PySide2: "MouseReleaseEvernt" not triggered in QGraphicsItem - pyqt

Question

I have a custom QGraphicsItem on a QgraphicsScene. And I'm trying to draw on QGraphicsItem following mouse events. Here is what I implemented:
The custom QGraphicsItem in implemented in GraphicsItem_custom.py
from PySide2.QtWidgets import QGraphicsItem
from PySide2.QtCore import QPointF, QRectF, Qt, QRect, QPoint
from PySide2.QtGui import QPen, QPainter, QPixmap
class GraphicsItem_custom(QGraphicsItem):
def __init__(self,*args, **kwargs):
super().__init__(*args, **kwargs)
self.start, self.end = QPoint(), QPoint()
self.rectangles = []
def boundingRect(self):
return QRectF(0, 0, 500, 300)
def paint(self, painter, option, widget):
painter.setPen(QPen(Qt.red, 6, Qt.SolidLine))
painter.drawText(QPointF(0, 10), "Hiya")
painter.drawRect(self.boundingRect())
for rectangle in self.rectangles:
painter.drawRect(rectangle)
if not self.start.isNull() and not self.end.isNull() and self.start != self.end:
rect = QRect(self.start, self.end)
print("paintEvent start: " + str(self.start) + ", end: " + str(self.end))
print("")
painter.drawRect(rect.normalized())
def mousePressEvent(self, event):
super().mousePressEvent(event)
print("Pressed")
if event.buttons() and Qt.LeftButton:
self.start = event.pos()
self.end = self.start
self.update()
print("left button")
def mouseMoveEvent(self, event):
print("Move")
super().mouseMoveEvent(event)
if event.buttons() and Qt.LeftButton:
self.end = event.pos()
self.update()
def mouseReleaseEvent(self, event):
super().mouseReleaseEvent(event)
print("Released")
if event.button() and Qt.LeftButton:
if self.start != self.end:
r = QRect(self.start, self.end).normalized()
self.rectangles.append(r)
self.start = self.end = QPoint()
self.update()
Then, in main.py I create the scene and view, and add the item onto the scene:
class MyApp(QMainWindow):
def __init__(self):
super().__init__()
self.window_width, self.window_height = 1200, 800
self.setMinimumSize(self.window_width, self.window_height)
self.scene = QGraphicsScene(self)
self.view = QGraphicsView(self.scene, self)
self.view.setGeometry(0,0,1000, 700)
self.item = GraphicsItem_custom()
self.scene.addItem(self.item)
if __name__ == '__main__':
# don't auto scale when drag app to a different monitor.
# QApplication.setAttribute(Qt.HighDpiScaleFactorRoundingPolicy.PassThrough)
app = QApplication(sys.argv)
app.setStyleSheet('''
QWidget {
font-size: 30px;
}
''')
myApp = MyApp()
myApp.show()
try:
sys.exit(app.exec_())
except SystemExit:
print('Closing Window...')
When I run this code, the mousePressEvent() is triggered when I press, however, mouseReleaseEvent() is not when triggered when I release. Can anyone tell me what I did wrong?

Answer 1

The documentation of mousePressEvent() explains:
If you do reimplement this function, event will by default be accepted (see QEvent::accept()), and this item is then the mouse grabber. This allows the item to receive future move, release and doubleclick events.
If you call the base implementation like this:
super().mousePressEvent(event)
the result will the same as not reimplementing the function (you're calling the default behavior), so the event will not accepted, meaning that the item will not receive move, release and doubleclick events, unless you've set specific flags that trigger those event handlers due to their nature (like Qt.ItemIsMovable).
You have to carefully decide if you actually need to call the base implementation or not (usually depending on the flags), and eventually add this at some point in the mousePressEvent() override:
event.setAccepted(True)