 most presentation types, the printer and parser are duals. That is,
1547 the printer, when given an object to print, produces output that the
1548 parser can interpret to arrive back at the original object.
1549 \code{accept} does not need a parser in every case, and for simplicity
1550 the programmer might choose not to provide a parser at all.
1551
1552 We took the easy way out with our parser. \CLIM{} provides a
1553 completion facility and instead of reading the day name of the stream
1554 and parsing it into an integer, we provided an exhaustive set of input
1555 to object mappings. The \macro{completing-from-suggestions} macro
1556 collects all suggestions made by the parser via
1557 \code{suggest}. \code{suggest} takes an input and object as
1558 suggestion. In our case, day name taken from \variable{*days*} is the
1559 input and the number of the week \variable{i} is the object. \CLIM{}
1560 will match the input given by the user with the suggestions and in
1561 case of a match will return the corresponding object to the caller.
1562
1563 We define an application frame for the appointment browser in Figure
1564 \ref{fig-scheduler1}. The frame defines state variables to hold the
1565 list of appointments and the current day. This information is kept in
1566 slots on the frame, so that multiple copies of the application can be
1567 run, each with its own appointment list. The application represents
1568 the appointment data as an alist containing an entry for each day of
1569 the week, with each entry containing a list of the appointments for
1570 the day. Test data is provided as a default initform.
1571
1572 Just as in the previous example, the appointment application defines
1573 two panes, an interactor and an output display pane. The appointment
1574 application defines two commands. The ``Show Summary'' command resets
1575 the display back to the weekly summary mode by setting the
1576 \variable{current-day} slot to \constant{nil}. The ``Select Day''
1577 command sets \variable{current-day} to the value of an argument that
1578 is specified to be a weekday. This presentation type specification
1579 allows the command processor to make all presented weekday active when
1580 it is filling in this argument, as well as, provide completion
1581 assistance to the user.
1582
1583 The command ``Select Day'' has the following argument specification:
1584 \begin{lstlisting}
1585 ((day 'weekday :gesture :select))
1586 \end{lstlisting}
1587 It takes an argument \code{day} of type \class{weekday}. Up until now,
1588 our command arguments have looked similarly to specialized lambda
1589 lists, but next to the type information the arguments of commands are
1590 also allowed to have various keyword-value pairs for selection further
1591 options. In this case, we supplied the value \code{:select} for the
1592 option \keyword{:gesture}. The macro \macro{define-command} parses
1593 this keyword-value pair and generate a presentation translator. More
1594 precisely, a presentation-to-command translator is defined that is
1595 equal in functionality to the one we have seen in the file-browser
1596 example. Whenever a presentation of the type \code{pathname} is
1597 selected (e.g.{} with pointer clicks) in a \class{command} input
1598 context, it is translated into a command invocation of ``Select
1599 Weekday''.
1600
1601 Finally, we turn to the display the appointment information. The
1602 display function, \method{display-appointments}, shown in Figure
1603 \ref{fig-scheduler2}, is somewhat more complex than our earlier
1604 example. It can display two different sets of information: a weekly
1605 summary showing the days of the week and the number of appointments
1606 for each day, or a detailed description of one day's appointments.
1607
1608 \method{display-appointments} decides which set of information to
1609 display by examining the application state variable
1610 \variable{current-day}. The table formatting facility is used to
1611 present the weekly summary information neatly organized. The daily
1612 appointment list, by contrast, is displayed using
1613 \method{write-string}. Note, however, that whenever a day of the week
1614 is displayed, it is done with a call to \code{present} using the
1615 weekday presentation type. This allows the printed weekdays to be
1616 selected either as a command or as a weekday argument. This example
1617 illustrates how an application with interesting UI behavior can be
1618 constructed from a high-level specification of its functionality.
1619
1620 % \section{The benefits of the extended output stream}
1621 %
1622 % At the first glance, the extended output stream facilities does not
1623 % look more exciting than the output capabilities of sheets. In fact,
1624 % the output streams use the output facility of sheets to do their
1625 % job, thus the variaty of graphic operations is less or equal to that
1626 % of the lower facility. But an output stream helps to organize output
1627 % can do many things, we are not able to achieve that easy with the
1628 % low-level output tools.
1629 %
1630 % Output Recording, Table/Graph Formatting,and Incremental Redisplay
1631 % are tools that are built on top of the stream characteristic in the
1632 % extended output stream. We have already seen Table Formatting at
1633 % work in the scheduler example.
