Qt-interest Archive, January 2008
(Qt 4.3.2) Does QWidget::setGeometry() cause a QWidget::update()?
Message 1 in thread
Good morning,
As my first posting I thought I'd start with an easy one.
I have subclassed a QAbstractScrollArea, and I paint directly on its
viewport in the paintEvent() method. I have implemented my own
scrollContentsBy() to optimise repainting of the viewport, and by
drawing the QPaintEvent::region() on screen I know this is all working
as expected, and it's working very nicely.
My only problem occurs when resizing the scroll area itself (even when
the scroll area is a top level widget). Ideally I only want the newly
exposed parts of the viewport to be updated, so I set WA_StaticContents
to be true on both the viewport and the scroll area. Unfortunately, this
doesn't seem to work, and instead the entire viewport is updated. Going
through the code for QAbstractScrollArea, the only thing I can find that
might be the culprit is in the layoutChildren() method, where the
viewport is resized with a call to QWidget::setGeometry(). Does this
always cause a full QWidget::update()?
If this is causing the full repaint, why isn't it adhering to the
WA_StaticContents attribute?
If this isn't causing the repaint, is there something else I may be
missing that is?
Any help would be much appreciated.
Yours,
Dave
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Message 2 in thread
David Thorne wrote:
> If this is causing the full repaint, why isn't it adhering to the
> WA_StaticContents attribute?
Hi, David. Since 4.1, as long as propagation is enabled (i.e., as long as
the widget has a background), static contents resizing doesn't work
unfortunately. It's something we'd like to improve/reintroduce in the 4.x
series but unfortunately right now the best you can do to get this behavior
is to implement your own pixmap caching, and reblit and reexpose on resize.
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Message 3 in thread
Andreas, thanks for the response. I do have one question though.
> Since 4.1, as long as propagation is enabled (i.e., as long as
> the widget has a background), static contents resizing doesn't work
> unfortunately.
Your response implies that with a particular combination of propagation
and background settings the static contents resizing would work. Is that
the case? When I make the viewport a top level widget, static contents
works fine, and I must profess that I don't understand the interplay
between the conflicting / complementary attributes that affect such
things...
Qt::WA_StaticContents,
Qt::WA_OpaquePaintEvent,
QWidget::setAutoFillBackground(...),
QWidget::setBackgroundRole(...).
If it helps answer the question, the viewport itself is solely used as a
canvas on which I paint information. It has no child widgets of its own,
no controls, just pretty colours ;)
> It's something we'd like to improve/reintroduce in the 4.x
> series but unfortunately right now the best you can do to get this behavior
> is to implement your own pixmap caching, and reblit and reexpose on resize.
>
As it happens I had to implement pixmap caching anyway otherwise
scrolling the widget would have also crawled to a halt (my application
has to render a window full of coloured and tessellated rectangles, and
when those rectangles are zoomed out to 2x2 pixels and I have my window
maximised on my 30" display, that's a lot of real estate to have to
repaint interactively).
Yours,
Dave
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Message 4 in thread
David Thorne wrote:
> Andreas, thanks for the response. I do have one question though.
>> Since 4.1, as long as propagation is enabled (i.e., as long as
>> the widget has a background), static contents resizing doesn't work
>> unfortunately.
> Your response implies that with a particular combination of propagation
> and background settings the static contents resizing would work. Is that
> the case?
No. Static contents does not work for child widgets, only for top-level
widgets. But as Andreas said, this is something we will look into fixing
for some future release.
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