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<h1>6 Default Values</h1>
<h3><a name="id66044">6.1
The Default Value Model</a></h3>
<p>When a new object is created, a set of options is provided by the application. Options which are not explicitly given are taken care of by the parent (the container object).</p>
<div class="example"><pre>
B=gs:create(button,Win,[{x,0},{label,{text,"press Me"}}]).
</pre></div>
<p>In the example shown above, the window provides default values for options like location and background color. If an application cannot use the default values provided by GS, new ones can be configured. For example, the following code creates a red button at location y=30.</p>
<div class="example"><pre>
gs:config(Win,[{default,button,{y,30}},
{default,button,{font,{courier,18}}}]),
B=gs:create(button,Win,[{x,0},{label,{text,"press Me"}}]).
</pre></div>
<p>The syntax for the default option is <span class="code">{default,Objecttype,{Option,DefaultValue}}</span>, where <span class="code">Objecttype</span> is the name of any GS object. The special keywords <span class="code">all</span> or <span class="code">buttons</span> which denote button, radio button, and check button can be used.</p>
<p>The semantics for the default option can be expressed as follows: If an object of kind <span class="code">Objecttype</span> is created and no value for <span class="code">Option</span> is given, then use <span class="code">DefaultValue</span> as the value. Only options of <span class="code">{Key,Value}</span> syntax can be given a default values. Default values may be inherited in several steps. In the following example, the button will show the text "Cancel".</p>
<div class="example"><pre>
gs:config(Win,[{default,button,{label,{text,"Cancel"}}}]),
F=gs:create(frame,Win,[]),
B=gs:create(button,F,[]).
</pre></div>
<p>Default values are inherited so that changed default values only affect new objects, not existing objects.</p>
<p>Default values only have meaning when creating child objects, since objects which cannot have children cannot have default options. An example is buttons.</p>
<p>The following example illustrates how default options can be used:</p>
<a name="gs_fig7"></a>
<img alt="IMAGE MISSING" src="images/ex16.gif"><br>
<em>Figure
6.1:
Example of Default Options</em>
<div class="example"><pre>
-module(ex16).
-copyright('Copyright (c) 1991-97 Ericsson Telecom AB').
-vsn('$Revision: /main/release/3 $ ').
-export([start/0,init/0]).
start() -> spawn(ex16, init, []).
init() ->
I=gs:start(),
Win=gs:create(window, I,
[{width, 200},{height, 200},
{title,"Default Demo"},{map, true}]),
gs:create(canvas, can1,Win,
[{x,0},{y, 0},{width,200},{height,200},
{default,text,{font,{courier,bold,19}}},
{default,text,{fg,blue}},
{default,rectangle,{fill,red}},{default,text,{text,"Pow!"}},
{default,oval,{fill,green}}]),
{A,B,C} = erlang:now(),
random:seed(A,B,C),
loop().
loop() ->
receive
{gs,_Id,destroy,_Data,_Arg} -> bye
after 500 ->
XY = {random:uniform(200),random:uniform(200)},
draw(random:uniform(3),XY),
loop()
end.
draw(1,XY) ->
gs:create(text,can1,[{coords,[XY]}]);
draw(2,XY) ->
XY2 = {random:uniform(200),random:uniform(200)},
gs:create(rectangle,can1,[{coords,[XY,XY2]}]);
draw(3,XY) ->
XY2 = {random:uniform(200),random:uniform(200)},
gs:create(oval,can1,[{coords,[XY,XY2]}]).
</pre></div>
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