Fix crash with StackView::initialItem

The following binding

    initialView: Qt.createComponent("blah.qml")

works with QQC1 but crashes with QQC2 as soon as the garbage collector
kicks in. In QQC1 StackView was implemented in a .qml file and the
property was declared as

    property var initialItem

For such declared properties the QML engine takes care of ensuring that
the reference assigned is a strong reference towards the GC and it also
tracks the life-time of the QObject in case it's explicitly deleted by
somebody else.

In QQC2 the property continues to be documented as "var" property,
however it is implemented in C++ as QVariant property. The QVariant is
not known to the GC at all and it also doesn't do life-cycle tracking.

The C++ equivalent to "var" in QML is QJSValue, which is a strong
reference and also ends up using a QPointer internally when holding a
QObject.

Task-number: QTBUG-67118
Change-Id: I43e473c7bf2c40f9843e9d50fe4fd805b54fd256
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

4 files changed, 23 insertions, 8 deletions

diff --git a/src/quicktemplates2/qquickstackview.cpp b/src/quicktemplates2/qquickstackview.cpp
index 1abc506b..6dae65c8 100644
--- a/src/quicktemplates2/qquickstackview.cpp
+++ b/src/quicktemplates2/qquickstackview.cpp
@@ -898,13 +898,13 @@ void QQuickStackView::clear(Operation operation)
 
     \sa push()
 */
-QVariant QQuickStackView::initialItem() const
+QJSValue QQuickStackView::initialItem() const
 {
     Q_D(const QQuickStackView);
     return d->initialItem;
 }
 
-void QQuickStackView::setInitialItem(const QVariant &item)
+void QQuickStackView::setInitialItem(const QJSValue &item)
 {
     Q_D(QQuickStackView);
     d->initialItem = item;
@@ -1081,9 +1081,9 @@ void QQuickStackView::componentComplete()
     QQuickStackElement *element = nullptr;
     QString error;
     int oldDepth = d->elements.count();
-    if (QObject *o = d->initialItem.value<QObject *>())
+    if (QObject *o = d->initialItem.toQObject())
         element = QQuickStackElement::fromObject(o, this, &error);
-    else if (d->initialItem.canConvert<QString>())
+    else if (d->initialItem.isString())
         element = QQuickStackElement::fromString(d->initialItem.toString(), this, &error);
     if (!error.isEmpty()) {
         d->warn(error);
diff --git a/src/quicktemplates2/qquickstackview_p.h b/src/quicktemplates2/qquickstackview_p.h
index e347ba9d..ba6fe106 100644
--- a/src/quicktemplates2/qquickstackview_p.h
+++ b/src/quicktemplates2/qquickstackview_p.h
@@ -65,7 +65,7 @@ class Q_QUICKTEMPLATES2_PRIVATE_EXPORT QQuickStackView : public QQuickControl
     Q_PROPERTY(bool busy READ isBusy NOTIFY busyChanged FINAL)
     Q_PROPERTY(int depth READ depth NOTIFY depthChanged FINAL)
     Q_PROPERTY(QQuickItem *currentItem READ currentItem NOTIFY currentItemChanged FINAL)
-    Q_PROPERTY(QVariant initialItem READ initialItem WRITE setInitialItem FINAL)
+    Q_PROPERTY(QJSValue initialItem READ initialItem WRITE setInitialItem FINAL)
     Q_PROPERTY(QQuickTransition *popEnter READ popEnter WRITE setPopEnter NOTIFY popEnterChanged FINAL)
     Q_PROPERTY(QQuickTransition *popExit READ popExit WRITE setPopExit NOTIFY popExitChanged FINAL)
     Q_PROPERTY(QQuickTransition *pushEnter READ pushEnter WRITE setPushEnter NOTIFY pushEnterChanged FINAL)
@@ -93,8 +93,8 @@ public:
     };
     Q_ENUM(Status)
 
-    QVariant initialItem() const;
-    void setInitialItem(const QVariant &item);
+    QJSValue initialItem() const;
+    void setInitialItem(const QJSValue &item);
 
     QQuickTransition *popEnter() const;
     void setPopEnter(QQuickTransition *enter);
diff --git a/src/quicktemplates2/qquickstackview_p_p.h b/src/quicktemplates2/qquickstackview_p_p.h
index d86f35cd..7f1b77b1 100644
--- a/src/quicktemplates2/qquickstackview_p_p.h
+++ b/src/quicktemplates2/qquickstackview_p_p.h
@@ -96,7 +96,7 @@ public:
 
     bool busy;
     QString operation;
-    QVariant initialItem;
+    QJSValue initialItem;
     QQuickItem *currentItem;
     QSet<QQuickStackElement*> removing;
     QList<QQuickStackElement*> removed;
diff --git a/tests/auto/controls/data/tst_stackview.qml b/tests/auto/controls/data/tst_stackview.qml
index 3c0f0273..3827354b 100644
--- a/tests/auto/controls/data/tst_stackview.qml
+++ b/tests/auto/controls/data/tst_stackview.qml
@@ -1230,4 +1230,19 @@ TestCase {
         touch.release(0, control).commit()
         verify(!ma.pressed)
     }
+
+    // Separate function to ensure that the temporary value created to hold the return value of the Qt.createComponent()
+    // call is out of scope when the caller calls gc().
+    function stackViewFactory()
+    {
+        return createTemporaryObject(stackView, testCase, {initialItem: Qt.createComponent("TestItem.qml")})
+    }
+
+    function test_initalItemOwnership()
+    {
+        var control = stackViewFactory()
+        verify(control)
+        gc()
+        verify(control.initialItem)
+    }
 }