<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <HTML> <HEAD> <TITLE>Programming in XPCE/Prolog: Section 10.3</TITLE><LINK REL=home HREF="index.html"> <LINK REL=contents HREF="Contents.html"> <LINK REL=index HREF="DocIndex.html"> <LINK REL=summary HREF="summary.html"> <LINK REL=previous HREF="exeobjects.html"> <LINK REL=next HREF="whoiswho.html"> <STYLE type="text/css"> /* Style sheet for SWI-Prolog latex2html */ dd.defbody { margin-bottom: 1em; } dt.pubdef { background-color: #c5e1ff; } pre.code { margin-left: 1.5em; margin-right: 1.5em; border: 1px dotted; padding-top: 5px; padding-left: 5px; padding-bottom: 5px; background-color: #f8f8f8; } div.navigate { text-align: center; background-color: #f0f0f0; border: 1px dotted; padding: 5px; } div.title { text-align: center; padding-bottom: 1em; font-size: 200%; font-weight: bold; } div.author { text-align: center; font-style: italic; } div.abstract { margin-top: 2em; background-color: #f0f0f0; border: 1px dotted; padding: 5px; margin-left: 10%; margin-right:10%; } div.abstract-title { text-align: center; padding: 5px; font-size: 120%; font-weight: bold; } div.toc-h1 { font-size: 200%; font-weight: bold; } div.toc-h2 { font-size: 120%; font-weight: bold; margin-left: 2em; } div.toc-h3 { font-size: 100%; font-weight: bold; margin-left: 4em; } div.toc-h4 { font-size: 100%; margin-left: 6em; } span.sec-nr { } span.sec-title { } span.pred-ext { font-weight: bold; } /* Footnotes */ sup.fn { color: blue; text-decoration: underline; } span.fn-text: { display: none; } sup.fn span {display: none;} sup:hover span { display: block !important; position: absolute; top: auto; left: auto; width: 80%; color: #000; background: white; border: 2px solid; padding: 5px; margin: 10px; z-index: 100; font-size: smaller; } </STYLE> </HEAD> <BODY BGCOLOR="white"> <DIV class="navigate"><A class="nav" href="index.html"><IMG SRC="home.gif" BORDER=0 ALT="Home"></A> <A class="nav" href="Contents.html"><IMG SRC="index.gif" BORDER=0 ALT="Contents"></A> <A class="nav" href="DocIndex.html"><IMG SRC="yellow_pages.gif" BORDER=0 ALT="Index"></A> <A class="nav" href="summary.html"><IMG SRC="info.gif" BORDER=0 ALT="Summary"></A> <A class="nav" href="exeobjects.html"><IMG SRC="prev.gif" BORDER=0 ALT="Previous"></A> <A class="nav" href="whoiswho.html"><IMG SRC="next.gif" BORDER=0 ALT="Next"></A> </DIV> <H2><A NAME="sec:10.3"><SPAN class="sec-nr">10.3</SPAN> <SPAN class="sec-title">Defining global named objects</SPAN></A></H2> <A NAME="sec:namedref"></A> <P>As explained before, named references should be restricted to debugging and reusable objects. A couple of problems related to their creation and usage can be identified: <P> <UL> <LI><I>Creating</I><BR> Objects need to be created before they can be used. Reusable objects generally are used from various places in the application. How are they best declared and when are they best created? <LI><I>Versioning</I><BR> Symbolic programming languages generally allow the programmer to change the program when it is running. This property makes them suitable for rapid-prototyping. Global objects are created from the Prolog system. It is desirable that the global object changes if the source-code that declares it has been changed. </UL> <P>Various alternatives to creating global objects have been tried. We will present some of the straight-forward approaches below and describe the (dis)advantages. <A class="sec" href="namedref.html">Section 10.3.3</A> describes the <A NAME="idx:pceglobal2:333"></A><A class="pred" href="sec-D.1.html#pce_global/2">pce_global/2</A> directive to solve this problem. We will use a particular fill-pattern (image) as an example. <H3><A NAME="sec:10.3.1"><SPAN class="sec-nr">10.3.1</SPAN> <SPAN class="sec-title">Using directives</SPAN></A></H3> <P>Using a directive to create a reusable global object appears to be a logical way of dealing with them. This leads to the following code: <PRE class="code"> :- new(@stones_image, image(stones)). ..., send(Box, fill_pattern, @stones_image), ... </PRE> <P>This code