python - 在启动UI之后以及以后按下按钮时如何使用QtPainter？

To draw directly onto a widget you can override it's paintEvent. The thing to remember when doing so is that paintEvent fires every time the Widget deems it necessary to redraw itself e.g. when it has been resized or moved. This means that when implementing your own version of paintEvent you need to draw all the shapes you want drawn on your widget. Note that paintEvent is rarely called directly. If you want the widget to redraw itself, you should call update() which will schedule a paintEvent for you.

这是一个简单的示例，其中在单击按钮时将任意矩形添加到画布。矩形存储在Canvas对象的数组中。在中Canvas.paintEvent，我们创建的实例，QPainter并使用此对象绘制数组中的所有矩形。

from PyQt5 import QtWidgets, QtGui, QtCore
from random import randrange

class Canvas(QtWidgets.QWidget):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.rectangles = []

    def add_rectangle(self, rect, color):
        self.rectangles.append((rect, color))
        self.update()

    def paintEvent(self, event):
        painter = QtGui.QPainter(self)
        brush = QtGui.QBrush(QtCore.Qt.white)
        painter.setBrush(brush)
        painter.drawRect(event.rect())

        pen = QtGui.QPen()
        pen.setWidth(3)
        for rect, color in self.rectangles:
            pen.setColor(color)
            painter.setPen(pen)
            brush.setColor(color)
            painter.setBrush(brush)
            painter.drawRect(rect)

class MyWindow(QtWidgets.QWidget):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.canvas = Canvas(self)
        self.button = QtWidgets.QPushButton('Add rectangle')
        self.button.clicked.connect(self.add_rectangle)

        self.layout = QtWidgets.QVBoxLayout(self)
        self.layout.addWidget(self.canvas)
        self.layout.addWidget(self.button)
        self.resize(500,500)

    def add_rectangle(self):
        w = self.canvas.width()
        h = self.canvas.height()
        x0, y0 = randrange(w), randrange(h)
        x1, y1 = randrange(w), randrange(h)

        shape = QtCore.QRect(min(x0,x1), min(y0,y1), abs(x0-x1), abs(y0-y1))
        color = QtGui.QColor.fromRgb(*(randrange(256) for i in range(3)), 180)
        self.canvas.add_rectangle(shape, color)


if __name__ == "__main__":
    app = QtWidgets.QApplication([])
    window = MyWindow()
    window.show()
    app.exec()