Fix ItemParticle constant CPU usage

Takes particleSystem enabled and running states and particleItem
enabled and its parent enabled into account with some connections

Fixes: QTBUG-117923
Pick-to: 6.5
Change-Id: I3921759cf09a3aea2b8bfa51f847c5a909bb534b
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit c6fad0e79ec7768e1bbd852dd85a594e4318f8f7)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 3fbca326e5bd4c0fabec79bfc7ff7439b30fc2d0)

2 files changed, 63 insertions, 4 deletions

diff --git a/src/particles/qquickitemparticle.cpp b/src/particles/qquickitemparticle.cpp
index cd48095b18..f6d352d478 100644
--- a/src/particles/qquickitemparticle.cpp
+++ b/src/particles/qquickitemparticle.cpp
@@ -94,7 +94,11 @@ QQuickItemParticle::QQuickItemParticle(QQuickItem *parent) :
 {
     setFlag(QQuickItem::ItemHasContents);
     clock = new Clock(this);
-    clock->start();
+    connect(this, &QQuickItemParticle::systemChanged, this, &QQuickItemParticle::reconnectSystem);
+    connect(this, &QQuickItemParticle::parentChanged, this, &QQuickItemParticle::reconnectParent);
+    connect(this, &QQuickItemParticle::enabledChanged, this, &QQuickItemParticle::updateClock);
+    reconnectSystem(m_system);
+    reconnectParent(parent);
 }
 
 QQuickItemParticle::~QQuickItemParticle()
@@ -230,9 +234,10 @@ QSGNode* QQuickItemParticle::updatePaintNode(QSGNode* n, UpdatePaintNodeData* d)
     if (m_pleaseReset)
         m_pleaseReset = false;
 
-    prepareNextFrame();
-
-    update();//Get called again
+    if (clockShouldUpdate()) {
+        prepareNextFrame();
+        update(); //Get called again
+    }
     if (n)
         n->markDirty(QSGNode::DirtyMaterial);
     return QQuickItem::updatePaintNode(n,d);
@@ -285,6 +290,52 @@ QQuickItemParticleAttached *QQuickItemParticle::qmlAttachedProperties(QObject *o
     return new QQuickItemParticleAttached(object);
 }
 
+bool QQuickItemParticle::clockShouldUpdate() const
+{
+    QQuickItem *parentItem = qobject_cast<QQuickItem *>(parent());
+    return (m_system && m_system->isRunning() && !m_system->isPaused() && m_system->isEnabled()
+            && ((parentItem && parentItem->isEnabled()) || !parentItem) && isEnabled());
+}
+
+void QQuickItemParticle::reconnectParent(QQuickItem *parentItem)
+{
+    updateClock();
+    disconnect(m_parentEnabledStateConnection);
+    if (parentItem) {
+        m_parentEnabledStateConnection = connect(parentItem, &QQuickParticleSystem::enabledChanged,
+                                                 this, &QQuickItemParticle::updateClock);
+    }
+}
+
+void QQuickItemParticle::reconnectSystem(QQuickParticleSystem *system)
+{
+    updateClock();
+    disconnect(m_systemRunStateConnection);
+    disconnect(m_systemPauseStateConnection);
+    disconnect(m_systemEnabledStateConnection);
+    if (system) {
+        m_systemRunStateConnection = connect(m_system, &QQuickParticleSystem::runningChanged, this, [this](){
+            QQuickItemParticle::updateClock();
+        });
+        m_systemPauseStateConnection = connect(m_system, &QQuickParticleSystem::pausedChanged, this, [this](){
+            QQuickItemParticle::updateClock();
+        });
+        m_systemEnabledStateConnection = connect(m_system, &QQuickParticleSystem::enabledChanged, this,
+                                                 &QQuickItemParticle::updateClock);
+    }
+}
+
+void QQuickItemParticle::updateClock()
+{
+    if (clockShouldUpdate()) {
+        if (!clock->isRunning())
+            clock->start();
+    } else {
+        if (clock->isRunning())
+            clock->pause();
+    }
+}
+
 QT_END_NAMESPACE
 
 #include "moc_qquickitemparticle_p.cpp"
diff --git a/src/particles/qquickitemparticle_p.h b/src/particles/qquickitemparticle_p.h
index cec3432c46..acb40d8769 100644
--- a/src/particles/qquickitemparticle_p.h
+++ b/src/particles/qquickitemparticle_p.h
@@ -71,6 +71,10 @@ protected:
     void initialize(int gIdx, int pIdx) override;
     void prepareNextFrame();
 private:
+    bool clockShouldUpdate() const;
+    void updateClock();
+    void reconnectSystem(QQuickParticleSystem *system);
+    void reconnectParent(QQuickItem *parent);
     void processDeletables();
     void tick(int time = 0);
     QSet<QQuickItem* > m_deletables;
@@ -85,6 +89,10 @@ private:
 
     typedef QTickAnimationProxy<QQuickItemParticle, &QQuickItemParticle::tick> Clock;
     Clock *clock;
+    QMetaObject::Connection m_systemRunStateConnection;
+    QMetaObject::Connection m_systemPauseStateConnection;
+    QMetaObject::Connection m_systemEnabledStateConnection;
+    QMetaObject::Connection m_parentEnabledStateConnection;
 };
 
 class QQuickItemParticleAttached : public QObject