QQuickWindow: use QVector eventDeliveryTargets to avoid repeated delivery

If no handler or item accepts a newly-pressed point, the event will be
re-delivered in deliverTouchEvent().  It doesn't make sense to
re-deliver to the same handlers though.

A temporary QSet isn't cheap to create, whereas it seems we will need
to keep track of handlers which have already been visited, in order to
avoid visiting passively-grabbing handlers multiple times.  Since both
a QVector and a QSet are heap-allocated, and we expect to have a limited
quantity of handlers grabbing at one time, a retained QVector (cleared
between events) seems to be the cheapest data structure.

Change-Id: I831e9a2576b2fcb9095e065795f2baff58115a49
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>

3 files changed, 8 insertions, 1 deletions

diff --git a/src/quick/items/qquickevents.cpp b/src/quick/items/qquickevents.cpp
index a68e72b0b7..d7b5fe79de 100644
--- a/src/quick/items/qquickevents.cpp
+++ b/src/quick/items/qquickevents.cpp
@@ -731,6 +731,7 @@ QQuickPointerEvent *QQuickPointerMouseEvent::reset(QEvent *event)
         return this;
 
     m_device = QQuickPointerDevice::genericMouseDevice();
+    m_device->eventDeliveryTargets().clear();
     m_button = ev->button();
     m_pressedButtons = ev->buttons();
     Qt::TouchPointState state = Qt::TouchPointStationary;
@@ -765,6 +766,7 @@ QQuickPointerEvent *QQuickPointerTouchEvent::reset(QEvent *event)
         return this;
 
     m_device = QQuickPointerDevice::touchDevice(ev->device());
+    m_device->eventDeliveryTargets().clear();
     m_button = Qt::NoButton;
     m_pressedButtons = Qt::NoButton;
 
diff --git a/src/quick/items/qquickevents_p_p.h b/src/quick/items/qquickevents_p_p.h
index bbfc34dacf..1751e366b2 100644
--- a/src/quick/items/qquickevents_p_p.h
+++ b/src/quick/items/qquickevents_p_p.h
@@ -553,6 +553,8 @@ public:
     static QQuickPointerDevice *genericMouseDevice();
     static QQuickPointerDevice *tabletDevice(qint64);
 
+    QVector<QQuickPointerHandler *> &eventDeliveryTargets() { return m_eventDeliveryTargets; }
+
 private:
     QQuickPointerDevice(DeviceType devType, PointerType pType, Capabilities caps, int maxPoints, int buttonCount, const QString &name, qint64 uniqueId = 0)
       : m_deviceType(devType), m_pointerType(pType), m_capabilities(caps)
@@ -579,6 +581,7 @@ private:
     QPointingDeviceUniqueId m_uniqueId;
     // the device-specific event instance which is reused during event delivery
     QQuickPointerEvent *m_event;
+    QVector<QQuickPointerHandler *> m_eventDeliveryTargets; // during delivery, handlers which have already seen the event
 
     Q_DISABLE_COPY(QQuickPointerDevice)
     friend struct ConstructableQQuickPointerDevice;
diff --git a/src/quick/items/qquickitem.cpp b/src/quick/items/qquickitem.cpp
index 46e381db7d..9b91b4bda4 100644
--- a/src/quick/items/qquickitem.cpp
+++ b/src/quick/items/qquickitem.cpp
@@ -5095,12 +5095,14 @@ bool QQuickItemPrivate::handlePointerEvent(QQuickPointerEvent *event, bool avoid
 {
     Q_Q(QQuickItem);
     bool delivered = false;
+    QVector<QQuickPointerHandler *> &eventDeliveryTargets = event->device()->eventDeliveryTargets();
     if (extra.isAllocated()) {
         for (QQuickPointerHandler *handler : extra->pointerHandlers) {
             qCDebug(lcPointerHandlerDispatch) << "   delivering" << event << "to" << handler << "on" << q;
-            if (!avoidGrabber || !event->hasGrabber(handler)) {
+            if ((!avoidGrabber || !event->hasGrabber(handler)) && !eventDeliveryTargets.contains(handler)) {
                 handler->handlePointerEvent(event);
                 delivered = true;
+                eventDeliveryTargets.append(handler);
             }
         }
     }