How should viewportEvent be implemented in a QAbstractScrollArea? - qt

Question

I'm having a lot of problems getting the details right for my QAbstractScrollArea. This is my current implementation of viewportEvent:
def viewportEvent(self, event):
if event.type() in [QEvent.MouseButtonPress,
QEvent.MouseMove,
QEvent.MouseButtonRelease,
QEvent.ContextMenu,
QEvent.KeyPress,
QEvent.KeyRelease]:
return self.my_viewport.event(event)
if event.type() == QEvent.Resize:
self.my_viewport.resizeEvent(event)
return super().viewportEvent(event)
if event.type() in [QEvent.UpdateLater,
QEvent.UpdateRequest]:
self.my_viewport.event(event)
if event.type() == QEvent.Paint:
self.my_viewport.paintEvent(event)
return super().viewportEvent(event)
The idea is to pass through (to the viewport widget) things like key and mouse presses. Resize events need to be passed through and sent to the abstract-scroll-area itself? What about the size for the scroll bars? Shouldn't the resize event's size be changed. If I don't pass paint events through, the viewport widget doesn't paint.
Minimum working example of broken QOpenGLWidget with QAbstractScrollArea:
import sys
from PyQt5.QtCore import QEvent
from PyQt5.QtWidgets import (QAbstractScrollArea, QApplication, QMainWindow,
QOpenGLWidget)
class MainWindow(QMainWindow):
def __init__(self):
super().__init__()
self.scope_view_widget = ScrollingScopeView()
self.setCentralWidget(self.scope_view_widget)
class ScopeView(QOpenGLWidget):
def paintGL(self):
super().paintGL()
print("Painting")
class ScrollingScopeView(QAbstractScrollArea):
def __init__(self):
super().__init__()
self.set_my_viewport(ScopeView())
def set_my_viewport(self, new_viewport):
self.my_viewport = new_viewport
self.setViewport(self.my_viewport)
def viewportEvent(self, event):
# Uncommenting this breaks painting.
if event.type() == QEvent.Paint:
self.my_viewport.paintEvent(event)
return super().viewportEvent(event)
application = QApplication(sys.argv)
main_window = MainWindow()
main_window.show()
sys.exit(application.exec_())
Filed as a Qt bug: https://bugreports.qt.io/browse/QTBUG-53269

Answer 1

My previous answer was absolutely wrong. Kudos to the OP for his investigation.
Per the docs:
When inheriting QAbstractScrollArea, you need to do the following:
Control the scroll bars by setting their range, value, page step, and tracking their movements.
Draw the contents of the area in the viewport according to the values of the scroll bars.
Handle events received by the viewport in viewportEvent() - notably resize events.
Use viewport->update() to update the contents of the viewport instead of update() as all painting operations take place on the viewport.
Unless you need to do other event management, the very short viewportEvent() in your MCVE is correct. Take a look at the code (a better look than I did) and you'll see that most events (including paint events) are not passed to the viewport. Curiously, the code does make an exception to properly resize QOpenGLWidget.
I realize now the logic behind not painting by default is to allow you to update only the region of the viewport currently visible.
In short, the below is fine. I would recommend checking to ensure the paint event only includes the rect currently visible (check the value of rect() in the paint event), otherwise you'll be painting areas not currently visible in your viewport.
def viewportEvent(self, event):
# Uncommenting this breaks painting.
if event.type() == QEvent.Paint:
self.my_viewport.paintEvent(event)
return super().viewportEvent(event)
Apologies for my screwup. I hope this is helpful.