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, 1 insertions, 0 deletions

diff --git a/src/quicktemplates2/qquickcontrol_p_p.h b/src/quicktemplates2/qquickcontrol_p_p.h
index 4dd6f32a..42924875 100644
--- a/src/quicktemplates2/qquickcontrol_p_p.h
+++ b/src/quicktemplates2/qquickcontrol_p_p.h
@@ -95,6 +95,7 @@ public:
     virtual void resizeContent();
 
     virtual QQuickItem *getContentItem();
+    void setContentItem_helper(QQuickItem *item, bool notify = true);
 
 #if QT_CONFIG(accessibility)
     void accessibilityActiveChanged(bool active) override;