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<H2><A NAME="sec:4.4"><SPAN class="sec-nr">4.4</SPAN> <SPAN class="sec-title">Modal
dialogs: prompting for answers</SPAN></A></H2>
<A NAME="sec:modal"></A>
<P><A NAME="idx:modaldialog:144"></A><A NAME="idx:askquestions:145"></A><A NAME="idx:prompt:146"></A>A <EM>modal</EM>
dialog is a dialog that is displayed and blocks the application until
the user has finished answering the questions posed in the dialog. Modal
dialogs are often used to prompt for values needed to create a new
entity in the application or for changing settings.
<P>Modal windows are implemented using the methods <A NAME="idx:framegetconfirm:147"></A>`<B>frame<CODE><-</CODE>confirm</B>'
and
<A NAME="idx:framesendreturn:148"></A>`<B>frame<CODE>-></CODE>return</B>'. <A NAME="idx:framegetconfirm:149"></A>`<B>Frame<CODE><-</CODE>confirm</B>'
invokes <A NAME="idx:framesendopen:150"></A>`<B>frame<CODE>-></CODE>open</B>'
if the frame is not visible and then starts reading events and
processing them.
<A NAME="idx:framesendreturn:151"></A>`<B>Frame<CODE>-></CODE>return</B>:
value' causes <A NAME="idx:framegetconfirm:152"></A>`<B>frame<CODE><-</CODE>confirm</B>'
to return with the value passed as argument to <B><CODE>-></CODE>return</B>.
The following code is a very simple example opening a dialog and waiting
for the user to enter a name and press <font size=-1>RETURN</font> or
the <B>Ok</B> button.
<PRE class="code">
ask_name(Name) :-
new(D, dialog('Prompting for name')),
send(D, append,
new(TI, text_item(name, ''))),
send(D, append,
button(ok, message(D, return,
TI?selection))),
send(D, append,
button(cancel, message(D, return, @nil))),
send(D, default_button, ok), % Ok: default button
get(D, confirm, Answer), % This blocks!
send(D, destroy),
Answer \== @nil, % canceled
Name = Answer.
?- ask_name(X).
X = 'Bob Worker'
</PRE>
<P>See also <A class="sec" href="application.html">section 10.5</A> for
a discussion on how frames may be related, as well as alternatives for
keeping control in Prolog.
<H3><A NAME="sec:4.4.1"><SPAN class="sec-nr">4.4.1</SPAN> <SPAN class="sec-title">Example:
a simple editor for multiple fonts</SPAN></A></H3>
<P>The following example allows the user to select text in an editor and
change its appearance. The application is shown in <A class="fig" href="modal.html#fig:wysiwyg">figure
10</A>.
<P><A NAME="fig:wysiwyg"></A>
<CENTER>
<IMG SRC="wysiwyg.gif">
</CENTER>
<TABLE ALIGN=center WIDTH="75%"><TR><TD>
<B>Figure 10 : </B>Very simple WYSIWYG editor</TABLE>
<P>
<HR>
A typical XPCE/Prolog module header. Always make sure to load module
library(pce) explicitly if you want to write modules portable to the
various Prolog dialects supported by XPCE.
<PRE class="code">
:- module(wysiwyg,
[ wysiwyg/1 % +File
]).
:- use_module(library(pce)).
:- use_module(library(pce_style_item)).</PRE>
<P>Create the main window, consisting of a frame holding a dialog window
with a button for defining new styles and a menu for setting the style
of the selection. Both dialog items use call-back to <A NAME="idx:prolog:153"></A><B>@prolog</B>.
<PRE class="code"><A NAME="idx:wysiwyg1:154"></A>
wysiwyg(File) :-
new(Fr, frame(File)),
send(Fr, append, new(D, dialog)),
send(new(V, view), below, D),
send(V, font, normal),
send(D, append,
button(define_style,<A NAME="idx:prolog:155"></A>
message(@prolog, define_style, Fr))),
send(D, append,
menu(style, toggle,<A NAME="idx:prolog:156"></A>
and(message(@prolog, set_style, Fr, @arg1),
message(V, selection, 0, 0),<A NAME="idx:receiver:157"></A>
message(@receiver, clear_selection))),
right),
append_style(Fr, bold, style(font := bold)),
append_style(Fr, italic, style(font := italic)),
send(V, load, File),
send(Fr, open).</PRE>
<P>Set the style for the current selection. Simply pick the selection
start and end and make a fragment using the selection parameters and the
style-name.
<PRE class="code"><A NAME="idx:setstyle2:158"></A>
set_style(Fr, Style) :-
get(Fr, member, view, V),
get(V, selection, point(Start, End)),
( Start == End
-> send(Fr, report, warning, 'No selection')
; get(V, text_buffer, TB),
new(_, fragment(TB, Start, End-Start, Style))
).</PRE>
<P>Define a new style and add it to the menu and the view.
<PRE class="code"><A NAME="idx:definestyle1:159"></A>
define_style(Fr) :-
ask_style(Fr, Name, Style),
append_style(Fr, Name, Style).
<A NAME="idx:appendstyle3:160"></A>
append_style(Fr, Name, Style) :-
get(Fr, member, dialog, D),
get(D, member, style, Menu),
send(Menu, append, Name),<A NAME="idx:on:161"></A>
send(Menu, active, @on),
get(Fr, member, view, View),
send(View, style, Name, Style).</PRE>
<P>Prompt for the style-name and style-object. Class style_item is
defined in the library(pce_style_item). <A NAME="idx:framesendtransientfor:162"></A>`frame <B><CODE>-></CODE>transient_for</B>'
tells the window manager the dialog is a supporting frame for the main
application.
<A NAME="idx:framegetconfirmcentered:163"></A>`frame <B><CODE><-</CODE>confirm_centered</B>'
opens the frame centered around the given location and starts processing
events until <A NAME="idx:framesendreturn:164"></A>`frame <B><CODE>-></CODE>return</B>'
is activated.
<PRE class="code"><A NAME="idx:askstyle3:165"></A>
ask_style(Fr, Name, Style) :-
new(D, dialog('Define Style')),
send(D, append,
new(N, text_item(name, ''))),
send(D, append,
new(S, style_item(style))),
send(D, append,
button(ok, message(D, return, ok))),
send(D, append,
button(cancel, message(D, return, cancel))),
send(D, default_button, ok),
send(D, transient_for, Fr),
repeat,
get(D, confirm_centered, Fr?area?center, Answer),
( Answer == ok
-> get(N, selection, Name),
( Name == ''
-> send(D, report, error,
'Please enter a name'),
fail
; !,
get(S, selection, Style),<A NAME="idx:on:166"></A>
send(Style, lock_object, @on),
send(D, destroy)
)
; !,
send(D, destroy),
fail
).</PRE>
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