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

[Lars Friedrich Lars]

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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