QQuickListView: implement support for reusing items

This patch will implement delegate item recycling in
ListView. The API will be the same as used in TableView, except
that it will be off by default since the behavior of a delegate
that is reused is not compatible with legacy applications
which were not written with this in mind.

Most importantly:

- Component.onCompleted will only be called on a delegate the
first time it's created, and never when it's reused.

- Any user-declared properties in the delegate (that is, not
model roles, index, etc) will not be cleared or updated when an
item is reused. The application must do this manually upon
receiving the pooled or reused signal in the item.

[ChangeLog][ListView] ListView now has support for reusing delegate
items. This can be switched on by setting the reuseItems property of
ListView to true.

Task-number: QTBUG-80507
Change-Id: I68cc8300b050e4a1f89feebb1d31a2fd9189c793
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

1 files changed, 21 insertions, 6 deletions

diff --git a/src/quick/items/qquicklistview.cpp b/src/quick/items/qquicklistview.cpp
index 778f28bdd5..9206628716 100644
--- a/src/quick/items/qquicklistview.cpp
+++ b/src/quick/items/qquicklistview.cpp
@@ -92,7 +92,7 @@ public:
 
     FxViewItem *newViewItem(int index, QQuickItem *item) override;
     void initializeViewItem(FxViewItem *item) override;
-    bool releaseItem(FxViewItem *item) override;
+    bool releaseItem(FxViewItem *item, QQmlInstanceModel::ReusableFlag reusableFlag) override;
     void repositionItemAt(FxViewItem *item, int index, qreal sizeBuffer) override;
     void repositionPackageItemAt(QQuickItem *item, int index) override;
     void resetFirstItemPosition(qreal pos = 0.0) override;
@@ -139,6 +139,8 @@ public:
     bool flick(QQuickItemViewPrivate::AxisData &data, qreal minExtent, qreal maxExtent, qreal vSize,
                QQuickTimeLineCallback::Callback fixupCallback, qreal velocity) override;
 
+    QQuickItemViewAttached *getAttachedObject(const QObject *object) const override;
+
     QQuickListView::Orientation orient;
     qreal visiblePos;
     qreal averageSize;
@@ -634,15 +636,15 @@ void QQuickListViewPrivate::initializeViewItem(FxViewItem *item)
     }
 }
 
-bool QQuickListViewPrivate::releaseItem(FxViewItem *item)
+bool QQuickListViewPrivate::releaseItem(FxViewItem *item, QQmlInstanceModel::ReusableFlag reusableFlag)
 {
     if (!item || !model)
-        return QQuickItemViewPrivate::releaseItem(item);
+        return QQuickItemViewPrivate::releaseItem(item, reusableFlag);
 
     QPointer<QQuickItem> it = item->item;
     QQuickListViewAttached *att = static_cast<QQuickListViewAttached*>(item->attached);
 
-    bool released = QQuickItemViewPrivate::releaseItem(item);
+    bool released = QQuickItemViewPrivate::releaseItem(item, reusableFlag);
     if (released && it && att && att->m_sectionItem) {
         // We hold no more references to this item
         int i = 0;
@@ -682,7 +684,7 @@ bool QQuickListViewPrivate::addVisibleItems(qreal fillFrom, qreal fillTo, qreal
         int newModelIdx = qBound(0, modelIndex + count, model->count());
         count = newModelIdx - modelIndex;
         if (count) {
-            releaseVisibleItems();
+            releaseVisibleItems(reusableFlag);
             modelIndex = newModelIdx;
             visibleIndex = modelIndex;
             visiblePos = itemEnd + count * (averageSize + spacing);
@@ -737,7 +739,7 @@ void QQuickListViewPrivate::removeItem(FxViewItem *item)
         releasePendingTransition.append(item);
     } else {
         qCDebug(lcItemViewDelegateLifecycle) << "\treleasing stationary item" << item->index << (QObject *)(item->item);
-        releaseItem(item);
+        releaseItem(item, reusableFlag);
     }
 }
 
@@ -1772,6 +1774,12 @@ void QQuickListViewPrivate::setSectionHelper(QQmlContext *context, QQuickItem *s
         sectionItem->setProperty("section", section);
 }
 
+QQuickItemViewAttached *QQuickListViewPrivate::getAttachedObject(const QObject *object) const
+{
+    QObject *attachedObject = qmlAttachedPropertiesObject<QQuickListView>(object);
+    return static_cast<QQuickItemViewAttached *>(attachedObject);
+}
+
 //----------------------------------------------------------------------------
 
 /*!
@@ -3186,6 +3194,13 @@ void QQuickListView::keyPressEvent(QKeyEvent *event)
 void QQuickListView::geometryChanged(const QRectF &newGeometry, const QRectF &oldGeometry)
 {
     Q_D(QQuickListView);
+
+    if (d->model) {
+        // When the view changes size, we force the pool to
+        // shrink by releasing all pooled items.
+        d->model->drainReusableItemsPool(0);
+    }
+
     if (d->isRightToLeft()) {
         // maintain position relative to the right edge
         qreal dx = newGeometry.width() - oldGeometry.width();