qt4 - Qt 4.5 - Is emitting signal a function call, or a thread, and does it blocks?

Question

I am not sure about the nature of the signal/slot mechanism in Qt 4.5. When a signal is emitted, is it a blocking function call or a thread? Say this

emit GrabLatestData();

// proceed with latest data

Will all the signal/slot chain be resolved before proceeding to the next line?

question from:https://stackoverflow.com/questions/1264887/qt-4-5-is-emitting-signal-a-function-call-or-a-thread-and-does-it-blocks

Answer 1

It depends. From the documentation:

When a signal is emitted, the slots connected to it are usually executed immediately, just like a normal function call. When this happens, the signals and slots mechanism is totally independent of any GUI event loop. Execution of the code following the emit statement will occur once all slots have returned. The situation is slightly different when using queued connections; in such a case, the code following the emit keyword will continue immediately, and the slots will be executed later.

So in normal cases, it will be synchronous and blocking, and with queued connections it will be asynchronous and non-blocking.