[Qt-interest] qt event loop in a DLL for thread signal slot mechanism

[Lars Friedrich Lars]

Hi Alex,

unfortunately replacing QApplication by QCoreApplication does not fix the problem. The behavior is exactly the same.

regards,

lars

-------- Original-Nachricht --------
> Datum: Tue, 3 Nov 2009 15:25:43 -0800
> Von: "Malyushytsky, Alex" <alex at wai.com>
> An: "qt-interest at trolltech.com" <qt-interest at trolltech.com>
> Betreff: Re: [Qt-interest] qt event loop in a DLL for thread signal	slot	mechanism

> >> - Why does the Qt event loop 'steal' the keypress events although it is
> not a GUI application?
> 
> Never done what you are trying to do, but QApplications always means GUI
> related initialization.
> If you don't need GUI, try to use QCoreApplication instead and see if it
> solves your problem.
> 
> Regards,
>    Alex
> 
> -----Original Message-----
> From: qt-interest-bounces at trolltech.com
> [mailto:qt-interest-bounces at trolltech.com] On Behalf Of Lars Friedrich Lars
> Sent: Tuesday, November 03, 2009 3:00 PM
> To: qt-interest at trolltech.com
> Subject: [Qt-interest] qt event loop in a DLL for thread signal slot
> mechanism
> 
> Hello qt developers,
> 
> I have some serious problems with Qt event loop's behavior when using it
> in a windows DLL as an interface for a Delphi5-application.
> 
> I am wrapping some image conversion capabilities (furthermore using the
> open source toolkits ITK and VTK with cross-platform build tool CMake) in a
> DLL which incorporates Qt-threads. The DLL is more or less a tool which gets
> used by a Delphi5-application. The Delphi-application calls a function of
> the DLL which internally initiates a QThread which does some parametrized
> image conversion tasks in the background. In order to detect the end of a
> background thread I use QThread's finished()-signal and connect it to a
> DLL-internal slot which furthermore triggers a DLL-callback so that the calling
> Delphi-application gets notified as well.
> After some tests and studying the Qt-documentation, I noticed that I need
> an active Qt event loop within the DLL to make the signal/slot mechanism
> work. I decided to instantiate a DLL-global QApplication instance and call
> its (blocking!) exec()-method after loading the DLL internally. I implemented
> that by initiating a timer in the Delphi-application which calls the
> blocking (!!!) QApplication's exec()-mehod using the DLL (as I read in Qt's
> documentation that the event loop can only be started from the main thread!).
> However, that seems to work as the signal/slot mechanism seems to work
> correctly (I verified that with a log file).
> 
> Now my problem: as soon as the Qt event loop is running it seems to
> seriously influence the Delphi-application's event loop (its GUI behavior):
> - Delphi's TApplication OnIdle event is no longer invoked (this problem is
> more or less solved because I can generate this event by myself using a
> timer and analyzing the message queue state)
> - (some essential but not all) keypress/keydown/keyup events of the
> Delphi-application's GUI seem to be 'stolen' by Qt's event loop - they do not
> arrive in the Delphi application (for example the tab-key is no longer
> recognized by the Delphi-application to navigate through its GUI's controls)
> 
> Now, my qestions:
> - Did some of you tackle a similar problem?
> - Why does the Qt event loop 'steal' the keypress events although it is
> not a GUI application?
> - Can I 'pipe' the keypress events back to the Delphi-application's main
> thread (message queue)? Or could I connect the DLL's event loop directly to
> the Delphi-application's event loop?
> - Or did I completely misunderstand some Qt-fundamentals? Could I
> implement the signal/slot mechanism with some simpler Qt-tools than a QApplication?
> Maybe a lightweight event loop specific to thread signal/slot mechanism?
> 
> Any comments on this issue are really really appreciated!!!
> Please help me.
> 
> Thank you!!!!!!
> 
> 
> best regards,
> 
> lars
> 
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