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

Thu Nov 5 10:56:08 CET 2009

Hello Mike,

thank you very much for your suggestion!
I modified the code of producerconsumer2.tar.gz a bit to simulate my needs and packed it into my DLL. Tests showed that this mechanism avoids the Qt event loops to 'eat' my key press events of the calling Delphi-application, and that is exactly what I need.
However, now I have to adapt my real code to conform to the suggested lines of code ...
Thank you again!

regards,

lars

-------- Original-Nachricht --------
> Datum: Wed, 4 Nov 2009 10:47:42 -0500
> Von: Mike Jackson <imikejackson at gmail.com>
> An: qt-interest at trolltech.com
> Betreff: Re: [Qt-interest] qt event loop in a DLL for thread signal slot	mechanism

> I ran into something similar in the last few weeks. I was trying to 
> send signals across threads and needed the event loop running in the 
> thread.
> 
> http://labs.trolltech.com/blogs/2007/07/05/qthreads-no-longer-abstract/
> 
> http://chaos.troll.no/~bhughes/producerconsumer2.tar.gz
> 
> That code more or less worked for what I wanted to do.
> 
> Mike Jackson
> 
> 
> On 2009-11-04 02:23:00 -0500, "Lars Friedrich Lars" 
> <lars-friedrich at gmx.net> said:
> 
> > 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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