python - pyqt add rectangle in Qgraphicsscene

Question

I have a scene like this

class Scene(QtWidgets.QGraphicsScene):
    def __init__(self, parent=None):
        super(Scene, self).__init__(parent)

    def mousePressEvent(self, event):
        print('scene pressed')
        self.wid = MyRect(event.pos(), event.pos())
        self.addItem(self.wid)
        self.wid.show()

I would like class MyRect(QtWidgets.QGraphicsRectItem) with painter, mouse event and so on to be a draggable rectangle. all stuff in MyRect

So then I could have many Rectangle to the scene and even after draw line between them and so on (kind of diagram app), but keeping objects related editable options in MyRect, MyLine , ....

I thought :

class MyRect(QtWidgets.QGraphicsRectItem):
    def __init__(self, begin, end, parent=None):
        super().__init__(parent)
        self.begin = begin
        self.end = end

    def paintEvent(self, event):
        print('painting')
        qp = QtGui.QPainter(self)
        qp.drawRect(QtCore.QRect(self.begin, self.end))

    def mousePressEvent(self, event):
        self.begin = event.pos()
        self.end = event.pos()
        self.update()

    def mouseMoveEvent(self, event):
        self.end = event.pos()
        self.update()

    def mouseReleaseEvent(self, event):
        self.begin = event.pos()
        self.end = event.pos()
        self.update()

But I does not work (paint event not initiated whereas mousepressed event in scene is intiated)

I did not find what I wanted through the web so started totry do it by myself. I'm pretty sure it is a must known starting point but I cannot find it

Answer 1

First of all a QGraphicsItem is not a QWidget, so it has those events and does not handle them directly, that's what QGraphicsView and QGraphicsScene do. For example you say that you want to have a moveable rectangle because that task is simple is QGraphicsView, it is not necessary to overwrite:

from PyQt5 import QtCore, QtWidgets

class MainWindow(QtWidgets.QMainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)
        scene = QtWidgets.QGraphicsScene(self)
        view = QtWidgets.QGraphicsView(scene)
        self.setCentralWidget(view)

        rect_item = QtWidgets.QGraphicsRectItem(QtCore.QRectF(0, 0, 100, 100))
        rect_item.setFlag(QtWidgets.QGraphicsItem.ItemIsMovable, True)
        scene.addItem(rect_item) 


if __name__ == '__main__':
    import sys

    app = QtWidgets.QApplication(sys.argv)
    w = MainWindow()
    w.resize(640, 480)
    w.show()
    sys.exit(app.exec_())

---

If you want to change the way you paint the rectangle you must overwrite the paint() method as shown below:

from PyQt5 import QtCore, QtGui, QtWidgets


class RectItem(QtWidgets.QGraphicsRectItem):
    def paint(self, painter, option, widget=None):
        super(RectItem, self).paint(painter, option, widget)
        painter.save()
        painter.setRenderHint(QtGui.QPainter.Antialiasing)
        painter.setBrush(QtCore.Qt.red)
        painter.drawEllipse(option.rect)
        painter.restore()

class MainWindow(QtWidgets.QMainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)
        scene = QtWidgets.QGraphicsScene(self)
        view = QtWidgets.QGraphicsView(scene)
        self.setCentralWidget(view)

        rect_item = RectItem(QtCore.QRectF(0, 0, 100, 100))
        rect_item.setFlag(QtWidgets.QGraphicsItem.ItemIsMovable, True)
        scene.addItem(rect_item) 


if __name__ == '__main__':
    import sys

    app = QtWidgets.QApplication(sys.argv)
    w = MainWindow()
    w.resize(640, 480)
    w.show()
    sys.exit(app.exec_())

Update:

from PyQt5 import QtCore, QtGui, QtWidgets

class GraphicsScene(QtWidgets.QGraphicsScene):
    def __init__(self, parent=None):
        super(GraphicsScene, self).__init__(QtCore.QRectF(-500, -500, 1000, 1000), parent)
        self._start = QtCore.QPointF()
        self._current_rect_item = None

    def mousePressEvent(self, event):
        if self.itemAt(event.scenePos(), QtGui.QTransform()) is None:
            self._current_rect_item = QtWidgets.QGraphicsRectItem()
            self._current_rect_item.setBrush(QtCore.Qt.red)
            self._current_rect_item.setFlag(QtWidgets.QGraphicsItem.ItemIsMovable, True)
            self.addItem(self._current_rect_item)
            self._start = event.scenePos()
            r = QtCore.QRectF(self._start, self._start)
            self._current_rect_item.setRect(r)
        super(GraphicsScene, self).mousePressEvent(event)

    def mouseMoveEvent(self, event):
        if self._current_rect_item is not None:
            r = QtCore.QRectF(self._start, event.scenePos()).normalized()
            self._current_rect_item.setRect(r)
        super(GraphicsScene, self).mouseMoveEvent(event)

    def mouseReleaseEvent(self, event):
        self._current_rect_item = None
        super(GraphicsScene, self).mouseReleaseEvent(event)


class MainWindow(QtWidgets.QMainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)
        scene =GraphicsScene(self)
        view = QtWidgets.QGraphicsView(scene)
        self.setCentralWidget(view)


if __name__ == '__main__':
    import sys

    app = QtWidgets.QApplication(sys.argv)
    w = MainWindow()
    w.resize(640, 480)
    w.show()
    sys.exit(app.exec_())