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>

1 files changed, 9 insertions, 4 deletions

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));
 }