Invoke slot asynchronously without connecting to it using clear line of code

Question

I have encountered quite freaky bug - QAction::trigger caused blocking dialog to appear, which caused my server which called trigger to go stuck (eg. not able to process socket signals until dialog was closed).

I figured out a workaround. I connect signal void triggerWorkaround() to slot QAction::trigger using Qt::QueuedConnection and I emit it:

QObject::connect(this, &HackClass::triggerWorkaround, targetAction_.data(), &QAction::trigger, Qt::QueuedConnection);
emit triggerWorkaround();
QObject::disconnect(this, nullptr, targetAction_.data(), nullptr);

But that's three lines of confusing code. Is there a non-confusing method to do this? I have found QMetaObject::invokeMethod, but frankly, that's 10 times more confusing than my current solution. Also, I don't want to ever use method name as string!

Answer 1

You can separate that into a function QueuedInvoke like this:

//overload for methods/slots
//the slot gets invoked in the thread where the QObject lives
template <typename Object, typename T>
void QueuedInvoke(Object* object, T (Object::* f)()){
    QObject signalSource;
    QObject::connect(&signalSource, &QObject::destroyed,
                     object, f, Qt::QueuedConnection);
}
//overload for functors
//the functor gets invoked in the thread where the contextObject lives
//or in the current thread if no contextObject is provided
template <typename Func>
void QueuedInvoke(Func&& f, QObject* contextObject = QAbstractEventDispatcher::instance()){
    QObject signalSource;
    QObject::connect(&signalSource, &QObject::destroyed, 
                     contextObject, std::forward<Func>(f), Qt::QueuedConnection);
}

This will leverage the destroyed() signal emitted from the temporary QObject to post a queued event into the event loop. The slot/functor is actually invoked when the event loop processes that event.

So, Instead of the 3 lines you posted, You can use the above function like this:

QueuedInvoke(targetAction_.data(), &QAction::trigger);

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^{My answer is based on this great answer about executing a functor in a given QThread. You can refer to it for more details.}