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    <p>The solution is to combine various things that you're already doing. Instrument <code>itemChange</code>, looking for and count the items with updated geometry. Once you've counted as many items as there are in the current selection, fire off a signal that will have everything ready for updating your status. Make sure you've set the <code>QGraphicsItem::ItemSendsGeometryChanges</code> flag on all your items! </p> <p>This code was edited to remove the lag inherent in using a zero-timer approach. Below is a <a href="http://sscce.org" rel="nofollow noreferrer">sscce</a> that demonstrates it.</p> <p>You create circles of random radius by clicking in the window. The selection is toggled with Ctrl-click or ⌘-click. When you move the items, a centroid diamond follows the centroid of the selected group. This gives a visual confirmation that the code does indeed work. When the selection is empty, the centroid is not displayed.</p> <p>I've gratuitously added code to show how to leverage Qt's property system so that the items can be generic and leverage the <code>notifier</code> property of a scene if it has one. In its absence, the items simply don't notify, and that's it.</p> <p><a href="https://i.stack.imgur.com/Y6omA.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Y6omA.png" alt="screenshot of the example"></a></p> <pre><code>// https://github.com/KubaO/stackoverflown/tree/master/questions/scenemod-11232425 #include &lt;QtWidgets&gt; const char kNotifier[] = "notifier"; class Notifier : public QObject { Q_OBJECT int m_count = {}; public: int count() const { return m_count; } void inc() { m_count ++; } void notify() { m_count = {}; emit notification(); } Q_SIGNAL void notification(); }; typedef QPointer&lt;Notifier&gt; NotifierPointer; Q_DECLARE_METATYPE(NotifierPointer) template &lt;typename T&gt; class NotifyingItem : public T { protected: QVariant itemChange(QGraphicsItem::GraphicsItemChange change, const QVariant &amp;value) override { QVariant v; if (change == T::ItemPositionHasChanged &amp;&amp; this-&gt;scene() &amp;&amp; (v=this-&gt;scene()-&gt;property(kNotifier)).isValid()) { auto notifier = v.value&lt;NotifierPointer&gt;(); notifier-&gt;inc(); if (notifier-&gt;count() &gt;= this-&gt;scene()-&gt;selectedItems().count()) { notifier-&gt;notify(); } } return T::itemChange(change, value); } }; // Note that all you need to make Circle a notifying item is to derive from // NotifyingItem&lt;basetype&gt;. class Circle : public NotifyingItem&lt;QGraphicsEllipseItem&gt; { QBrush m_brush; public: Circle(const QPointF &amp; c) : m_brush(Qt::lightGray) { const qreal r = 10.0 + (50.0*qrand())/RAND_MAX; setRect({-r, -r, 2.0*r, 2.0*r}); setPos(c); setFlags(QGraphicsItem::ItemIsMovable | QGraphicsItem::ItemIsSelectable | QGraphicsItem::ItemSendsGeometryChanges); setPen({Qt::red}); setBrush(m_brush); } }; class View : public QGraphicsView { Q_OBJECT QGraphicsScene scene; QGraphicsSimpleTextItem text; QGraphicsRectItem centroid{-5, -5, 10, 10}; Notifier notifier; int deltaCounter = {}; public: explicit View(QWidget *parent = {}); protected: Q_SLOT void gotUpdates(); void mousePressEvent(QMouseEvent *event) override; }; View::View(QWidget *parent) : QGraphicsView(parent) { centroid.hide(); centroid.setRotation(45.0); centroid.setPen({Qt::blue}); centroid.setZValue(2); scene.addItem(&amp;centroid); text.setPos(5, 470); text.setZValue(1); scene.addItem(&amp;text); setRenderHint(QPainter::Antialiasing); setScene(&amp;scene); setSceneRect(0,0,500,500); scene.setProperty(kNotifier, QVariant::fromValue(NotifierPointer(&amp;notifier))); connect(&amp;notifier, &amp;Notifier::notification, this, &amp;View::gotUpdates); connect(&amp;scene, &amp;QGraphicsScene::selectionChanged, &amp;notifier, &amp;Notifier::notification); } void View::gotUpdates() { if (scene.selectedItems().isEmpty()) { centroid.hide(); return; } centroid.show(); QPointF centroid; qreal area = {}; for (auto item : scene.selectedItems()) { const QRectF r = item-&gt;boundingRect(); const qreal a = r.width() * r.height(); centroid += item-&gt;pos() * a; area += a; } if (area &gt; 0) centroid /= area; auto st = QStringLiteral("delta #%1 with %2 items, centroid at %3, %4") .arg(deltaCounter++).arg(scene.selectedItems().count()) .arg(centroid.x(), 0, 'f', 1).arg(centroid.y(), 0, 'f', 1); this-&gt;centroid.setPos(centroid); text.setText(st); } void View::mousePressEvent(QMouseEvent *event) { const auto center = mapToScene(event-&gt;pos()); if (! scene.itemAt(center, {})) scene.addItem(new Circle{center}); QGraphicsView::mousePressEvent(event); } int main(int argc, char *argv[]) { QApplication app{argc, argv}; View v; v.show(); return app.exec(); } #include "main.moc" </code></pre>
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    1. PONeed QGraphicsScene signal or event for _after_ change
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    1. COThank you for your answer! It seems to work but would be very cumbersome. I like Slavik81's idea better. He uses a 0ms timer.
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    2. CORun the code and see how it performs. A previous version (check the edits) used a 0ms timer and lagged -- as one would expect. Run that one too and see for yourself. To see the effect you must have graphical feedback -- your vision will notice the lag. I don't see how it's cumbersome -- you can factor out the `itemChange` into a separate template class in the [CRTP](http://en.wikipedia.org/wiki/Curiously_recurring_template_pattern) fashion. I'm editing the code to show how.
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    3. COIOW: If you think this is "cumbersome", you've seen nothin'. Is adding `NotifyngItem<baseclass>` "cumbersome"? I'd say it's the cleanest way to do it given existing Qt implementation, and given that you don't want any lag. Try it, and also try the code with 0ms timer :)
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