ScrollView: fix default Flickable content item creation

Short version: do not emit contentItemChanged() while contentItem()
is being called to avoid binding loops.

Long version: ScrollView is a thin wrapper around Flickable, and
when necessary, instantiates a Flickable behind the scenes. However,
ScrollView cannot instantiate one unconditionally, because it has to
work with existing Flickables, such as ListView and GridView, too.

  ScrollView { Item { } } // creates a Flickable
  ScrollView { Flickable { } } // does not create a Flickable
  ScrollView { contentItem: Flickable { } } // does not create a Flickable

When a Flickable is created behind the scenes, it is assigned as
the contentItem of the ScrollView. However, when the Flickable is
created lazily as a result of ScrollView::contentItem being accessed,
it must NOT emit contentItemChanged() while the contentItem() getter
is being called, because that results to a binding loop. This problem
was exposed by the recent attempts to enable QML caching at build time
(QTBUG-58571).

Change-Id: I712f2f30da454a6c22a722afe8a00ae240733571
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

1 files changed, 25 insertions, 14 deletions

diff --git a/src/quicktemplates2/qquickcontrol.cpp b/src/quicktemplates2/qquickcontrol.cpp
index c039253d..a2b16a10 100644
--- a/src/quicktemplates2/qquickcontrol.cpp
+++ b/src/quicktemplates2/qquickcontrol.cpp
@@ -281,6 +281,27 @@ QQuickItem *QQuickControlPrivate::getContentItem()
     return contentItem;
 }
 
+void QQuickControlPrivate::setContentItem_helper(QQuickItem *item, bool notify)
+{
+    Q_Q(QQuickControl);
+    if (contentItem == item)
+        return;
+
+    q->contentItemChange(item, contentItem);
+    destroyDelegate(contentItem, q);
+    contentItem = item;
+
+    if (item) {
+        if (!item->parentItem())
+            item->setParentItem(q);
+        if (componentComplete)
+            resizeContent();
+    }
+
+    if (notify)
+        emit q->contentItemChanged();
+}
+
 #if QT_CONFIG(accessibility)
 void QQuickControlPrivate::accessibilityActiveChanged(bool active)
 {
@@ -1243,25 +1264,15 @@ void QQuickControl::setBackground(QQuickItem *background)
 QQuickItem *QQuickControl::contentItem() const
 {
     QQuickControlPrivate *d = const_cast<QQuickControlPrivate *>(d_func());
-    return d->getContentItem();
+    if (!d->contentItem)
+        d->setContentItem_helper(d->getContentItem(), false);
+    return d->contentItem;
 }
 
 void QQuickControl::setContentItem(QQuickItem *item)
 {
     Q_D(QQuickControl);
-    if (d->contentItem == item)
-        return;
-
-    contentItemChange(item, d->contentItem);
-    QQuickControlPrivate::destroyDelegate(d->contentItem, this);
-    d->contentItem = item;
-    if (item) {
-        if (!item->parentItem())
-            item->setParentItem(this);
-        if (isComponentComplete())
-            d->resizeContent();
-    }
-    emit contentItemChanged();
+    d->setContentItem_helper(item, true);
 }
 
 void QQuickControl::classBegin()