has a serious disadvantage. Whenever this file is reloaded after the Prolog code inside it has changed, the directive will be executed as well. The predicate <A NAME="idx:new2:334"></A><A class="pred" href="sec-D.1.html#new/2">new/2</A> will generate a warning on an attempt to create an object with an existing name. <H3><A NAME="sec:10.3.2"><SPAN class="sec-nr">10.3.2</SPAN> <SPAN class="sec-title">Inline testing</SPAN></A></H3> <P>Another common approach is to test inline. For the example, this would look like: <PRE class="code"> ..., ( object(@stones_image) -> true ; new(@stones_image, image(stones)) ), send(Box, fill_pattern, @stones_image), ... </PRE> <P>This approach is bad programming style. If <A NAME="idx:stonesbitmap:335"></A><B>@stones_bitmap</B> is required at various places in the source files this construct needs to be repeated at various places. <H3><A NAME="sec:10.3.3"><SPAN class="sec-nr">10.3.3</SPAN> <SPAN class="sec-title">The `pce_global' directive</SPAN></A></H3> <A NAME="sec:global"></A> <P>This approach is based on <A NAME="idx:exceptionhandling:336">exception-handling</A>. If PCE translates a named reference into an internal reference and the named reference does not exist it will raise an exception. The <A NAME="idx:pceglobal2:337"></A><A class="pred" href="sec-D.1.html#pce_global/2">pce_global/2</A> directive installs an exception handler dealing with a specific global reference. The example becomes: <PRE class="code"> :- pce_global(@stones_image, new(image(stones))). ..., send(Box, fill_pattern, @stones_image), ... </PRE> <P><A NAME="idx:globalobjectswithreconsult:338"></A>This directive applies some heuristics to take care of redefinitions when the file is reconsulted: if the definition is still the same it will not complain. If the definition has modified and the object is already created it will rename the object using `object <B><CODE>-></CODE>name_reference</B>'. A later reference to the object will trap the exception handler again, creating a new object according to the current specification. The directive prints diagnostics messages on redefinitions and other possible problems during compilation. See <A class="sec" href="interface.html">appendix D</A> for details on <A NAME="idx:pceglobal2:339"></A><A class="pred" href="sec-D.1.html#pce_global/2">pce_global/2</A>. <H3><A NAME="sec:10.3.4"><SPAN class="sec-nr">10.3.4</SPAN> <SPAN class="sec-title">Global objects for recognisers</SPAN></A></H3> <P>Recogniser objects (see <A class="sec" href="recogniser.html">section 5.5</A>) that make graphical objects sensitive to mouse events are often created with a global reference. Suppose an application requires box objects that can be moved, resized and that have a popup menu. The recogniser may be defined as: <PRE class="code"> :- pce_global(@mybox_recogniser, make_mybox_recogniser). make_mybox_recogniser(R) :- Gr = @arg1, new(P, popup), send_list(P, append, [ menu_item(cut, message(Gr, free)) ... ]), new(R, handler_group(new(resize_gesture), new(move_gesture), popup_gesture(P))).</PRE> <P>This recogniser object may be attached to the graphical objects either using <A NAME="idx:graphicalsendrecogniser:340"></A>`<B>graphical<CODE>-></CODE>recogniser</B>' or by redefining the <A NAME="idx:graphicalsendevent:341"></A>`<B>graphical<CODE>-></CODE>event</B>' method. In the first case, the recogniser is an integral part of the graphical object and cannot be changed easily during the lifetime of the object. In the second case, the reference to the gesture is through the Prolog implementation of the method and replacing the global object will make all members of the class behave according to the new definition immediately. <P>If the latter approach is taken and the recogniser is under development, you may wish to use <A NAME="idx:free1:342"></A><A class="pred" href="sec-D.1.html#free/1">free/1</A> to make sure the new definition is created: <PRE class="code"> :- free(@mybox_recogniser). :- pce_global(@mybox_recogniser, make_mybox_recogniser). </PRE> <P></BODY></HTML>