bundles/org.eclipse.platform.doc.isv/guide/swt_layouts_custom.htm - platform/eclipse.platform.common - Git at Google

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 <H3>Custom layouts</H3>

 <P>Occasionally you may need to write your own custom
 <a href="../reference/api/org/eclipse/swt/widgets/Layout.html"><strong>Layout</strong></a>
 class. This is most appropriate when you have a complex layout that is used
 in many different places in your application.   Note that unless you are
 writing a very generic layout that will be used by several
 <strong><a href="../reference/api/org/eclipse/swt/widgets/Composite.html">Composite</a></strong>
 widgets, it is sometimes simpler and easier to calculate sizes and position
 children in a resize listener.</P>

 <P>Layouts are responsible for implementing two methods:</P>

 <ul>
   <li><strong>computeSize(...)</strong> calculates the width and height of a
       rectangle that encloses all of the composite's children once they have
       been sized and placed according to the layout algorithm. The hint
       parameters allow the width and/or height to be constrained.  For example,
       a layout may choose to grow in one dimension if constrained in another.</li>
   <li><strong>layout(...)</strong> positions and sizes the composite's children.
       A layout can choose to cache layout-related information, such as the
       preferred extent of each of the children. The <strong>flushCache</strong>
       parameter tells the <a href="../reference/api/org/eclipse/swt/widgets/Layout.html"><strong>Layout</strong></a>
       to flush cached data, which is necessary when other factors besides the
       size of the composite have changed, such as the creation or removal of
       children, or a change in the widget's font.</li>
 </ul>

 <p>A third method, <strong>flushCache(...)</strong>, can be optionally
 implemented to clear any cached data associated with a specific control.
 Often, the computeSize() method of a widget can be expensive, and so layouts
 can cache results to improve performance.</p>

 <p>Further discussion of custom layouts can be found in
 <a href="http://www.eclipse.org/articles/Understanding%20Layouts/Understanding%20Layouts.htm">Understanding
 layouts in SWT</a>.</p>

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	<H3>Custom layouts</H3>

	<P>Occasionally you may need to write your own custom
	<a href="../reference/api/org/eclipse/swt/widgets/Layout.html"><strong>Layout</strong></a>
	class. This is most appropriate when you have a complex layout that is used
	in many different places in your application. Note that unless you are
	writing a very generic layout that will be used by several
	<strong><a href="../reference/api/org/eclipse/swt/widgets/Composite.html">Composite</a></strong>
	widgets, it is sometimes simpler and easier to calculate sizes and position
	children in a resize listener.</P>

	<P>Layouts are responsible for implementing two methods:</P>

	<ul>
	<li><strong>computeSize(...)</strong> calculates the width and height of a
	rectangle that encloses all of the composite's children once they have
	been sized and placed according to the layout algorithm. The hint
	parameters allow the width and/or height to be constrained. For example,
	a layout may choose to grow in one dimension if constrained in another.</li>
	<li><strong>layout(...)</strong> positions and sizes the composite's children.
	A layout can choose to cache layout-related information, such as the
	preferred extent of each of the children. The <strong>flushCache</strong>
	parameter tells the <a href="../reference/api/org/eclipse/swt/widgets/Layout.html"><strong>Layout</strong></a>
	to flush cached data, which is necessary when other factors besides the
	size of the composite have changed, such as the creation or removal of
	children, or a change in the widget's font.</li>
	</ul>

	<p>A third method, <strong>flushCache(...)</strong>, can be optionally
	implemented to clear any cached data associated with a specific control.
	Often, the computeSize() method of a widget can be expensive, and so layouts
	can cache results to improve performance.</p>

	<p>Further discussion of custom layouts can be found in
	<a href="http://www.eclipse.org/articles/Understanding%20Layouts/Understanding%20Layouts.htm">Understanding
	layouts in SWT</a>.</p>

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