add std::function overload to qmlRegisterSingletonType

This changes enables passing stateful lambdas to
qmlRegisterSingletonType, which helps when porting away from
setContextProperty.

Unfortunately, we cannot directly add an overload for std::function, as
this causes ambiguity in the overload set when a lambda of the form
auto f = [](QQmlEngine*, QJSEngine*) -> QObject*
is passed to the function (which is what the examples do)

We therefore use a template to support abribtrary callables f, then SFINAE
them out if f is not convertible to the desired std::function, or when
f is convertible to a plain C function pointer, thus removing the
ambiguity

Change-Id: I6ca95ad692d8bb785e420b85bf3d8c1d0007ce17
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
 
 
 
 

1 files changed, 3 insertions, 1 deletions

diff --git a/src/qml/qml/qqmltype_p.h b/src/qml/qml/qqmltype_p.h
index 1d65a08c8f..158fefad2c 100644
--- a/src/qml/qml/qqmltype_p.h
+++ b/src/qml/qml/qqmltype_p.h
@@ -51,6 +51,8 @@
 // We mean it.
 //
 
+#include <functional>
+
 #include <private/qtqmlglobal_p.h>
 #include <private/qqmlrefcount_p.h>
 
@@ -145,7 +147,7 @@ public:
     struct Q_QML_PRIVATE_EXPORT SingletonInstanceInfo
     {
         QJSValue (*scriptCallback)(QQmlEngine *, QJSEngine *) = nullptr;
-        QObject *(*qobjectCallback)(QQmlEngine *, QJSEngine *) = nullptr;
+        std::function<QObject *(QQmlEngine *, QJSEngine *)> qobjectCallback = {};
         const QMetaObject *instanceMetaObject = nullptr;
         QString typeName;
         QUrl url; // used by composite singletons