Information and control on emitted signals and slots (Qt)

Question

Does Qt offer functionality to know the number of queued signals that are pending for a given slot to process? Is there a way to clear them? For example, if several emits are done on a signal connected to a given slot, how could someone know the amount of these emitted signals?

QMetaObject::Connection class has a laconic interface and does not seem to offer related functionality. Deleting the object that receives the signals, hence destroying the connections, solves the problem. But is there a way to do this without disconnecting the slots or deleting the receiving objects?

Answer 1

The reason you ask the question most likely indicates that your design is broken. The signals and slots are a mechanism to decouple code. The objects connected together should behave themselves no matter how many senders or receiver are there, and certainly shouldn't attempt to track such matters!

It'd be wiser to fix the issue at the source by altering the design. If you are uffering from an event storm e.g. due to changing a widget's data in a slot, the slot should be very lightweight and only schedule an update of the widget by calling update(), but never forcing an immediate repaint. This leverages repaint event compression done by Qt. You might wish to compress your own events too.

Connection types in Qt

Signals and slots in Qt can be delivered using a direct, queued or blocking connection. The automatic type is not really a fixed connection type. It is an instruction to resolve the type for every receiver, at every signal emission into either a direct or queued type.

The direct connection is like any indirect function call: nothing is queued, and the slot is called from within the body of the signal method:

// all direct-connected slots/functors are invoked before mySignal() returns
emit mySignal();

The queued connection posts a QMetaCallEvent to the receiving object thread's event loop. That event contains the arguments of the call, or carries the functor. Is is handled by QObject::event(). You can certainly intercept such events. For details, see this question.