Hi all
I have a relatively simple Qt program which utilises the PlPlot Qt widget
(QtExtWidget). QtExtWidget is a component of the main application window
and as such is constructed during program startup, prior to the calling of
QApplication::exec() and before any GUI elements have been made visible with
show(). I have noticed that during program startup I will sometimes get the
following warnings:
QColor::setRgb: RGB parameters out of range
This doesn't appear all the time and seems to be fairly sensitive to timing,
making it difficult to debug. However, after much trial and error I have
determined that the warning is triggered from the QtExtWidget constructor.
Further exploration showed it was coming from QtPLWidget's constructor
(from which QtExtWidget is derived).
Looking at the source of QtPLWidget::QtPLWidget() in plqt.cpp, I noticed a
call to QApplication::processEvents() which didn't seem right to me.
Commenting this out resolves the warning. Importantly, the warning remains
when any of the other Qt functions (such as resize(), QPen()) are commented
out.
The reason I'm suspicious of this call is that there is no guarantee that
any event loop has even been initialised at the time the constructors are
called. Certainly in my case the event loop isn't running because
QApplication::exec() has not yet been called at the time of construction.
Similarly, the widget tree is not visible at event creation time, so it
seems reasonable that some parts of the GUI environment (such as QColor
objects) might not have been allocated yet. At a fundamental level, I
expect that the processEvents() call could be triggering a draw of the as
yet incompletely constructed widget. I also recall reading in the past that
draw actions cannot be triggered in constructors due to the incomplete
nature of the widget being built.
Regardless, I can't think of any reason for calling
QApplication::processEvents() in the constructor of a Qt widget. Does
anyone know why this call is being made in QtPLWidget's constructor? Based
on tests here and the reasons given above I think it should be removed. The
trivial patch to do this is included at the end of this email.
What do others think?
Regards
jonathan
--- a/plplot-5.10.0/bindings/qt_gui/plqt.cpp 2013-12-10 01:09:59.000000000
+1030
+++ b/plplot-5.10.0/bindings/qt_gui/plqt.cpp 2014-07-03 11:42:26.884803875
+0930
@@ -653,7 +653,6 @@
resize( i_iWidth, i_iHeight );
lastColour.r = -1;
setVisible( true );
- QApplication::processEvents();
redrawFromLastFlush = false;
redrawAll = true;
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