tdf#130857 qt weld: Implement simple drawing in QtInstanceDrawingArea

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>

3 files changed, 62 insertions, 0 deletions

diff --git a/include/vcl/qt/QtUtils.hxx b/include/vcl/qt/QtUtils.hxx
index 368424a8cbe7..87f21d96315c 100644
--- a/include/vcl/qt/QtUtils.hxx
+++ b/include/vcl/qt/QtUtils.hxx
@@ -21,6 +21,7 @@
 #include <rtl/ustring.hxx>
 #include <vcl/filter/PngImageWriter.hxx>
 #include <vcl/image.hxx>
+#include <vcl/outdev.hxx>
 
 #include <QtCore/QString>
 #include <QtGui/QPixmap>
@@ -53,6 +54,11 @@ inline QPixmap toQPixmap(const css::uno::Reference<css::graphic::XGraphic>& rIma
     return toQPixmap(aImage);
 }
 
+inline QPixmap toQPixmap(const OutputDevice& rDevice)
+{
+    return toQPixmap(Image(rDevice.GetBitmapEx(Point(), rDevice.GetOutputSizePixel())));
+}
+
 inline QPixmap loadQPixmapIcon(const OUString& rIconName)
 {
     BitmapEx aIcon(rIconName);
diff --git a/vcl/inc/qt5/QtInstanceDrawingArea.hxx b/vcl/inc/qt5/QtInstanceDrawingArea.hxx
index 757174330c13..e730464ee83e 100644
--- a/vcl/inc/qt5/QtInstanceDrawingArea.hxx
+++ b/vcl/inc/qt5/QtInstanceDrawingArea.hxx
@@ -45,6 +45,12 @@ public:
     virtual a11yrelationset get_accessible_relation_set() override;
     virtual AbsoluteScreenPixelPoint get_accessible_location_on_screen() override;
 
+    virtual bool eventFilter(QObject* pObject, QEvent* pEvent) override;
+
+private:
+    void handlePaintEvent();
+    void handleResizeEvent();
+
 private:
     virtual void click(const Point&) override;
 };
diff --git a/vcl/qt5/QtInstanceDrawingArea.cxx b/vcl/qt5/QtInstanceDrawingArea.cxx
index 0ae6baa2b087..3046c890022d 100644
--- a/vcl/qt5/QtInstanceDrawingArea.cxx
+++ b/vcl/qt5/QtInstanceDrawingArea.cxx
@@ -10,12 +10,17 @@
 #include <QtInstanceDrawingArea.hxx>
 #include <QtInstanceDrawingArea.moc>
 
+#include <vcl/qt/QtUtils.hxx>
+
 QtInstanceDrawingArea::QtInstanceDrawingArea(QLabel* pLabel)
     : QtInstanceWidget(pLabel)
     , m_pLabel(pLabel)
     , m_xDevice(DeviceFormat::WITHOUT_ALPHA)
 {
     assert(m_pLabel);
+
+    // install event filter, so eventFilter() can handle widget events
+    m_pLabel->installEventFilter(this);
 }
 
 void QtInstanceDrawingArea::queue_draw()
@@ -74,4 +79,49 @@ AbsoluteScreenPixelPoint QtInstanceDrawingArea::get_accessible_location_on_scree
 
 void QtInstanceDrawingArea::click(const Point&) { assert(false && "Not implemented yet"); }
 
+bool QtInstanceDrawingArea::eventFilter(QObject* pObject, QEvent* pEvent)
+{
+    if (pObject != m_pLabel)
+        return false;
+
+    SolarMutexGuard g;
+    assert(GetQtInstance().IsMainThread());
+
+    switch (pEvent->type())
+    {
+        case QEvent::Paint:
+            handlePaintEvent();
+            return false;
+        case QEvent::Resize:
+            handleResizeEvent();
+            return false;
+        default:
+            return false;
+    }
+}
+
+void QtInstanceDrawingArea::handlePaintEvent()
+{
+    tools::Rectangle aRect(0, 0, m_pLabel->width(), m_pLabel->height());
+    aRect = m_xDevice->PixelToLogic(aRect);
+    m_xDevice->Erase(aRect);
+    m_aDrawHdl.Call(std::pair<vcl::RenderContext&, const tools::Rectangle&>(*m_xDevice, aRect));
+    QPixmap aPixmap = toQPixmap(*m_xDevice);
+
+    // set new pixmap if it changed
+#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
+    if (aPixmap.toImage() != m_pLabel->pixmap().toImage())
+#else
+    if (aPixmap.toImage() != m_pLabel->pixmap(Qt::ReturnByValue).toImage())
+#endif
+        m_pLabel->setPixmap(aPixmap);
+}
+
+void QtInstanceDrawingArea::handleResizeEvent()
+{
+    const Size aSize = toSize(m_pLabel->size());
+    m_xDevice->SetOutputSizePixel(aSize);
+    m_aSizeAllocateHdl.Call(aSize);
+}
+
 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */