PyQt5 animation 行为作为 class 属性

Question

I made a mini-game with python and pyqt5, a moving box that you can move with the arrow keys, but the animation only works when it is defined as a class attribute.

Here is the code:

    class Game(QMainWindow):
        # some code
        def move(self, direction):
            animation = QPropertyAnimation(self.box, b'geometry')
            x, y = self.box.x(), self.box.y()
            if(direction=='right'):
                x+=10
            # directions left, up and down
            animation.setEndValue(QRect(x, y, 20, 20))
            box.move(x, y)
            animation.setDuration(20)
            animation.start()
            animation.finished.connect(lambda: self.move(direction)
        # capture keypress to move the box

When I replace animation with self.animation, it works fine. I don't have a problem using self.animation, but it took me some time to realize that the reason my code wasn't working is because it's not an attribute, why does animation only works when it's an attribute?

Answer 1

The problem is that "animation" is a local variable so it will be eliminated when the function that created it finishes executing, in your case when it finishes executing "move" that is an instant after starting the animation.

To understand this we can use the following example, because QPropertyAnimation is a QObject, it has the destroyed signal that indicates when a QObject is destroyed.

import sys

from PyQt5 import QtCore, QtGui, QtWidgets


class MainWindow(QtWidgets.QMainWindow):
    def __init__(self, parent=None):
        super().__init__(parent)

        self.box = QtWidgets.QPushButton("QPushButton", self)

    def move(self, pos):
        animation = QtCore.QPropertyAnimation(self.box, b"pos")
        animation.setStartValue(self.box.pos())
        animation.setEndValue(pos)
        animation.setDuration(20)
        animation.start()
        animation.destroyed.connect(lambda obj: print("destroyed", obj))


if __name__ == "__main__":

    app = QtWidgets.QApplication(sys.argv)
    w = MainWindow()
    w.show()
    w.move(QtCore.QPoint(40, 40))
    sys.exit(app.exec_())

Output:

destroyed <PyQt5.QtCore.QObject object at 0x7f412abffd70>

---

So the solution in general is that it is not eliminated at least until the animation ends, for this there are several options:

1. Make it an attribute of the class since it will have the same life cycle as the class, and that is the method you used.

2. Since QPropertyAnimation is a QObject, then one way to extend the life cycle is to pass it a parent, but in this case it is better to destroy it as soon as the animation is finished:

def move(self, direction):
    animation = QPropertyAnimation(self.box, b'geometry', )
    # ...
    animation.start()
    animation.finished.connect(lambda: self.move(direction))