[Interest] Relationship between a QEventLoop and QCoreApplication::exec().

[Thiago Macieira]

On quarta-feira, 3 de agosto de 2016 11:22:36 PDT Jorge Fierro wrote:
> What exactly is the relationship between an event loop entered by
> calling QEventLoop::exec() and *the* main event loop (the one running
> when you can QCoreApplication::exec())?

Both functions as well as QThread::exec() are windows into the actual event 
dispatcher that runs behind the scenes. All three control the 
QAbstractEventDispatcher that either QCoreApplication or QThread creates.

I think these two links answer your question best:

https://code.woboq.org/qt5/qtbase/src/corelib/kernel/
qcoreapplication.cpp.html#_ZN16QCoreApplication4execEv (line 1258)
https://code.woboq.org/qt5/qtbase/src/corelib/thread/
qthread.cpp.html#_ZN7QThread4execEv (line 506)

As you can see, both QCoreApplication::exec or QThread::exec create a 
QEventLoop and call exec on it. The same is also true, for that matter, for 
QDialog::exec.

> There are a number of related questions:
> 
> - Does QEventLoop::exec() enter *the* main event loop? Or,
> - Does QEventLoop::exec() enter a different event loop independent
> from QCoreApplication::exec()?

There's only one event dispatcher per thread. 

> - Can QEventLoop::exec() be called outside QCoreApplication::exec()
> (i.e., in a call stack that didn't originate from QCoreApplication::exec())?

Sure. You could even use QEventLoop::exec() in place of 
QCoreApplication::exec() in your main function or QThread::exec() in your run 
function. The two are very similar, but not identical. QCoreApplication's loop 
emits aboutToQuit and its descendant classes react to the last window closing, 
while QEventLoop does not.

> - If QEventLoop::exec() spins the main loop then any event at all can
> be generated. Is this why one must use it very carefully to avoid
> reentrancy issues?

Yes. Avoid nesting event loops like the plague.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center