Fix interaction of QQuickItems with the garbage collector

The QObject ownership of QQuickItem objects is not accessible / mutable
in QML, because the parent property maps to the (dynamic) visual parent.

There are use-cases of creating QQuickItem objects without a QObject parent and
to support this, the visual parent needs to mark its visual children in order
to provide intuitive semantics.

[ChangeLog][QtQuick][Import Behavior Changes] A QQuick Item is now strongly
referenced by its visual parent item, so it doesn't require a QObject parent to
stay alive.

Task-number: QTBUG-35913
Change-Id: Ief2d40ac76298a0cf241ca73ff654c4ecfa12748
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

2 files changed, 79 insertions, 0 deletions

diff --git a/tests/auto/quick/qquickitem/data/visualParentOwnership.qml b/tests/auto/quick/qquickitem/data/visualParentOwnership.qml
new file mode 100644
index 0000000000..644d14ba43
--- /dev/null
+++ b/tests/auto/quick/qquickitem/data/visualParentOwnership.qml
@@ -0,0 +1,14 @@
+import QtQuick 2.0
+
+Item {
+    Component {
+        id: factory
+        Item {}
+    }
+
+    property Item keepAliveProperty;
+
+    function createItemWithoutParent() {
+        return factory.createObject(/*parent*/ null);
+    }
+}
diff --git a/tests/auto/quick/qquickitem/tst_qquickitem.cpp b/tests/auto/quick/qquickitem/tst_qquickitem.cpp
index ad3c4fc208..f4f2374183 100644
--- a/tests/auto/quick/qquickitem/tst_qquickitem.cpp
+++ b/tests/auto/quick/qquickitem/tst_qquickitem.cpp
@@ -49,6 +49,7 @@
 #include <qpa/qwindowsysteminterface.h>
 #include <QDebug>
 #include <QTimer>
+#include <QQmlEngine>
 #include "../../shared/util.h"
 
 class TestItem : public QQuickItem
@@ -167,6 +168,8 @@ private slots:
 
     void acceptedMouseButtons();
 
+    void visualParentOwnership();
+
 private:
 
     enum PaintOrderOp {
@@ -1754,6 +1757,68 @@ void tst_qquickitem::acceptedMouseButtons()
     QCOMPARE(item.releaseCount, 3);
 }
 
+static void gc(QQmlEngine &engine)
+{
+    engine.collectGarbage();
+    QCoreApplication::sendPostedEvents(0, QEvent::DeferredDelete);
+    QCoreApplication::processEvents();
+}
+
+void tst_qquickitem::visualParentOwnership()
+{
+    QQuickView view;
+    view.setSource(testFileUrl("visualParentOwnership.qml"));
+
+    QQuickItem *root = qobject_cast<QQuickItem*>(view.rootObject());
+    QVERIFY(root);
+
+    QVariant newObject;
+    {
+        QVERIFY(QMetaObject::invokeMethod(root, "createItemWithoutParent", Q_RETURN_ARG(QVariant, newObject)));
+        QPointer<QQuickItem> newItem = qvariant_cast<QQuickItem*>(newObject);
+        QVERIFY(!newItem.isNull());
+
+        QVERIFY(!newItem->parent());
+        QVERIFY(!newItem->parentItem());
+
+        newItem->setParentItem(root);
+
+        gc(*view.engine());
+
+        QVERIFY(!newItem.isNull());
+        newItem->setParentItem(0);
+
+        gc(*view.engine());
+        QVERIFY(newItem.isNull());
+    }
+    {
+        QVERIFY(QMetaObject::invokeMethod(root, "createItemWithoutParent", Q_RETURN_ARG(QVariant, newObject)));
+        QPointer<QQuickItem> firstItem = qvariant_cast<QQuickItem*>(newObject);
+        QVERIFY(!firstItem.isNull());
+
+        firstItem->setParentItem(root);
+
+        QVERIFY(QMetaObject::invokeMethod(root, "createItemWithoutParent", Q_RETURN_ARG(QVariant, newObject)));
+        QPointer<QQuickItem> secondItem = qvariant_cast<QQuickItem*>(newObject);
+        QVERIFY(!firstItem.isNull());
+
+        secondItem->setParentItem(firstItem);
+
+        gc(*view.engine());
+
+        delete firstItem;
+
+        root->setProperty("keepAliveProperty", newObject);
+
+        gc(*view.engine());
+        QVERIFY(!secondItem.isNull());
+
+        root->setProperty("keepAliveProperty", QVariant());
+
+        gc(*view.engine());
+        QVERIFY(secondItem.isNull());
+    }
+}
 
 QTEST_MAIN(tst_qquickitem)