/usr/share/doc/swi-prolog-doc/UserGuide/fileviewer.html

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<H2><A NAME="sec:2.8"><SPAN class="sec-nr">2.8</SPAN> <SPAN class="sec-title">Example: 
show files in directory</SPAN></A></H2>

<A NAME="sec:fileviewer"></A>

<P>In this section we illustrate the above with some simple examples. We 
also show how the GUI can call procedures in the Prolog system.

<P>The demo creates a list of files from a given directory and allows 
the user to view a file. <A class="fig" href="fileviewer.html#fig:fileviewer">Figure 
2</A> shows the result.

<P><A NAME="fig:fileviewer"></A>
<CENTER>
<IMG SRC="fileviewer.gif">
</CENTER>
<TABLE ALIGN=center WIDTH="75%"><TR><TD>
<B>Figure 2 : </B>The FileViewer demo</TABLE>

<PRE class="code">
fileviewer(Dir) :-
        new(DirObj, directory(Dir)),
        new(F, frame('File Viewer')),
        send(F, append(new(B, browser))),
        send(new(D, dialog), below(B)),
        send(D, append(button(view,
                              message(@prolog, view,
                                      DirObj, B?selection?key)))),
        send(D, append(button(quit,
                              message(F, destroy)))),
        send(B, members(DirObj?files)),
        send(F, open).

view(DirObj, F) :-
        send(new(V, view(F)), open),
        get(DirObj, file(F), FileObj),
        send(V, load(FileObj)).</PRE>

<P>The main window of the application consists of a <A class="" href="summary.html#class:frame">frame</A> 
holding two specialised <A class="" href="summary.html#class:window">window</A> 
instances. A <A class="" href="summary.html#class:frame">frame</A> is a 
collection of <EM>tiled</EM> windows. Any opened window in <font size=-1>XPCE</font> 
is enclosed in a <A class="" href="summary.html#class:frame">frame</A>. 
The windows are positioned using the methods
<B><CODE>-&gt;</CODE>above</B>, <B><CODE>-&gt;</CODE>below</B>, <B><CODE>-&gt;</CODE>right</B> 
and <B><CODE>-&gt;</CODE>left</B>. The frame ensures that its member 
windows are properly aligned and handles resizing of the frame. See also <A class="sec" href="framelayout.html">section 
10.6</A>.

<P>In line 3, a <A class="" href="summary.html#class:browser">browser</A> 
is <B><CODE>-&gt;</CODE>append</B>ed to the frame object. A browser is a 
window specialised for displaying a list of objects. Next, a <A class="" href="summary.html#class:dialog">dialog</A> 
is positioned below the browser. A dialog is a window specialised in 
handling the layout of controllers, or
<A class="" href="summary.html#class:dialog_item">dialog_item</A> 
objects as <font size=-1>XPCE</font> calls them.

<P>Line 5 adds a <A class="" href="summary.html#class:button">button</A> 
to the dialog. `view' specifies the <EM>name</EM> of the button. <font size=-1>XPCE</font> 
defines a central mapping from `dialog_item
<B><CODE>&lt;-&gt;</CODE>name</B>' to the label displayed. The default 
mapping capitalises the name and replaces underscores with spaces. In <A class="sec" href="mlingual.html">section 
11.7</A>, we describe how this can be used to realise multi-lingual 
applications. The second argument of the button specifies the <EM>action</EM> 
associated with the button. A <A class="" href="summary.html#class:message">message</A> 
is a dormant send-operation. When pressed, the button executes

<PRE class="code">
send(@prolog, view, DirObj, B?selection?key)
</PRE>

<P>If a message is sent to <A NAME="idx:prolog:70"></A><B>@prolog</B>, 
this calls a predicate with the name of the <EM>selector</EM> of the 
message and an arity that equals the number of arguments to the message 
(2 here).

<P>The second argument is a term with principal functor <A class="" href="summary.html#class:?">?</A> 
and defines an `obtainer', a dormant get-operation. It is defined as a 
Prolog infix operator of type <TT>yfx</TT>, priority 500. This implies 
that
<TT>B?selection?key</TT> should be read as<SUP class="fn">4<SPAN class="fn-text">Initial 
versions of <font size=-1>XPCE</font> lacked the obtainer. In this case 
the browser <EM>B</EM> would be passed to the predicate <A NAME="idx:view1:71"></A><SPAN class="pred-ext">view/1</SPAN>, 
which would extract the current filename from the browser. Obtainers 
improve the readability and avoid the need to mix UI related code with 
application code.</SPAN></SUP>

<PRE class="code">
?(?(B, selection), key)
</PRE>

<P>The result of the get-operation <B><CODE>&lt;-</CODE>selection</B> on 
a browser returns the
<A class="" href="summary.html#class:dict_item">dict_item</A> object 
currently selected. Dict-items are the elements of a <A class="" href="summary.html#class:dict">dict</A>, 
the underlying data object of a browser. A <A class="" href="summary.html#class:dict_item">dict_item</A> 
consists of a <B><CODE>&lt;-</CODE>key</B> (its identifier), a <B><CODE>&lt;-</CODE>label</B> 
(the visual text) and optionally an associated <B><CODE>&lt;-</CODE>object</B>.

<P>Line 8 appends a second button to the dialog window. The dialog 
window will automatically align this one to the right of the first. The 
action sends a message directly to another <font size=-1>XPCE</font> 
object and <B><CODE>-&gt;</CODE>destroys</B> the frame object if the 
quit button is pressed. Note that this will erase all UI objects 
associated with the frame. The garbage collector destroys all related 
objects.

<P>Line 10 fills the <A class="" href="summary.html#class:browser">browser</A> 
with the files from the specified directory. The expression <TT>DirObj?files</TT> 
defines an obtainer operating on an instance of class <A class="" href="summary.html#class:directory">directory</A>. 
The obtainer evaluates to a <A class="" href="summary.html#class:chain">chain</A>, <font size=-1>XPCE</font>'s 
notion of a list, holding the names of all files in the directory. This 
chain is sent to the members-method of the browser <VAR>B</VAR>.

<P>Again, the garbage collector takes care of the directory and chain 
objects. The browser automatically converts the given names to
<A class="" href="summary.html#class:dict_item">dict_item</A> objects.<SUP class="fn">5<SPAN class="fn-text">This 
conversion is implemented with class <A class="" href="summary.html#class:dict_item">dict_item</A>. 
In fact, the browser just specifies the desired type and the message 
passing kernel invokes the conversion method of class <A class="" href="summary.html#class:dict_item">dict_item</A>.</SPAN></SUP>

<P>Finally, the frame is <B><CODE>-&gt;</CODE>open</B>ed. This will 
cause the frame to ask each of the windows to compute its desired size, 
after which the frame aligns the windows, decides on their final 
geometry and creates the Window-system counterpart.

<P>The <A NAME="idx:view2:72"></A><SPAN class="pred-ext">view/2</SPAN> 
callback predicate opens an instance of <A class="" href="summary.html#class:view">view</A>, 
a window specialised in text-editing, gets the file-object for the given 
filename F. and loads the file into the view. The method <A NAME="idx:viewsendload:73"></A>`<B>view<CODE>-&gt;</CODE>load</B>' 
expects an instance of class <A class="" href="summary.html#class:file">file</A>. 
Again, the type-conversion will deal with this.

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