[PyQt] Re: question about resizing behavior

[Darren Dale]

On Fri, Jul 3, 2009 at 3:37 AM, Ole Streicher <ole-usenet-spam at gmx.net>wrote:

> Hello Darren,
>
> Darren Dale <dsdale24 at gmail.com> writes:
> > Somebody reported some strange resizing behavior at the matplotlib
> mailing
> > list.
>
> The "somebody" was me :-)
>
> Here some additional information:
>
> If you printout the resize (overwritten) events of the scrollbar and the
> matplotlib widgets (see code below), you will see that sometimes the
> events are not processed in-order:
>
> ScrollBar start 640
> ScrollBar   end 640
>  Diagram start 640
>  Diagram   end 640
>
> occurs when I just started, which is correct. But when one changes the
> horizontal size, the following happens (printouts which obviously belong
> to the same resize event are marked with the same number of stars):
>
> *     Diagram start 633
> **    Diagram start 608
> [...]
> **    Diagram   end 608
> **  ScrollBar start 608
> **  ScrollBar   end 608
> *     Diagram   end 633
> *   ScrollBar start 633
> *   ScrollBar   end 633
>
> What you see is that
>
> - the matplotlib FigureCanvasQTAgg gets its first resize event
> - during its processing inside FigureCanvasQTAgg a second event
>  occurred
> - this second event is processed *faster* by FigureCanvasQTAgg than
>  the first event
> - thus, the second event occurs *first* on the scrollbar
> - the first resize event only occurs after the second on the scrollbar
> - this leads to a wrong size of the scrollbar
> - this may occur even nested (removed "[...]" in the printout above)
>
> This may be a bug in FigureCanvasQTAgg (it is not synchonized in the
> processing of resize events, allowing events to bypass), or in
> Qt/QVBoxLayout (same argument there). I am not deep enough inside Qt to
> know the API details here.
>
> Best regards
>
> Ole
> -----------------------------8<------------------------------------------
> import sys
>
> from PyQt4 import QtGui, QtCore
> from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg
> from matplotlib.figure import Figure
>
> class MyDiagram(FigureCanvasQTAgg):
>    def __init__(self, fig):
>        FigureCanvasQTAgg.__init__(self, fig)
>
>    def resizeEvent(self, event):
>        print '  Diagram start', event.size().width()
>        FigureCanvasQTAgg.resizeEvent(self, event)
>        print '  Diagram   end', event.size().width()
>
>
> class MyScrollBar(QtGui.QScrollBar):
>    def __init__(self, parent):
>        QtGui.QScrollBar.__init__(self, QtCore.Qt.Horizontal, parent)
>
>    def resizeEvent(self, event):
>        print 'ScrollBar start', event.size().width()
>        QtGui.QScrollBar.resizeEvent(self, event)
>        print 'ScrollBar   end', event.size().width()
>
> class DiagramWidget(QtGui.QWidget):
>   def __init__(self, parent=None):
>       QtGui.QWidget.__init__(self, parent)
>
>        self.scrollbar = MyScrollBar(self)
>
>       fig = Figure()
>       axes = fig.add_subplot(111)
>       axes.plot(xrange(100))
>        self.diagram = MyDiagram(fig)
>        self.diagram.setParent(self)
>
>       layout = QtGui.QVBoxLayout(self)
>       self.setLayout(layout)
>       layout.addWidget(self.diagram)
>       layout.addWidget(self.scrollbar)
>
> a = QtGui.QApplication(sys.argv)
> w = DiagramWidget()
> w.show()
> a.exec_()
>
> -----------------------------8<------------------------------------------
>


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 also tried adding a call to qApp.processEvents() in
FigureCanvasQT.resizeEvent, and that seemed to make the problem less
frequent, but the problem still exists.

Darren
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