[Lazarus] Message Loop paused when no interaction with mouse?

Henry Vermaak henry.vermaak at gmail.com
Wed Apr 20 11:35:25 CEST 2011

On 20/04/11 10:26, Bernd wrote:
> Hi,
> I am sure I have read some discussion about this already but I cannot
> find it anymore, so I will ask here, I don't know whether it is a bug
> or intentional and I am only using it wrongly. The following is what I
> have and what I am trying to do:
> * Linux (KDE3.5, windowmanager is kwin) Lazarus is latest SVN, LCL
> widgetset is GTK+
> * a TForm with a TImage
> * a TThread (started when the form is created, running forever,
> automatically drawing to the TImage)
> The GUI thread is idle, the form is responsive, all the work is done
> in the separate TThread. This thread will wait for RGB data from a
> camera on a named pipe and read it and then write pixels in the TImage
> via Image.Picture.Bitmap.RawImage.Data[somewhere]^ . It will call
> Image.Picture.Bitmap.BeginUpdate before it starts and EndUpdate after
> it is done and this is all working fine.
> Exactly after each frame i can see there is the OnPictureChanged event
> triggered, I can verify this with:
> procedure TForm1.DisplayPictureChanged(Sender: TObject);
> begin
>    writeln('\n*** frame ***');
> end;
> It will print '*** frame ***'' three times a second, exactly the frame
> rate of my camera. If I move the mouse around on the form I can see my
> TImage updating at the same rate. I just have to move the mouse. As
> soon as I stop moving the mouse over the form the TImage will not
> update anymore. The events will still be fired, I can still see it
> printing *** frame *** nicely every 300ms to the console, only the
> TImage won't update.
> I tried to call the Repaint method of the TImage (from within its
> OnPictureChanged handler) but this will freeze it (infinite loop?), I
> tried Image.Invalidate, I tried the same with the entire form, I tried
> calling Application.HandleMessage (giving me GTK warnings about the
> wrong thread (?) in the console), some things froze it, some crash it
> and some have no effect at all.
> I have also tried several things with Synchonize() and somehow managed
> to freeze it but since I now already have my TImage firing its event
> I figured out this would be the right place from where I should try to
> update the GUI. But I can't find it.
> Is this a bug or am I just missing something? The only way I have
> found working is if I put  a timer onto the form and then only this:
> procedure TForm1.Timer1Timer(Sender: TObject);
> begin
>    //
> end;
> This empty event method is already enough to wake up the form and
> "simulate" what I did with my mouse move. But I don't want to have a
> timer firing every few milliseconds (it has to be really fast because
> I can set my camera to much shorter exposure and higher frame rate),
> this simply does not "look right" to me, I would rather have it call
> the right method exactly after a new frame has arrived.
> Since I cannot find anything with google relating to this I have
> written this rather lengthy essay, hoping all possible search terms
> are now included and the next one with this problem will find this
> thread.

See here for a related issue:


You'll have to wake up the main thread after the frame (which is what 
your timer is doing).  For gtk you can use 
g_main_context_wakeup(g_main_context_default), for qt there is 


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