Ensure identical behavior for singleton types defined in C++ and QML

Singleton types defined in C++ have no QML context. Therefore, both
qmlContext(obj) and qmlEngine(obj) return zero. Although documented,
this behavior is surprising and inconsistent with singleton types defined
in QML. The current behavior was decided upon in QTBUG-23116.

This patch puts C++ singleton types into a sub-context of the root context
by default, just like it is the case for QML-defined singleton types. This
doesn't cause any harm, but avoids surprises. It also fixes a bug in
QmlTypeWrapper that returned an invalid QVariant for QJSValue singletons.

Task-number: QTBUG-38583
Change-Id: Id1d48ecdc49f0e22714857a1b49b457885889e5e
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

3 files changed, 20 insertions, 29 deletions

diff --git a/src/qml/doc/src/qmlfunctions.qdoc b/src/qml/doc/src/qmlfunctions.qdoc
index 55ca040af6..6445a003a1 100644
--- a/src/qml/doc/src/qmlfunctions.qdoc
+++ b/src/qml/doc/src/qmlfunctions.qdoc
@@ -538,30 +538,6 @@
    }
    \endqml
 
-   Since singleton types do not have an associated QQmlContext object, then within the functions of a QObject-derived
-   type that is registered as a singleton type implementation the QML context and engine information is not available.
-   The QQmlEngine::contextForObject() function returns NULL when supplied with a pointer to an QObject that
-   implements a singleton type.
-
-   Extending the above example:
-
-   \code
-   class SingletonTypeExample : public QObject
-   {
-       ...
-
-       Q_INVOKABLE void doSomethingElse()
-       {
-           // QML Engine/Context information is not accessible here:
-           Q_ASSERT(QQmlEngine::contextForObject(this) == 0);
-           Q_ASSERT(qmlContext(this) == 0);
-           Q_ASSERT(qmlEngine(this) == 0);
-       }
-
-       ...
-   }
-   \endcode
-
    \sa {Choosing the Correct Integration Method Between C++ and QML}
 */
 
diff --git a/src/qml/qml/qqmlmetatype.cpp b/src/qml/qml/qqmlmetatype.cpp
index 0429df6185..307f9a9ec6 100644
--- a/src/qml/qml/qqmlmetatype.cpp
+++ b/src/qml/qml/qqmlmetatype.cpp
@@ -264,7 +264,14 @@ void QQmlMetaTypeData::registerType(QQmlTypePrivate *priv)
 void QQmlType::SingletonInstanceInfo::init(QQmlEngine *e)
 {
     if (scriptCallback && scriptApi(e).isUndefined()) {
-        setScriptApi(e, scriptCallback(e, e));
+        QJSValue value = scriptCallback(e, e);
+        if (value.isQObject()) {
+            QObject *o = value.toQObject();
+            // even though the object is defined in C++, qmlContext(obj) and qmlEngine(obj)
+            // should behave identically to QML singleton types.
+            e->setContextForObject(o, new QQmlContext(e->rootContext(), e));
+        }
+        setScriptApi(e, value);
     } else if (qobjectCallback && !qobjectApi(e)) {
         QObject *o = qobjectCallback(e, e);
         setQObjectApi(e, o);
@@ -273,6 +280,9 @@ void QQmlType::SingletonInstanceInfo::init(QQmlEngine *e)
         }
         // if this object can use a property cache, create it now
         QQmlData::ensurePropertyCache(e, o);
+        // even though the object is defined in C++, qmlContext(obj) and qmlEngine(obj)
+        // should behave identically to QML singleton types.
+        e->setContextForObject(o, new QQmlContext(e->rootContext(), e));
     } else if (!url.isEmpty() && !qobjectApi(e)) {
         QQmlComponent component(e, url, QQmlComponent::PreferSynchronous);
         QObject *o = component.beginCreate(e->rootContext());
diff --git a/src/qml/qml/qqmltypewrapper.cpp b/src/qml/qml/qqmltypewrapper.cpp
index 26f035705c..7270cffb00 100644
--- a/src/qml/qml/qqmltypewrapper.cpp
+++ b/src/qml/qml/qqmltypewrapper.cpp
@@ -95,12 +95,17 @@ QObject* QQmlTypeWrapper::singletonObject() const
 
 QVariant QQmlTypeWrapper::toVariant() const
 {
-    QObject *qobjectSingleton = singletonObject();
-    if (qobjectSingleton)
+    // Only Singleton type wrappers can be converted to a variant.
+    if (!isSingleton())
+        return QVariant();
+
+    QQmlEngine *e = engine()->qmlEngine();
+    QQmlType::SingletonInstanceInfo *siinfo = d()->type().singletonInstanceInfo();
+    siinfo->init(e);
+    if (QObject *qobjectSingleton = siinfo->qobjectApi(e))
         return QVariant::fromValue<QObject*>(qobjectSingleton);
 
-    // only QObject Singleton Type can be converted to a variant.
-    return QVariant();
+    return QVariant::fromValue<QJSValue>(siinfo->scriptApi(e));
 }