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<h1>3 Options</h1>
<h3><a name="id58188">3.1
The Option Concept</a></h3>
<p>Each object has a set of options. The options are key-value tuples and the key is an atom. Depending on the option, the value can be any Erlang term. Typical options are: x, y, width, height, text, and color. A list of options should be supplied when an object is created . It is also possible to reconfigure an object with the function <span class="code">gs:config/2</span>. The following example shows one way to create a red button with the text "Press Me" on it:</p>
<div class="example"><pre>
Butt = gs:create(button,Win, [{x,10},{y,10}]),
gs:config(Butt, [{width,50},{height,50},{bg,red}]),
gs:config(Butt, [{label, {text,"Press Me"}},{y,20}]),
</pre></div>
<p>The evaluation order of options is not defined. This implies that the grouping of options shown in the following example is not recommended:</p>
<div class="example"><pre>
Rect = gs:create(rectangle,Can, [{coords,[{10,10},{20,20}]},
{move,{5,5}}]),
</pre></div>
<p>After the operation, the rectangle can be at position <span class="code">[{10,10},{20,20}]</span> or <span class="code">[{15,15},{25,25}]</span>. The following example produces a deterministic behaviour:</p>
<div class="example"><pre>
Rect = gs:create(rectangle,Can,[{coords,[{10,10},{20,20}]},
gs:config(Rect,[{move,{5,5}}]),
</pre></div>
<p>The value of each option can be read individually with the <span class="code">read/2</span> function as shown in the following example:</p>
<div class="example"><pre>
Value = gs:read(ObjectId,Option)
</pre></div>
<p>The next example shows how to read the text and the width options from a button:</p>
<div class="example"><pre>
Text = gs:read(Butt, text),
Width = gs:read(Butt, width),
</pre></div>
<h3><a name="id64276">3.2
The Option Tables</a></h3>
<p>Each object is described in terms of its options. The options are listed in a table as is shown in the following example:</p>
<table border="1" cellpadding="2" cellspacing="0">
<tr>
<td align="left" valign="middle"><strong>{Option,Value}</strong></td>
<td align="left" valign="middle"><strong>Default</strong></td>
<td align="left" valign="middle"><strong>Description</strong></td>
</tr>
<tr>
<td align="left" valign="middle">{fg, Color}</td>
<td align="left" valign="middle"><unspec></td>
<td align="left" valign="middle">Foreground color of the object</td>
</tr>
<tr>
<td align="left" valign="middle">{map, Bool}</td>
<td align="left" valign="middle">false</td>
<td align="left" valign="middle">Visibility on the screen</td>
</tr>
<tr>
<td align="left" valign="middle">...</td>
<td align="left" valign="middle">...</td>
<td align="left" valign="middle">...</td>
</tr>
</table>
<em>Table
3.1:
Options</em>
<p>The <unspec> default value means that either <span class="code">gs</span> or the back-end provides the default value. For example, the <span class="code">fg</span> option can be used as follows:</p>
<div class="example"><pre>
Rect = gs:create(rectangle, Window, [{fg, red}]),
Color = gs:read(Rect, fg),
</pre></div>
<h3><a name="id64432">3.3
Config-Only Options</a></h3>
<p>Most options are read/write key-value tuples such as <span class="code">{select,true|false}</span> and <span class="code">{map,true|false</span>, but some options are by nature write-only, or read-only. For example, buttons can flash for a short time and canvas objects can be moved dx, dy. The following table exemplifies some config-only options:</p>
<table border="1" cellpadding="2" cellspacing="0">
<tr>
<td align="left" valign="middle"><strong>Config-Only</strong></td>
<td align="left" valign="middle"><strong>Description</strong></td>
</tr>
<tr>
<td align="left" valign="middle">flash</td>
<td align="left" valign="middle">Causes the object to flash for 2 seconds.</td>
</tr>
<tr>
<td align="left" valign="middle">raise</td>
<td align="left" valign="middle">Raises the object on top of other overlapping objects.</td>
</tr>
<tr>
<td align="left" valign="middle">{move, {Dx, Dy}}</td>
<td align="left" valign="middle">Moves the object relative to its current position.</td>
</tr>
</table>
<em>Table
3.2:
Config-Only Options</em>
<p><span class="code">gs:config(Button,[flash]),</span> causes the button to flash.</p>
<h3><a name="id64553">3.4
