[Interest] Thread guarantee on slots+timers

[André Somers]

Sensei schreef op 11-4-2014 10:04:
> Dear all,
>
> quite a simple question here. I have an object living in a QThread, and
> this object has a timer executing a public slot every X seconds.
>
> If by chance I force calling the slot and meanwhile the timer emits a
> timeout() (or the other way round FWIW), am I guaranteed that the slot
> executes (by force) and next the slots gets run (by the timer) ?
>
> My guess is: yes, since I have only one event loop. However, asking for
> confirmation is better :)
>
Not quite, and it is not such a simple question at all. It depends on 
which thread actually triggers the slot, and how. If you were to trigger 
the slot, either by a direct method call or using a direct connection, 
from two different threads, there are no guarantees of it being run 
consequtively. It may run twice at the same time from two different 
threads, or interleaved, or... There is just no telling.

If your slot is exclusively ran in a single thread, either because it is 
only triggered from that thread or because you use queued connections 
from other threads to trigger it, it will be ran consecutively for each 
invocation. However, invocations that find their source in the event 
queue (timers, but also queued connections) may be triggered later than 
those that result from direct method calls or direct (or automatic, 
within the same thread) connections. This is because the event  queue is 
only processed again if you return to it. Before that, timer events or 
queued signals are not processed and thus don't trigger the slots.

André