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POWhy doesn't setPos() move the QGraphicsItems in the scene?
 

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  1. POWhy doesn't setPos() move the QGraphicsItems in the scene?
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    <p>In the code below, I have three <code>QGraphicsItem</code>s that are laid out by <code>QGraphicsLinearLayout</code>, which is set as layout to a <code>QGraphicsWidget</code>.</p> <pre><code>#include &lt;QApplication&gt; #include &lt;QBrush&gt; #include &lt;QDebug&gt; #include &lt;QGraphicsItem&gt; #include &lt;QGraphicsLayoutItem&gt; #include &lt;QGraphicsLinearLayout&gt; #include &lt;QGraphicsRectItem&gt; #include &lt;QGraphicsScene&gt; #include &lt;QGraphicsView&gt; #include &lt;QGraphicsWidget&gt; #include &lt;QPen&gt; class MyShape : public QGraphicsRectItem, public QGraphicsLayoutItem { public: MyShape(void) { setPen(QPen(QBrush(Qt::black), 1)); setBrush(QBrush(Qt::green)); setRect(0, 0, 20, 20); } virtual QSizeF sizeHint(Qt::SizeHint which, const QSizeF&amp; constraint = QSizeF()) const { Q_UNUSED(which); Q_UNUSED(constraint); return boundingRect().size(); } virtual void setGeometry(const QRectF&amp; rect) { setPos(rect.topLeft()); } }; int main(int argc, char** argv) { QApplication app(argc, argv); QGraphicsScene scene; MyShape* shape1 = new MyShape; MyShape* shape2 = new MyShape; MyShape* shape3 = new MyShape; scene.addItem(shape1); scene.addItem(shape2); scene.addItem(shape3); QGraphicsLinearLayout* layout = new QGraphicsLinearLayout; layout-&gt;addItem(shape1); layout-&gt;addItem(shape2); layout-&gt;addItem(shape3); QGraphicsWidget* container = new QGraphicsWidget; container-&gt;setLayout(layout); scene.addItem(container); container-&gt;setPos(300, 300); // This doesn't appear to have any affect // Item for indicating origin QGraphicsRectItem* tmp = scene.addRect(0, 0, 2, 2, QPen(), QBrush(Qt::green)); tmp-&gt;setPos(0, 0); qDebug() &lt;&lt; tmp-&gt;scenePos(); qDebug() &lt;&lt; container-&gt;scenePos(); QGraphicsView view; view.setScene(&amp;scene); view.centerOn(0, 0); view.show(); return app.exec(); } </code></pre> <p>Then, I try to move the <code>QGraphicsWidget</code> within the scene by calling <code>setPos()</code>, but it doesn't appear to work (the <code>QGraphicsItem</code>s remain in the same place). However, <em>something</em> appears to happen since the scrollbars change. It seems that the <code>QGraphicsWidget</code> moves, without taking the <code>QGraphicsItem</code>s along with it.</p> <p>Why?</p> <h2>EDIT:</h2> <p>By suggestion from <em>DerManu</em>, I changed the inheritance of <code>MyShape</code> to <code>QGraphicsWidget</code>, and with the following code, it works:</p> <pre><code>#include &lt;QApplication&gt; #include &lt;QBrush&gt; #include &lt;QDebug&gt; #include &lt;QGraphicsItem&gt; #include &lt;QGraphicsLayoutItem&gt; #include &lt;QGraphicsLinearLayout&gt; #include &lt;QGraphicsRectItem&gt; #include &lt;QGraphicsScene&gt; #include &lt;QGraphicsView&gt; #include &lt;QGraphicsWidget&gt; #include &lt;QPen&gt; class MyShape : public QGraphicsWidget { public: MyShape(void) { setMinimumSize(20, 20); setMaximumSize(20, 20); setPreferredSize(20, 20); } QRectF boundingRect(void) const { QRectF box(0, 0, 22, 22); box.translate(box.center() * -1); return box; } void paint(QPainter* painter, const QStyleOptionGraphicsItem* option, QWidget* widget) { Q_UNUSED(option); Q_UNUSED(widget); // Set box border appearance painter-&gt;setPen(QPen(Qt::black, 1)); // Set box background appearance painter-&gt;setBrush(QBrush(Qt::green)); QRectF box(0, 0, 20, 20); box.translate(box.center() * -1); painter-&gt;drawRect(box); } }; int main(int argc, char** argv) { QApplication app(argc, argv); QGraphicsScene scene; MyShape* shape1 = new MyShape; MyShape* shape2 = new MyShape; MyShape* shape3 = new MyShape; scene.addItem(shape1); scene.addItem(shape2); scene.addItem(shape3); QGraphicsLinearLayout* layout = new QGraphicsLinearLayout(Qt::Vertical); layout-&gt;addItem(shape1); layout-&gt;addItem(shape2); layout-&gt;addItem(shape3); QGraphicsWidget* container = new QGraphicsWidget; container-&gt;setLayout(layout); scene.addItem(container); container-&gt;setPos(200, 200); // This doesn't appear to have any affect scene.setSceneRect(-300, -300, 600, 600); // Item for indicating origin QGraphicsRectItem* tmp = scene.addRect(0, 0, 2, 2, QPen(), QBrush(Qt::green)); tmp-&gt;setPos(0, 0); qDebug() &lt;&lt; tmp-&gt;scenePos(); qDebug() &lt;&lt; container-&gt;scenePos(); QGraphicsView view; view.setScene(&amp;scene); view.centerOn(0, 0); view.show(); return app.exec(); } </code></pre> <p>So what is it that <code>QGraphicsWidget</code> provides that I missed in the original code such that using <code>setPos</code> on the container item correctly moves the position of the <code>MyShape</code> items?</p>
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    1. PTQuestion
    1. USgablin
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    1. CODoes this also happen when you don't add MyShape items, but for example three QGraphicsWidgets to the layout? I'm trying to figure out whether the setPos call in MyShape::setGeometry breaks anything.
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      1. POWhy doesn't setPos() move the QGraphicsItems in the scene?
      1. USDerManu
    2. CO@DerManu: Well, the problem with that is that there are currently no `QGraphicsWidgets` that implement `QGraphicsLayoutItem`. So I don't know how to go about testing that without having to implement a class (and if I do that I'm back to square one).
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      1. USgablin
    3. COI was mistaken; `QGraphicsWidget` _does_ implement `QGraphicsLayoutItem`. So I tried changing the inheritance from `public QGraphicsRectItem, public QGraphicsLayoutItem` to `public QGraphicsWidget`, override the `paint` and `boundingRect` methods, and _voilá_, now it works! I'm guessing that `QGraphicsWidget` implements some methods that I've forgotten. But which? I'm very curious to find out.
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      1. POWhy doesn't setPos() move the QGraphicsItems in the scene?
      1. USgablin
 

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