1634 %
1635 % \begin{figure*}
1636 % \hrulefill
1637 % \begin{lstlisting}
1638 % (let (a-string
1639 % sum-of-ints
1640 % (input-stream (frame-standard-input *application-frame*)))
1641 % (accepting-values (input-stream :own-window t)
1642 % (setq a-string (accept 'string :prompt "1st string" :stream input-stream))
1643 % (terpri input-stream)
1644 % (setq sum-of-ints
1645 % (+ (prog1 (or (accept 'integer :prompt "1st integer" :stream input-stream) 0)
1646 % (terpri input-stream))
1647 % (prog1 (or (accept 'integer :prompt "2nd integer" :stream input-stream) 0)
1648 % (terpri input-stream))))))
1649 % \end{lstlisting}
1650 % \hrulefill
1651 % \caption{Hairy example of accepting-values demonstrating the use of the blocking \method{accept} method}\label{accepting-values-example}
1652 % \end{figure*}
1653 %
1654 % With the \macro{accepting-values} even more is possible.
1655 % \figurename~\ref{accepting-values-example} demonstrates that it is
1656 % possible to build up a whole dialog window simply by requesting a
1657 % series of input types. It does that via \method{accepting-values}
1658 % which brings up a seperate dialog window to request a string and two
1659 % integers from the user.
1660 %
1661 % The trick of \method{accepting-value} is dual-evaluation. Its body
1662 % is evaluated twice. First to collect all accept calls and build up a
1663 % dialog. The second evaluation happens after the user has entered all
1664 % requested information and has hit the ``Ok'' button as confirmation.
1665 % In the second evaluation the entered values are returned from the
1666 % corresponding accept forms. Notice that we have to wrap
1667 % \method{accept} in an or statement for \method{+}, because when the
1668 % body is evaluated for the first time all \method{accept} forms
1669 % return \constant{nil} causing an exception if handed to
1670 % +.\footnote{In pratice, we would sum up the integers after
1671 % accepting-values, because this is much more easy to read later.}
1672 % Please do not try to mimic the code shown in
1673 % \figurename~\ref{accepting-values-example}. Nonetheless it is shown
1674 % to give the reader an insight how this sudden violation of
1675 % evaluation sequence is implemented.
1676 %
1677 % We should emphasize that only the body of accepting-values is
1678 % evaluated twice. The \method{let} statement in
1679 % \figurename~\ref{accepting-values-example} does not return before
1680 % \method{accepting-values} does. This is not a form of continuation.
1681 % \method{accepting-values} effectively blocks the event thread just
1682 % as a single \method{accept} would (\method{accepting-values} can
1683 % therefore be seen as something that aggregates several accept
1684 % statements). Despite this blocking, \CLIM{} manages to continue
1685 % event handling.
1686
1687 \section{Conclusion}
1688
1689 The series of examples presented in this article illustrates the broad
1690 range of functionality provided by \CLIM{}. The later examples,
1691 especially, demonstrate that complex user interfaces can be built
1692 economically and in a modular fashion using \CLIM{}. Many of the higher level
1693 facilities make it possible to separate the issues involved in
1694 designing an application's user interface from the functionality of
1695 the application.
1696
1697 On the other hand, these higher level facilities are not
1698 appropriate for all programmers. \CLIM{}'s lower level facilities and clean
1699 modularization of the higher level facilities provide these programmers with
1700 portable platform and a framework for implementing their own user
1701 interface toolkits and frameworks. In addition, \CLIM{}'s use of \CLOS{}
1702 to define explicit, documented protocols provides application
1703 programmers with the opportunity to customize \CLIM{} and support
1704 interfaces not anticipated by the \CLIM{} designers.
1705
1706 A free \CLIM{} implementation is available as \mcclim{}, found at
1707 \url{http://common-lisp.net/project/mcclim/}. In addition to the
1708 caveats in this document, note that \mcclim{} only works with an X
1709 windows backend as of January 2006. An experimental port called
1710 beagle to the native toolkit of Mac OS X is in progress.
1711
1712 \section*{Acknowledgments}
1713
1714 \paragraph*{Original article} \CLIM{} represents the cooperative
1715 effort of individuals at several companies. These individuals include
1716 Jim Veitch, John Irwin, and Chris Richardson of Franz; Richard Lamson,
1717 David Linden, and Mark Son-Bell of ILA; Paul Wieneke and Zack Smith of
1718 Lucid; Scott McKay, John Aspinall, Dave Moon and Charlie Hornig of
1719 Symbolics; and Gregor Kizcales and John Seely Brown of Xerox PARC.
1720 Mark Son-Bell and Jon L. White have help us improve this paper.
1721
1722 \paragraph*{2006 update} Clemens Fruhwirth thanks the developers for
1723 \mcclim{} for producing a free \CLIM{} implementation, and especially
1724 Robert Strandh for answering so many questions in conjunction with
1725 \mcclim{}.
1726
1727 \bibliographystyle{alpha}
1728 \bibliography{guided-tour}
1729
1730 \end{document}

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