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

[Mike Jackson]

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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