Lazy create QmlEngine on accessing rootContext

A common usecase appears to be to set variables in the rootContext
before loading a url in a QQuickWidget. We there need to ensure there
is a QmlEngine to set variables on when this is attempted.

Change-Id: I07aff2104313eeb3fab902ea3c6043c3c82c50f7
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/quickwidgets/qquickwidget.cpp

@@ -109,13 +109,13 @@ void QQuickWidgetPrivate::init(QQmlEngine* e)
     QObject::connect(renderControl, SIGNAL(sceneChanged()), q, SLOT(triggerUpdate()));
 }
 
-void QQuickWidgetPrivate::ensureEngine()
+void QQuickWidgetPrivate::ensureEngine() const
 {
-    Q_Q(QQuickWidget);
+    Q_Q(const QQuickWidget);
     if (!engine.isNull())
         return;
 
-    engine = new QQmlEngine(q);
+    engine = new QQmlEngine(const_cast<QQuickWidget*>(q));
     engine.data()->setIncubationController(offscreenWindow->incubationController());
 }
 
@@ -493,7 +493,8 @@ QUrl QQuickWidget::source() const
 QQmlEngine* QQuickWidget::engine() const
 {
     Q_D(const QQuickWidget);
-    return d->engine ? const_cast<QQmlEngine *>(d->engine.data()) : 0;
+    d->ensureEngine();
+    return const_cast<QQmlEngine *>(d->engine.data());
 }
 
 /*!
@@ -506,7 +507,8 @@ QQmlEngine* QQuickWidget::engine() const
 QQmlContext* QQuickWidget::rootContext() const
 {
     Q_D(const QQuickWidget);
-    return d->engine ? d->engine.data()->rootContext() : 0;
+    d->ensureEngine();
+    return d->engine.data()->rootContext();
 }
 
 /*!

src/quickwidgets/qquickwidget_p.h

@@ -105,7 +105,7 @@ public:
     QImage grabFramebuffer() Q_DECL_OVERRIDE;
 
     void init(QQmlEngine* e = 0);
-    void ensureEngine();
+    void ensureEngine() const;
     void handleWindowChange();
     void invalidateRenderControl();
 
@@ -115,7 +115,7 @@ public:
 
     QUrl source;
 
-    QPointer<QQmlEngine> engine;
+    mutable QPointer<QQmlEngine> engine;
     QQmlComponent *component;
     QBasicTimer resizetimer;
     QQuickWindow *offscreenWindow;

tests/auto/quickwidgets/qquickwidget/tst_qquickwidget.cpp

@@ -305,9 +305,13 @@ void tst_qquickwidget::grabBeforeShow()
 void tst_qquickwidget::nullEngine()
 {
     QQuickWidget widget;
-    QVERIFY(widget.engine() == Q_NULLPTR);
+    // Default should have no errors, even with a null qml engine
     QVERIFY(widget.errors().isEmpty());
     QCOMPARE(widget.status(), QQuickWidget::Null);
+
+    // A QML engine should be created lazily.
+    QVERIFY(widget.rootContext());
+    QVERIFY(widget.engine());
 }
 
 QTEST_MAIN(tst_qquickwidget)