Implement simple drawing in QtInstanceDrawingArea
by handling the QEvent::Resize and QEvent::Paint
events for the QLabel widget.
While the QLabel does not emit any signals when
these events occur, it's possible to handle them
by setting an event filter using QObject::installEventFilter [1].
Install QtInstanceDrawingArea as an event filter
for its label, and implement handling of the Resize
and Paint event in QtInstanceDrawingArea::eventFilter:
On the QEvent::Resize event, adjust the output size
of the output device and call the `weld::Widget::m_aSizeAllocateHdl`
handler. (See also GtkInstanceDrawingArea::signal_size_allocate
and SalInstanceDrawingArea::ResizeHdl to see what the
gtk3 and VCL implementations do.)
On the QEvent::Paint event, trigger drawing on the
output device via the weld::DrawingArea::m_aDrawHdl
handler, and get a QPixmap for what's painted on the
output device and set that QPixmap for the label if
it has changed.
With this in place, the GtkDrawingArea in the "Help"
-> "Show Tip of the Day" dialog displays the
image as expected, and gets updated when moving to
the next tip, once support for that dialog has
been declared, which will be done in an upcoming
commit.
For more complex cases, a more performant solution
might become necessary and can be investigated
once support for these is implemented.
For the "Tip of the Day" dialog, the `aRect` passed in
QtInstanceDrawingArea::handlePaintEvent is irrelevant
as CuiGraphicPreviewWindow::Paint doesn't make use
of the passed rectangle. (This might also have to be
reconsidered/adjusted in case a dialog actually making
use of the rectangle doesn't work as expected.)
[1] https://doc.qt.io/qt-6/qobject.html#installEventFilter
Change-Id: I651e00043fac04a4c77aae921e6397e8d97cc69c
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/176173
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
a3400e0471