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>

6 files changed, 142 insertions, 0 deletions

diff --git a/src/quick/items/qquickitem.cpp b/src/quick/items/qquickitem.cpp
index 8c94b9dd5b..72b0e588ec 100644
--- a/src/quick/items/qquickitem.cpp
+++ b/src/quick/items/qquickitem.cpp
@@ -6478,6 +6478,17 @@ void QQuickItemPrivate::incrementCursorCount(int delta)
 #endif
 }
 
+void QQuickItemPrivate::markObjects(QV4::ExecutionEngine *e)
+{
+    Q_Q(QQuickItem);
+    QQmlData *ddata = QQmlData::get(q);
+    if (ddata)
+        ddata->jsWrapper.markOnce(e);
+
+    foreach (QQuickItem *child, childItems)
+        QQuickItemPrivate::get(child)->markObjects(e);
+}
+
 #ifndef QT_NO_CURSOR
 
 /*!
diff --git a/src/quick/items/qquickitem_p.h b/src/quick/items/qquickitem_p.h
index ef5c63e40f..96cb9e8843 100644
--- a/src/quick/items/qquickitem_p.h
+++ b/src/quick/items/qquickitem_p.h
@@ -583,6 +583,9 @@ public:
     virtual void mirrorChange() {}
 
     void incrementCursorCount(int delta);
+
+    // recursive helper to let a visual parent mark its visual children
+    void markObjects(QV4::ExecutionEngine *e);
 };
 
 /*
diff --git a/src/quick/items/qquickview.cpp b/src/quick/items/qquickview.cpp
index d1fe489dcb..61f0f9bec2 100644
--- a/src/quick/items/qquickview.cpp
+++ b/src/quick/items/qquickview.cpp
@@ -56,6 +56,26 @@
 
 QT_BEGIN_NAMESPACE
 
+DEFINE_MANAGED_VTABLE(QQuickRootItemMarker);
+
+QQuickRootItemMarker::QQuickRootItemMarker(QQuickViewPrivate *view)
+    : QV4::Object(QQmlEnginePrivate::getV4Engine(view->engine.data()))
+    , view(view)
+{
+    setVTable(&static_vtbl);
+}
+
+void QQuickRootItemMarker::markObjects(QV4::Managed *that, QV4::ExecutionEngine *e)
+{
+    QQuickItem *root = static_cast<QQuickRootItemMarker*>(that)->view->root;
+    if (root) {
+        QQuickItemPrivate *rootPrivate = QQuickItemPrivate::get(root);
+        rootPrivate->markObjects(e);
+    }
+
+    QV4::Object::markObjects(that, e);
+}
+
 void QQuickViewPrivate::init(QQmlEngine* e)
 {
     Q_Q(QQuickView);
@@ -68,6 +88,13 @@ void QQuickViewPrivate::init(QQmlEngine* e)
     if (!engine.data()->incubationController())
         engine.data()->setIncubationController(q->incubationController());
 
+    {
+        QV4::ExecutionEngine *v4 = QQmlEnginePrivate::getV4Engine(engine.data());
+        QV4::Scope scope(v4);
+        QV4::Scoped<QQuickRootItemMarker> v(scope, new (v4->memoryManager) QQuickRootItemMarker(this));
+        rootItemMarker = v;
+    }
+
     if (QQmlDebugService::isDebuggingEnabled())
         QQmlInspectorService::instance()->addView(q);
 }
diff --git a/src/quick/items/qquickview_p.h b/src/quick/items/qquickview_p.h
index 170c93a6cf..e18b45dfbe 100644
--- a/src/quick/items/qquickview_p.h
+++ b/src/quick/items/qquickview_p.h
@@ -51,12 +51,17 @@
 #include <QtCore/QWeakPointer>
 
 #include <QtQml/qqmlengine.h>
+#include <private/qv4object_p.h>
 #include "qquickwindow_p.h"
 
 #include "qquickitemchangelistener_p.h"
 
 QT_BEGIN_NAMESPACE
 
+namespace QV4 {
+struct ExecutionEngine;
+}
+
 class QQmlContext;
 class QQmlError;
 class QQuickItem;
@@ -94,6 +99,23 @@ public:
     QQuickView::ResizeMode resizeMode;
     QSize initialSize;
     QElapsedTimer frameTimer;
+    QV4::PersistentValue rootItemMarker;
+};
+
+struct QQuickRootItemMarker : public QV4::Object
+{
+    Q_MANAGED
+
+    QQuickRootItemMarker(QQuickViewPrivate *view);
+
+    static void destroy(Managed *that)
+    {
+        static_cast<QQuickRootItemMarker*>(that)->~QQuickRootItemMarker();
+    }
+
+    static void markObjects(Managed *that, QV4::ExecutionEngine *e);
+
+    QQuickViewPrivate *view;
 };
 
 QT_END_NAMESPACE
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)