Connect QML signal to C++11 lambda slot (Qt 5)

Question

Connecting a QML signal to a regular C++ slot is easy:

// QML
Rectangle { signal foo(); }

// C++ old-style
QObject::connect(some_qml_container, SIGNAL(foo()), some_qobject, SLOT(fooSlot()); // works!

However, no matter what I try, I cannot seem to be able to connect to a C++11 lambda function slot.

// C++11
QObject::connect(some_qml_container, SIGNAL(foo()), [=]() { /* response */ }); // fails...
QObject::connect(some_qml_container, "foo()", [=]() { /* response */ }); // fails...

Both attempts fail with a function signature error (no QObject::connect overload can accept these parameters). However, the Qt 5 documentation implies that this should be possible.

Unfortunately, Qt 5 examples always connect a C++ signal to a C++ lambda slot:

// C++11
QObject::connect(some_qml_container, &QMLContainer::foo, [=]() { /* response */ }); // works!

This syntax cannot work for a QML signal, as the QMLContainer::foo signature is not known at compile-time (and declaring QMLContainer::foo by hand defeats the purpose of using QML in the first place.)

Is what I'm trying to do possible? If so, what is the correct syntax for the QObject::connect call?

Answer 1

Lambdas etc only work with new syntax. If you can't find a way to give QML signal as a pointer, then I think it is not directly possible.

If so, you have a workaround: create a dummy signal-routing QObject subclass, which only has signals, one for every QML signal you need to route. Then connect QML signals to corresponding signals of an instance of this dummy class, using the old connect syntax.

Now you have C++ signals you can use with the new syntax, and connect to lambdas.

The class could also have a helper method, to automate connections from QML to signals of the class, which would utilize QMetaObject reflection mechanisms and a suitable signal naming scheme, using same principle as QMetaObject::connectSlotsByName uses. Alternatively you can just hard-code the QML-router signal connections but still hide them inside a method of the router class.

Untested...

Answer 2

You can use a helper:

class LambdaHelper : public QObject {
  Q_OBJECT
  std::function<void()> m_fun;
public:
  LambdaHelper(std::function<void()> && fun, QObject * parent = {}) :
    QObject(parent),
    m_fun(std::move(fun)) {}
   Q_SLOT void call() { m_fun(); }
   static QMetaObject::Connection connect(
     QObject * sender, const char * signal, std::function<void()> && fun) 
   {
     if (!sender) return {};
     return connect(sender, signal, 
                    new LambdaHelper(std::move(fun), sender), SLOT(call()));
   }
};

Then:

LambdaHelper::connect(sender, SIGNAL(mySignal()), [] { ... });

The sender owns the helper object and will clean it up upon its destruction.