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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<title>GooCanvas Reference Manual: Underlying Architecture</title>
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<div class="refentry">
<a name="goocanvas-architecture"></a><div class="titlepage"></div>
<div class="refnamediv"><table width="100%"><tr>
<td valign="top">
<h2><span class="refentrytitle">Underlying Architecture</span></h2>
<p>Underlying Architecture — how the canvas fits together.</p>
</td>
<td valign="top" align="right"></td>
</tr></table></div>
<div class="refsect1">
<a name="architecture"></a><h2>Underlying Architecture</h2>
<div class="refsect2">
<a name="widget"></a><h3>The GooCanvas Widget</h3>
<p>
	<a class="link" href="GooCanvas.html" title="GooCanvas"><span class="type">GooCanvas</span></a> is a <span class="type">GtkWidget</span> (it is actually a subclass of
	<span class="type">GtkContainer</span>), and so can be placed in an interface just like
	any normal widget. Usually a <a class="link" href="GooCanvas.html" title="GooCanvas"><span class="type">GooCanvas</span></a> widget would be placed inside
	a <span class="type">GtkScrolledWindow</span> in order to enable scrolling of the canvas.
      </p>
<p>
	The size of the canvas can be set explicitly using
	<a class="link" href="GooCanvas.html#goo-canvas-set-bounds" title="goo_canvas_set_bounds ()"><code class="function">goo_canvas_set_bounds()</code></a>, or if the <a class="link" href="GooCanvas.html#GooCanvas--automatic-bounds" title='The "automatic-bounds" property'><span class="type">"automatic-bounds"</span></a>
	property is set to <code class="literal">TRUE</code> the bounds will be automatically calculated
	to include all of the canvas items. The units used in the canvas can
	be set with the <a class="link" href="GooCanvas.html#GooCanvas--units" title='The "units" property'><span class="type">"units"</span></a> property. The canvas units can be
	pixels, points, inches or millimeters and apply to the canvas and
	all items.
      </p>
</div>
<hr>
<div class="refsect2">
<a name="simple-structure"></a><h3>The Structure of the Simple Canvas</h3>
<p>
	The simple canvas consists of a hierarchy of canvas items.
	The root item is automatically created by the canvas and can be
	accessed using <a class="link" href="GooCanvas.html#goo-canvas-get-root-item" title="goo_canvas_get_root_item ()"><code class="function">goo_canvas_get_root_item()</code></a>. New items and groups can
	then be created and added to the root item.
      </p>
<p>
	Each item in the canvas keeps a <a class="link" href="goocanvas2-GooCanvas-Types.html#GooCanvasBounds" title="struct GooCanvasBounds"><span class="type">GooCanvasBounds</span></a> structure which
	stores the bounding rectangle of the item and all of its descendants.
	This makes it easy to find out which items in the canvas need repainting
	or which item the mouse is over. (The bounds are stored in the canvas
	coordinate space, which is the coordinate space of the entire canvas,
	after any item transformation matrices have been applied.)
      </p>
</div>
<hr>
<div class="refsect2">
<a name="model-view-structure"></a><h3>The Structure of the Model/View Canvas</h3>
<p>
	The model/view canvas consists of a hierarchy of item models, and an
	identical hierarchy of canvas items, with each canvas item
	corresponding to one item model.
      </p>
<p>
	The hierarchy of item models can be used in several <a class="link" href="GooCanvas.html" title="GooCanvas"><span class="type">GooCanvas</span></a>
	widgets, to allow multiple views of the same model.
	Though different canvas items will be used in each <a class="link" href="GooCanvas.html" title="GooCanvas"><span class="type">GooCanvas</span></a>.
      </p>
<p>
	The root item model is set with <a class="link" href="GooCanvas.html#goo-canvas-set-root-item-model" title="goo_canvas_set_root_item_model ()"><code class="function">goo_canvas_set_root_item_model()</code></a>.
	The canvas will automatically create canvas items to display
	the hierarchy of item models, and will automatically add and
	remove canvas items as the item model hierarchy is changed.
