[Qt-interest] QSignalTransition drops silently signals

[Bo Thorsen]

The only reasonable explanation I have for this is if your signal slots 
are direct.

You can test this by creating a proxy object in the gui thread like this:

class ProxyObject : public QObject {
     ...
signals:
     void retransmit();
};

...

// This MUST be done by objects in the GUI thread
connect(otherThread, SIGNAL(toT2()), new ProxyObject(this),
     SIGNAL(void retransmit()), Qt::QueuedConnection);

If this turns out to be a problem, it looks to me like you should send a 
bug report. If this does not fix the problem, then you have to look in 
your own code for it.

The proxy signal with the queued connection works by not handling the 
emit before the event loop runs again. Since the event loop is in the 
same thread as the state machine, the signal can not be lost.

Bo.

Den 14-06-2011 21:09, Peter Kümmel skrev:
> Recently I used the QStateMachine framework in a more complex
> than "animation-on-ui-event "multi-threaded environment and stumbled
> over a strange behavior of QSignalTransition, which at the first glace
> looks like a design error in QSignalTransition (haven't checked the
> other subclassesof QAbstractTransition).
>
> Assume following setup:
>
> QStates:
>       s1, s2, s3
>
> QSignalTransitions
>       t1: s1->s2 and t2: s2->s3
>       t1 and t2 are triggered by objects of another thread than QStateMachine ones
>
> Then this is possible:
> The state machine makes the transition from s1 to s2 because t1 was triggered,
> and while the state machine is busy with t1 the signal which should trigger t2
> is also emitted by an object (this is possible because the emitting object runs in a other thread).
> But because the connections of t2 to the signal is not created until s2 is entered,
> the signal for t2 is ignored and silently dropped. No deferring, no warning, nothing,
> the state machine simply will never leave t2.
>
> Here I wonder if this is a bug or a feature?
>
> Peter
>
>
> P.S.:
> Recently I also threw it into the black hole:
> http://bugreports.qt.nokia.com/browse/QTBUG-19789
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Bo Thorsen,
Fionia Software.

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