[PyQt] Re: question about resizing behavior

[Darren Dale]

Hi Ole,

On Tue, Jul 7, 2009 at 11:33 AM, Ole Streicher <ole-usenet-spam at gmx.net>wrote:

> Hi again,
>
> Darren Dale <dsdale24 at gmail.com> writes:
> > Nice demonstration of the problem Ole. I notice that if, after
> > resizing, I pause briefly before releasing the mouse button, the
> > scroll bar is more likely to resize properly. The problem is much more
> > apparent if I release the mouse button immediately after resizing or
> > while still dragging the window edge.
>
> I already created a Qt bug report for that (issue number #255433, not
> public
> for some reason). They asked me to compile a C++ only variant of the
> bug.
>
> However, I could not reproduce the problem in C++: the events occure here
> always in-order independent of the processing times in the (simulated)
> inner widget resize time.
>
> I could also not reproduce the bug in PyQt4 without matplotlib by
> replacing the FigureCanvasQTAgg with a QFrame and adding some random
> sleep to the resize function.
>
> But, with some stacktracing, I found the reason:
>
> in matplotlib.FigureCanvasQTAgg, the following piece of code is called
> on a resizeEvent:
>
> def draw( self ):
>
>    # ... some internal code
>
>    # Added following line to improve realtime pan/zoom on windows:
>    QtGui.qApp.processEvents()
>
> This makes the problem clear:
> - if we resize the window quickly, several resizeEvents are created and
>  are in the event queue of Qt
> - the first one gets processed by the QVBoxLayout which starts the
>  processing in its first (matplotlib) widget.
> - the matplotlib widget does what it should and at some point calls
>  matplotlib.FigureCanvasQTAgg.draw(self).
>  - this starts the processing of the next event in the queue which is
>    the next (2nd) resize event. BUT: we are still in the processing of
>    the first event!!!
>  - the 2nd event goes to the QVBoxLayout, from there to the matlotlib
>    widget (eventually starting some recursive behaviour here)
>  - if the 2nd event is finished, QVBoxLayout sends it to its other widget
>    (the scrollbar). The scrollbar is resized according to the *2nd event*.
> - if the processing of the 2nd event was finished and no more events are
>  in the queue, the matplotlib.FigureCanvasQTAgg.draw() finishes
> - at the end, this will finish the processing of the first resize event
>  in the matplotlib widget and start the *first resize event* for the
>  scrollbar
>
> This is exactly the behaviour that I described in my first posting. It
> is caused by the QtGui.qApp.processEvents() call in the draw() function.
>
> So, I think this is clearly a bug in matplotlib:
> QtGui.qApp.processEvents() should not be called while processing another
> event.
>
> Darren, I dont know your position with respect to matplotlib: are you a
> developer and thus aware of the bug or shall I post this again on the
> matplotlib mailing list?
>

I am a developer, but I think it would be good to post your summary there at
the matplotlib mailing list so we can continue this discussion there.

Darren
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