QQuickItemParticle: Only delete owned particles

A common pattern for ItemParticle (as shown in the particleview.qml
example) is to use it to manage movement of a model's delegates. In such
a case item management is handled by the user code (which bridges the
needs of the Model and the View) and deleting the items in ItemParticle
quickly leads to crashes.

This change maintains (and improves, due to handling resets) the
deletion of delegates created by the ItemParticles, as shown in
delegates.qml. This example was expanded with a click to reset feature
so as to more clearly see the impact of resets.

Task-number: QTBUG-37486
Change-Id: I9de935034c11a7dd2abdd60e7b3bd42867dede9c
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>

2 files changed, 54 insertions, 11 deletions

diff --git a/src/particles/qquickitemparticle.cpp b/src/particles/qquickitemparticle.cpp
index 4f81c0662b..b9a889cddb 100644
--- a/src/particles/qquickitemparticle.cpp
+++ b/src/particles/qquickitemparticle.cpp
@@ -65,11 +65,25 @@ QT_BEGIN_NAMESPACE
 /*!
     \qmlmethod QtQuick.Particles::ItemParticle::take(Item item, bool prioritize)
 
-    Asks the ItemParticle to take over control of item. It will be emitted when there is a logical particle available.
+    Asks the ItemParticle to take over control of item positioning temporarily.
+    It will follow the movement of a logical particle when one is available.
 
     By default items form a queue when waiting for a logical particle, but if prioritize is true then it will go immediately to the
     head of the queue.
+
+    ItemParticle does not take ownership of the item, and will relinquish
+    control when the logical particle expires. Commonly at this point you will
+    want to put it back in the queue, you can do this with the below line in
+    the delegate definition:
+    \code
+    ItemParticle.onDetached: itemParticleInstance.take(delegateRootItem);
+    \endcode
+    or delete it, such as with the below line in the delegate definition:
+    \code
+    ItemParticle.onDetached: delegateRootItem.destroy();
+    \endcode
 */
+
 /*!
     \qmlmethod QtQuick.Particles::ItemParticle::give(Item item)
 
@@ -88,8 +102,13 @@ QT_BEGIN_NAMESPACE
 /*!
     \qmlproperty Component QtQuick.Particles::ItemParticle::delegate
 
-    An instance of the delegate will be created for every logical
-    particle, and moved along with it.
+    An instance of the delegate will be created for every logical particle, and
+    moved along with it. As an alternative to using delegate, you can create
+    Item instances yourself and hand them to the ItemParticle to move using the
+    \l take method.
+
+    Any delegate instances created by ItemParticle will be destroyed when
+    the logical particle expires.
 */
 
 QQuickItemParticle::QQuickItemParticle(QQuickItem *parent) :
@@ -139,9 +158,8 @@ void QQuickItemParticle::commit(int, int)
 {
 }
 
-void QQuickItemParticle::tick(int time)
+void QQuickItemParticle::processDeletables()
 {
-    Q_UNUSED(time);//only needed because QTickAnimationProxy expects one
     foreach (QQuickItem* item, m_deletables){
         if (m_fade)
             item->setOpacity(0.);
@@ -149,26 +167,39 @@ void QQuickItemParticle::tick(int time)
         QQuickItemParticleAttached* mpa;
         if ((mpa = qobject_cast<QQuickItemParticleAttached*>(qmlAttachedPropertiesObject<QQuickItemParticle>(item))))
             mpa->detach();//reparent as well?
-        delete item;
+        int idx = -1;
+        if ((idx = m_managed.indexOf(item)) != -1) {
+            m_managed.takeAt(idx);
+            delete item;
+        }
         m_activeCount--;
     }
     m_deletables.clear();
+}
+
+void QQuickItemParticle::tick(int time)
+{
+    Q_UNUSED(time);//only needed because QTickAnimationProxy expects one
+    processDeletables();
 
     foreach (QQuickParticleData* d, m_loadables){
         Q_ASSERT(d);
         if (m_stasis.contains(d->delegate))
             qWarning() << "Current model particles prefers overwrite:false";
         //remove old item from the particle that is dying to make room for this one
-        if (d->delegate)
+        if (d->delegate) {
             m_deletables << d->delegate;
-        d->delegate = 0;
+            d->delegate = 0;
+        }
         if (!m_pendingItems.isEmpty()){
             d->delegate = m_pendingItems.front();
             m_pendingItems.pop_front();
         }else if (m_delegate){
             d->delegate = qobject_cast<QQuickItem*>(m_delegate->create(qmlContext(this)));
+            if (d->delegate)
+                m_managed << d->delegate;
         }
-        if (d->delegate && d){//###Data can be zero if creating an item leads to a reset - this screws things up.
+        if (d && d->delegate){//###Data can be zero if creating an item leads to a reset - this screws things up.
             d->delegate->setX(d->curX() - d->delegate->width()/2);//TODO: adjust for system?
             d->delegate->setY(d->curY() - d->delegate->height()/2);
             QQuickItemParticleAttached* mpa = qobject_cast<QQuickItemParticleAttached*>(qmlAttachedPropertiesObject<QQuickItemParticle>(d->delegate));
@@ -190,8 +221,18 @@ void QQuickItemParticle::reset()
 {
     QQuickParticlePainter::reset();
     m_loadables.clear();
-    //TODO: Cleanup items?
-    //deletables?
+
+    // delete all managed items which had their logical particles cleared
+    // but leave it alone if the logical particle is maintained
+    QSet<QQuickItem*> lost = QSet<QQuickItem*>::fromList(m_managed);
+    foreach (const QString group, m_groups){
+        int gIdx = m_system->groupIds[group];
+        foreach (QQuickParticleData* d, m_system->groupData[gIdx]->data)
+                lost.remove(d->delegate);
+    }
+    m_deletables.append(lost.toList());
+    //TODO: This doesn't yet handle calling detach on taken particles in the system reset case
+    processDeletables();
 }
 
 
diff --git a/src/particles/qquickitemparticle_p.h b/src/particles/qquickitemparticle_p.h
index 24c33319c9..04564e739a 100644
--- a/src/particles/qquickitemparticle_p.h
+++ b/src/particles/qquickitemparticle_p.h
@@ -88,8 +88,10 @@ protected:
     virtual void initialize(int gIdx, int pIdx);
     void prepareNextFrame();
 private:
+    void processDeletables();
     void tick(int time = 0);
     QList<QQuickItem* > m_deletables;
+    QList<QQuickItem* > m_managed;
     QList< QQuickParticleData* > m_loadables;
     bool m_fade;