Read-Only Options</a></h3>
<p>The opposite of config-only options are read-only options. The following table exemplifies some read-only options:</p>
<table border="1" cellpadding="2" cellspacing="0">
<tr>
<td align="left" valign="middle"><strong>Read-Only</strong></td>
<td align="left" valign="middle"><strong>Return</strong></td>
<td align="left" valign="middle"><strong>Description</strong></td>
</tr>
<tr>
<td align="left" valign="middle">size</td>
<td align="left" valign="middle">Int</td>
<td align="left" valign="middle">The number of items (entries).</td>
</tr>
<tr>
<td align="left" valign="middle">{get, Index}</td>
<td align="left" valign="middle">String</td>
<td align="left" valign="middle">The entry at index <span class="code">Index</span>.</td>
</tr>
</table>
<em>Table
3.3:
Read-Only Options</em>
<p><span class="code">EntryString = gs:read(Listbox,{get, Index}),</span> is an example.</p>
<h3><a name="id64674">3.5
Data Types</a></h3>
<p>As previously stated, each object is described in terms of its options. This section defines the data types for options.</p>
<dl>
<dt><strong><strong>Anchor|Align.</strong></strong></dt>
<dd><span class="code">n|w|s|e|nw|se|ne|sw|center</span></dd>
<dt><strong><strong>Atom.</strong></strong></dt>
<dd>An Erlang atom such as <span class="code">myWay</span>.</dd>
<dt><strong><strong>Bool.</strong></strong></dt>
<dd>
<span class="code">true</span> or <span class="code">false</span>
</dd>
<dt><strong><strong>Color.</strong></strong></dt>
<dd>
<span class="code">{R,G,B}</span>, or a the predefined name <span class="code">red</span>,<span class="code">green</span>, <span class="code">blue</span>, <span class="code">white</span>, <span class="code">black</span>, <span class="code">grey</span>, or <span class="code">yellow</span>. For example <span class="code">{0,0,0}</span> is black and <span class="code">{255,255,255}</span> is white.</dd>
<dt><strong><strong>Cursor.</strong></strong></dt>
<dd>A mouse cursor, or any of the following: <span class="code">arrow</span>, <span class="code">busy</span>, <span class="code">cross</span>, <span class="code">hand</span>, <span class="code">help</span>, <span class="code">resize</span>, <span class="code">text</span>, or <span class="code">parent</span>. <span class="code">parent</span> has a special meaning, namely that this object will have the same cursor as its <span class="code">parent</span>.</dd>
<dt><strong><strong>FileName.</strong></strong></dt>
<dd>
<span class="code">FileName</span> is a string. The file name may include a directory path and should point out a file of a suitable type. The path can be either absolute or relative to the directory from where Erlang was started.</dd>
<dt><strong><strong>Float.</strong></strong></dt>
<dd>Any float, for example 3.1415.</dd>
<dt><strong><strong>Font.</strong></strong></dt>
<dd>A Font is represented as a two or three tuple:<span class="code">{Family,Size}</span> or <span class="code">{Family,Style,Size}</span>, where <span class="code">Style</span> is <span class="code">bold</span>, <span class="code">italic</span>, or a combination of those in a list. <span class="code">Size</span> is an arbitrary integer. <span class="code">Family</span> is a typeface of type <span class="code">times</span>, <span class="code">courier</span>, <span class="code">helvetica</span>, <span class="code">symbol</span>, <span class="code">new_century_schoolbook</span>, or <span class="code">screen</span> (which is a suitable screen font).</dd>
<dt><strong><strong>Int.</strong></strong></dt>
<dd>Any integer number, for example 42.</dd>
<dt><strong><strong>Label.</strong></strong></dt>
<dd>A label can either be a plain text label <span class="code">{text, String}</span>, or an image <span class="code">{image, FileName}</span> where <span class="code">FileName</span> should point out a bitmap.</dd>
<dt><strong><strong>String.</strong></strong></dt>
<dd>An Erlang list of ASCII bytes. For example,
<span class="code">"Hi there"=[72,105,32,116,104,101,114,101]</span>
</dd>
<dt><strong><strong>Term.</strong></strong></dt>
<dd>Any Erlang term.</dd>
</dl>
<p>In cases where the type is self-explanatory, the name of the parameter is used. For example, <span class="code">{move, {Dx,Dy}}</span>.</p>
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