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Commit 4b001fa0 authored by edgrif's avatar edgrif
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initial version of foo canvas usage/coding doc.

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<HTML>
<HEAD>
<TITLE>The Foo Canvas Widget</TITLE>
</HEAD>
<BODY>
<H1>The Foo Canvas Widget</H1>
<H2>Overview</H2>
<H2>Coordinate Systems</H2>
<H2>Event Handling</H2>
<P>Event handling is not completely straight forward because event handling is split between
events that happen within the root item of the canvas and events that happen within the
canvas window but <b>not</b> within the root item. These two areas will often be disjoint
because the width and/or height of one will be larger than the other. By default the root item
is set to be the size of the bounding rectangle of all the items on the canvas. I guess in theory
we could resize the root item to be at least as big as the canvas window.
<P>Within the root item, event handling is consistent with a cascade of:
<pre><code>top item --> parent item --> ....... --> parent item --> root item
</code></pre>
<P>If any item event handler registered by the application returns <code>TRUE</code> the event is not
passed on to the items parent, otherwise it is until the root item is reached. At this point
however the event is <b>not</b> passed on to any other obvious handler. If you register an event
handler generally for the canvas window it gets called <b>before</b> the handlers registered
for the canvas items. This is even if you use the <code>g_signal_connect_after()</code> call
to register the handler. This is a pain for several reasons:
<ul>
<li>it means the general event handler for canvas events is not in the same system as the
other event handlers.
<li>interactions between this event handler and the others are complicated because this base
handler cannot act as the catch all for all events not handled by canvas items.
</ul>
<P>But perhaps is also good in that handling of canvas <b>window</b> events can be separated
from handling of canvas <b>item</b> events.
<P>Perhaps more thought required here.
<HR>
<ADDRESS><a href="mailto:edgrif@sanger.ac.uk">Ed Griffiths &lt;edgrif@sanger.ac.uk&gt;</a></ADDRESS>
<!-- hhmts start -->
Last modified: Mon Jan 17 15:16:35 GMT 2005
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</BODY>
</HTML>
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