How does the QT signal-slot mechanism work if you need to send a signal from an object of one thread to a slot on an object of another thread?

Question

Qt

Hello!
By itself, the principle of signal-slot operation in qt is clear to me (when one object sends a signal to the slot of another object in one thread). I got completely confused when I tried to figure out how to transfer a signal from an object located in one thread to a slot to an object located in another thread.
Who understands, please explain. I didn’t manage to google intelligible information, except for the one that completely confused me.
Thank you!

Answer 1

The key to understanding signaling is understanding the raison d'être of QThread:
> A QThread object manages one thread of control within the program. QThreads begin executing in run(). By default, run() starts the event loop by calling exec() and runs a Qt event loop inside the thread.
Launching your own message processing loop and making it possible to receive them, including from other threads. The feature of the signals / slots of the kit is that it is also a simple message queue system, similar to the Erlang one, if encountered. A call within a single thread is a direct call to signed slots on the same stack, but a QueuedConnection or an inter-thread call puts a message in the queue of the corresponding thread (these messages are in the same queue with messages from the OS), and the thread will retrieve them in the event-loop -e. Thus, you simply ask another thread to call the slot, and it will call it when it once again completes the message loop and it reaches the slot call message.
> It is safe to connect signals and slots across different threads, thanks to a mechanism called queued connections.
doc.qt.io/qt-5.4/qt.html#ConnectionType-enum
Qt::QueuedConnection - The slot is invoked when control returns to the event loop of the receiver's thread. The slot is executed in the receiver's thread.