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    <ul> <li><strong>how QGraphicsItem knows when the bounding rect changes?</strong></li> </ul> <p>I think <em>you know</em>. Let's say you have a QGraphicsItemGroup aggregating several child items, and you want to show only one child at a time. The bounding rect of the group item needs to be the same as the bounding rect of the currently selected item:</p> <pre><code>QRectF MyItemGroup::boundingRect() const { QRectF rect; if (currentItem_) { rect = currentItem_-&gt;boundingRect(); } return rect; } </code></pre> <p>Suppose you have a method to change which one of the children has to be shown:</p> <pre><code>void MyItemGroup::setCurrentItem(MyItem * item) { if (list_of_items_.contains(item)) { prepareGeometryChange(); currentItem_ = item; } } </code></pre> <p>If you comment out prepareGeometryChange, the view will not ask again for MyItemGroup's bounding rect, and the update of the item (triggered somewhere else) will involve the wrong rect.</p> <ul> <li><strong>how it knows when to call update()?</strong></li> </ul> <p>According with the QGraphicsItem sources, inside the <code>prepareGeometry</code> method update is called only in two cases:</p> <ol> <li>if someone is connected to the <code>changed</code> signal</li> <li>if the scene has no views</li> </ol> <p>more details <a href="http://qt.gitorious.org/qt/qt/blobs/4.8/src/gui/graphicsview/qgraphicsitem.cpp" rel="noreferrer">here</a></p> <ul> <li><strong>Are you ever supposed to call update() yourself after calling prepareGeometryChange() and then changing the bounding rectangle?</strong></li> </ul> <p>I think the answer is <em>No</em>. Aside from the call to <code>update</code>, <code>prepareGeometryChange</code> marks as dirty the item region on the scene, causing a repaint of the region itself. In my experience this sufficed in the 100% of the cases, so I didn't need to call update().</p>
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    1. POHow does QGraphicsItem::prepareGeometryChange() work?
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    1. POHow does QGraphicsItem::prepareGeometryChange() work?
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    1. COmore generally, you could precise it should be called whenever the input of `::boundingRect()` computation changes. Your example is right though.
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    2. COThanks for your answer. However, I'm still wondering *how* it works. For example, when does it call update()?
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    3. COanswer updated, hope this helps.
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