[Qt-interest] Correct way of connecting to the QCoreApplication::aboutToQuit() signal ?

[Thiago Macieira]

Em Segunda-feira 8. Fevereiro 2010, às 19.09.00, Julien Cugnière escreveu:
> The documentation mentions that one should connect to the
> QCoreApplication::aboutToQuit() signal in order to handle last minute
> cleanup when the application closes.
> 
> However, can I connect to this signal a slot that doesn't live in the main
> thread ? 

Why do you want to do that? I suggest you just place it in the main thread and 
be done with it.

Anyway, you also have to ensure that the thread is running while the signal is 
emitted. Otherwise, the signal will never be delivered to the slot.

> I'm basically afraid that if I connect with the default connection
> type (Qt::AutoConnection), Qt will not wait for my slot to finish before
> quitting the application... 

You're right, it won't. If the target slot lives in a separate thread, the 
AutoConnection decays to QueuedConnection. That means "fire-and-forget".

> So, given that I don't know from which thread
> this signal will be fired, do I need to connect to it using
> Qt::DirectConnection, or Qt::BlockingQueuedConnection ?

The signal will be fired from the main thread.

You choose which solution is best for you. If you use DirectConnection, your 
slot will be called in the main thread, just as if the object lived in the 
main thread. So to avoid any hassles, I recommend just placing the object in 
the main thread once and for all.

As for BlockingQueuedConnection, the slot will be called in its proper thread, 
which you must ensure to still be running. Emitting a signal that is connected 
via BlockingQueuedConnection to a thread that isn't running results in 
deadlock.

> NB : I know there are other way to achieve what I want, such as
> intercepting QCloseEvent, but I'm more interested in understanding the way
> aboutToQuit() is supposed to work.

aboutToQuit is just a signal. It doesn't work any differently from any other 
signal.

-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
  Senior Product Manager - Nokia, Qt Development Frameworks
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