Signal execution order with Qt::QueuedConnection

Question

I have two signals A and B emitted one after another from an object in thread X, and the two connected slots are in the Main thread. The connection is QueuedConnection (due

Answer 1

Both of your signals will be queued in a single event queue of the X thread, so corresponding slots will be executed in the order of signals were emitted.

But in the following case you can't rely on the slots execution order:

signal A connected to a slot in X thread
signal B connected to a slot in Y thread

Also, there is a Qt::BlockingQueuedConnection connection type. If you connect your first signal using it, your current thread will be blocked until the corresponding slot in another thread finishes its job.

Answer 2

According to current QT sources (5.*) they will be dispatched in sequence they were emitted using FIFO algorithm. But as stated in comment there is nothing in documentation which specifies this order so I wouldn't suggest to relay on that behaviour.