如何在 QObject::connect 中使用 QMetaMethod

Question

I have two instances of QObject subclasses and two QMetaMethod instances of signal in one of objects and slot in another object. I want to connect this signal and slot with each other.

I've looked through the qobject.h file and find that SIGNAL() and SLOT() macro are just add "1" or "2" character to the beginning of method signature so it looks like it should be possible to add the same character to the beginning of string returned by QMetaMethod::signature() but this approach depends on some undocumented internals of toolkit and may be broken at any time by a new version of Qt.

Does anybody know reliable way to connect signals and slots through their QMetaMethod reflection representation?

I've created suggestion in Qt issue tracker: https://bugreports.qt.io/browse/QTBUG-10637 If anybody also interested in this feature you can vote for this ticket there.

Answer 1

This has been fixed as of Qt 4.8.0:

https://bugreports.qt.io/browse/QTBUG-10637

Suppose we have a QObject* m_subject, and wish to connect the change-notification signal of a property to a propertyChanged() slot:

const QMetaObject* meta = m_subject->metaObject();
QMetaProperty prop = meta->property(meta->indexOfProperty("myProperty"));
if (prop.hasNotifySignal()) {
    QMetaMethod signal = prop.notifySignal();
    QMetaMethod updateSlot = metaObject()->method(
        metaObject()->indexOfSlot("propertyChanged()"));
    connect(m_subject, signal, this, updateSlot);
}

I successfully used this to make a QWidget subclass which finds all the properties of any QObject and creates a QLineEdit for each of them, with a connection to keep the QLineEdit updated whenever the corresponding property changes. (Because I didn't find a way to pass a propertyID value to propertyChanged() though, it was necessary to make a subclass of QLineEdit and implement propertyChanged() there. QSignalMapper didn't help, because all the properties are in the same object.)

Answer 2

It looks like there is no way to make it work without relying on internal implementation. If I were you, I'd submit feature request to Qt bug tracker , write a code that mimics current behavior SIGNAL/SLOT macros and add unit test that will fail when SIGNAL/SLOT behavior changes.

There might be a simpler solution to the problem you're trying to solve: describe what exactly are you trying to do without any implementation details.

Answer 3

Thanks to MBack, I now use metamethods to connect my view to my model's properties dynamically for MVVM or MVC pattern. In order to respect DRY, a boilerplate is required with something like this :

 void MyClass::connectSignalToSlot(QObject* sender, std::string signalName, QObject* receiver, std::string slotName)
 {
    int sigIdx = sender->metaObject()->indexOfSignal(signalName.c_str());
    auto signal = sender->metaObject()->method(sigIdx);
    int slotIdx = receiver->metaObject()->indexOfSlot(slotName.c_str());
    auto slot = receiver->metaObject()->method(slotIdx);
    connect(sender,signal,receiver,slot);
 }

void MyClass::connectPropertyChangedToSlot(QObject* sender, std::string propName, QObject* receiver, std::string slotName)
{
   int sigIdx = sender->metaObject()->indexOfProperty(propName.c_str());
   auto signal = sender->metaObject()->property(sigIdx ).notifySignal();
   int slotIdx = receiver->metaObject()->indexOfSlot(slotName.c_str());
   auto slot = receiver->metaObject()->method(slotIdx);
   return connect(sender, signal, receiver, slot);
}

Answer 4

If signature method is public in QMetaMethod then the result shouldn't be broken by trolls and it's safe to use it (documentation says nothing about "dangers" when using QMetaMethod::signature method). I think you can safely use it. Just to be sure, what version of Qt you are using right now ?