Canvas Item: Prevent polish loop when calling requestAnimationFrame

Currently if you call requestAnimationFrame within the callback given by
requestAnimationFrame there is a polish loop. This is however the common
way when using Canvas2D in the browser to get screen refresh rate
animations. This patch makes sure that polish won't be called when
processing polish items by calling polish indirectly.

Task-number: QTBUG-55778
Change-Id: Ib96e39f7ca3cecc33609976e9049cc309768f9f7
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

2 files changed, 21 insertions, 1 deletions

diff --git a/src/quick/items/context2d/qquickcanvasitem.cpp b/src/quick/items/context2d/qquickcanvasitem.cpp
index b772ed97d2..1167f408f5 100644
--- a/src/quick/items/context2d/qquickcanvasitem.cpp
+++ b/src/quick/items/context2d/qquickcanvasitem.cpp
@@ -640,6 +640,17 @@ void QQuickCanvasItem::releaseResources()
     }
 }
 
+bool QQuickCanvasItem::event(QEvent *event)
+{
+    switch (event->type()) {
+    case QEvent::PolishRequest:
+        polish();
+        return true;
+    default:
+        return QQuickItem::event(event);
+    }
+}
+
 void QQuickCanvasItem::invalidateSceneGraph()
 {
     Q_D(QQuickCanvasItem);
@@ -651,6 +662,12 @@ void QQuickCanvasItem::invalidateSceneGraph()
     d->textureProvider = 0;
 }
 
+void QQuickCanvasItem::schedulePolish()
+{
+    auto polishRequestEvent = new QEvent(QEvent::PolishRequest);
+    QCoreApplication::postEvent(this, polishRequestEvent);
+}
+
 void QQuickCanvasItem::componentComplete()
 {
     QQuickItem::componentComplete();
@@ -892,8 +909,9 @@ void QQuickCanvasItem::requestAnimationFrame(QQmlV4Function *args)
 
     d->animationCallbacks.insert(++id, QV4::PersistentValue(scope.engine, f->asReturnedValue()));
 
+    // QTBUG-55778: Calling polish directly here can lead to a polish loop
     if (isVisible())
-        polish();
+        schedulePolish();
 
     args->setReturnValue(QV4::Encode(id));
 }
diff --git a/src/quick/items/context2d/qquickcanvasitem_p.h b/src/quick/items/context2d/qquickcanvasitem_p.h
index 8af84d0e7c..217ae9bb69 100644
--- a/src/quick/items/context2d/qquickcanvasitem_p.h
+++ b/src/quick/items/context2d/qquickcanvasitem_p.h
@@ -182,6 +182,7 @@ private Q_SLOTS:
     void sceneGraphInitialized();
     void checkAnimationCallbacks();
     void invalidateSceneGraph();
+    void schedulePolish();
 
 protected:
     void componentComplete() Q_DECL_OVERRIDE;
@@ -190,6 +191,7 @@ protected:
     QSGNode *updatePaintNode(QSGNode *, UpdatePaintNodeData *) Q_DECL_OVERRIDE;
     void geometryChanged(const QRectF &newGeometry, const QRectF &oldGeometry) Q_DECL_OVERRIDE;
     void releaseResources() Q_DECL_OVERRIDE;
+    bool event(QEvent *event) Q_DECL_OVERRIDE;
 private:
     Q_DECLARE_PRIVATE(QQuickCanvasItem)
     Q_INVOKABLE void delayedCreate();