animate a QGraphicsPixmapItem

Question

I am trying to animate a QGraphicsPixmapItem in PyQt5. The program as is crashes without any error message, and removing the lines about the variable 'anime' makes the program work normally.

here is the QGraphicsPixMapItem:

class QStone(QGraphicsPixmapItem,QGraphicsObject):
    def __init__(self, color, movable):
        QGraphicsPixmapItem.__init__(self)
        QGraphicsObject.__init__(self)

        if movable:
            self.setFlag(QGraphicsItem.ItemIsMovable)

        white = QPixmap("ressources/white2.png")
        black = QPixmap("ressources/black_stone.png")
        empty = QPixmap("ressources/no_stone.jpg")


        if color == Player.white:
            self.setPixmap(white.scaled(60, 60, Qt.KeepAspectRatio))

        elif color == Player.black:
            self.setPixmap(black.scaled(60, 60, Qt.KeepAspectRatio))

        self.w = self.boundingRect().width()
        self.h = self.boundingRect().height()

    def hoverEnterEvent(self, event):
        self.setCursor(Qt.OpenHandCursor)
        self.setOpacity(0.5)
        event.accept()

    def hoverLeaveEvent(self, event):
        self.setCursor(Qt.ArrowCursor)
        self.setOpacity(1.)
        event.accept()

the QGraphicsObject inheritance seems to be required for using QPropertyAnimation.

here is the code containing this animation:(this method belongs to a QGraphicsView's subclass):

def display_stone(self, x, y, color=None):
    stone = ""
    # if color is None:
    #     stone = QStone("", True)
    if color == Player.white:
        stone = QStone(Player.white, False)
    elif color == Player.black:
        stone = QStone(Player.black, False)

    stone.setOpacity(0.0)
    anime = QPropertyAnimation(stone, b"opacity",self)
    anime.setDuration(800)
    anime.setStartValue(0.0)
    anime.setEndValue(1.0)
    anime.start()

    stone.setPos(x - stone.w / 2, y - stone.h / 2)
    self.scene.addItem(stone)
    stone.setZValue(10)

any idea? thank you

Answer 1

Unlike the Qt C++ API, PyQt does not allow double inheritance (except in exceptional cases⁽¹⁾) so you cannot implement a class that inherits from QGraphicsPixmapItem and QGraphicsObject.

In this case there are the following options:

1. In this case it is to create a QObject that handles the property you want to modify, and that is the object that is handled by the QPropertyAnimation:

class OpacityManager(QObject):
    opacityChanged = pyqtSignal(float)

    def __init__(self, initial_opacity, parent=None):
        super(OpacityManager, self).__init__(parent)
        self._opacity = initial_opacity

    @pyqtProperty(float, notify=opacityChanged)
    def opacity(self):
        return self._opacity

    @opacity.setter
    def opacity(self, v):
        if self._opacity != v:
            self._opacity = v
            self.opacityChanged.emit(self._opacity)


class QStone(QGraphicsPixmapItem):
    def __init__(self, color, movable=False):
        QGraphicsPixmapItem.__init__(self)

        self.manager = OpacityManager(self.opacity())
        self.manager.opacityChanged.connect(self.setOpacity)

        if movable:
            self.setFlag(QGraphicsItem.ItemIsMovable)
        # ...

# ...
anime = QPropertyAnimation(stone.manager, b"opacity", stone.manager)
# ...

2. Another option is QVariantAnimation:

# ...
anime = QVariantAnimation(self)
anime.valueChanged.connect(stone.setOpacity)
anime.setDuration(800)
# ...

^{(1) https://www.riverbankcomputing.com/static/Docs/PyQt5/qt_interfaces.html}