      </p>
</div>
<hr>
<div class="refsect2">
<a name="updates"></a><h3>The Update Procedure</h3>
<p>
	When items are added to the canvas or their properties are changed
	they may need to recalculate their bounds. To do this they set an
	internal flag such as <em class="parameter"><code>need_update</code></em>, and make a call to
	<a class="link" href="GooCanvasItem.html#goo-canvas-item-request-update" title="goo_canvas_item_request_update ()"><code class="function">goo_canvas_item_request_update()</code></a>.
      </p>
<p>
	<a class="link" href="GooCanvas.html" title="GooCanvas"><span class="type">GooCanvas</span></a> handles all the update requests at once, to avoid multiple
	redraws of the same parts of the canvas. To do this it installs
	an idle handler, <code class="function">goo_canvas_idle_handler()</code>, which is called as soon
	as the application is idle (and before any part of the canvas is
	redrawn). 
      </p>
<p>
	The idle handler calls <a class="link" href="GooCanvasItem.html#goo-canvas-item-update" title="goo_canvas_item_update ()"><code class="function">goo_canvas_item_update()</code></a> on the root item,
	which recursively calls <a class="link" href="GooCanvasItem.html#goo-canvas-item-update" title="goo_canvas_item_update ()"><code class="function">goo_canvas_item_update()</code></a> on any items as
	necessary, recalculating their bounds and requesting redraws as
	appropriate.
      </p>
<p>
	If a container item (e.g. <a class="link" href="GooCanvasGroup.html" title="GooCanvasGroup"><span class="type">GooCanvasGroup</span></a>) is changed it needs to
	ensure that all descendants recalculate their bounds so it
	calls <a class="link" href="GooCanvasItem.html#goo-canvas-item-update" title="goo_canvas_item_update ()"><code class="function">goo_canvas_item_update()</code></a> for all of its children with the
	<em class="parameter"><code>entire_tree</code></em> argument set to <code class="literal">TRUE</code>.
      </p>
</div>
<hr>
<div class="refsect2">
<a name="simple-updates"></a><h3>How Changes to Items are Handled</h3>
<p>
	When an item is changed (e.g. if the <a class="link" href="GooCanvasRect.html#GooCanvasRect--x" title='The "x" property'><span class="type">"x"</span></a> property of
	a <a class="link" href="GooCanvasRect.html" title="GooCanvasRect"><span class="type">GooCanvasRect</span></a> is changed), the item calls
	<a class="link" href="GooCanvasItemSimple.html#goo-canvas-item-simple-changed" title="goo_canvas_item_simple_changed ()"><code class="function">goo_canvas_item_simple_changed()</code></a> with a flag indicating if the
	bounds of the item need	to be recalculated.
      </p>
<p>
	If the bounds don't need to be recalculated, then
	<a class="link" href="GooCanvas.html#goo-canvas-request-redraw" title="goo_canvas_request_redraw ()"><code class="function">goo_canvas_request_redraw()</code></a> is called to simply request that the
	item is redrawn. This results in a call to <code class="function">gdk_window_invalidate_rect()</code>
	and the redraw proceeds just like a normal <span class="type">GtkWidget</span>.
      </p>
<p>
	However, if the bounds do need to be recalculated then
	<a class="link" href="GooCanvasItem.html#goo-canvas-item-request-update" title="goo_canvas_item_request_update ()"><code class="function">goo_canvas_item_request_update()</code></a> is called to request that the item
	be updated the next time the canvas performs an update.
      </p>
</div>
<hr>
<div class="refsect2">
<a name="model-view-updates"></a><h3>How Changes are Handled in the Model/View Canvas</h3>
<p>
	In the Model/View canvas it is the underlying item models which are
	initially changed. The item models emit "changed" signals which the
	items respond to. For the standard canvas items the
	<code class="function">goo_canvas_item_model_simple_changed()</code> signal handler is called,
	which calls <a class="link" href="GooCanvasItemSimple.html#goo-canvas-item-simple-changed" title="goo_canvas_item_simple_changed ()"><code class="function">goo_canvas_item_simple_changed()</code></a> and the
	procedure continues as in the simple canvas case above.
      </p>
</div>
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