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, 19 insertions, 0 deletions

diff --git a/tests/auto/qml/qqmlengine/tst_qqmlengine.cpp b/tests/auto/qml/qqmlengine/tst_qqmlengine.cpp
index 0cb6753020..8d0106b92c 100644
--- a/tests/auto/qml/qqmlengine/tst_qqmlengine.cpp
+++ b/tests/auto/qml/qqmlengine/tst_qqmlengine.cpp
@@ -1015,6 +1015,25 @@ void tst_qqmlengine::singletonInstance()
     }
 
     {
+        int data = 30;
+        auto id = qmlRegisterSingletonType<CppSingleton>("Qt.test",1,0,"CapturingLambda",[data](QQmlEngine*, QJSEngine*){ // register qobject singleton with capturing lambda
+                auto o = new CppSingleton;
+                o->setProperty("data", data);
+                return o;
+        });
+        QJSValue value = engine.singletonInstance<QJSValue>(id);
+        QVERIFY(!value.isUndefined());
+        QVERIFY(value.isQObject());
+        QObject *instance = value.toQObject();
+        QVERIFY(instance);
+        QCOMPARE(instance->metaObject()->className(), "CppSingleton");
+        QCOMPARE(instance->property("data"), data);
+    }
+    {
+        qmlRegisterSingletonType<CppSingleton>("Qt.test",1,0,"NotAmbiguous", [](QQmlEngine* qeng, QJSEngine* jeng) -> QObject* {return CppSingleton::create(qeng, jeng);}); // test that overloads for qmlRegisterSingleton are not ambiguous
+    }
+
+    {
         // Invalid types
         QJSValue value;
         value = engine.singletonInstance<QJSValue>(-4711);