QGraphicsScene鼠标绘制矩形问题：起始点大于终点时矩形不显示

问题1：拖动绘制时start_point坐标大于end_point导致矩形不显示 #

原因 #

用QRectF(start_point, end_point)构造矩形时，若起点的x/y坐标大于终点，会生成宽高为负数的无效矩形，QGraphicsRectItem不会渲染这类矩形。

修复 #

构造矩形后调用normalized()方法，将矩形归一化为左上角到右下角的有效区域：

self.graphics_line.setRect(QtCore.QRectF(self.start_point, self.end_point).normalized())

问题2：右键唤出菜单时出现不可见矩形 #

原因 #

1. mousePressEvent中右键判断逻辑错误：直接用QtCore.Qt.RightButton作为条件（非零值恒为真），导致只要self.graphics_line存在，左键点击也会进入右键分支，触发不必要操作；
2. clean_scene方法未实现完整，无法正确清理旧矩形项，导致残留无效项；
3. mouseReleaseEvent中按钮判断错误：使用event.buttons()（当前按住的按钮）而非event.button()（触发释放的按钮），逻辑不符合预期。

修复 #

1. 修正右键判断条件，明确判断触发事件的是右键：

elif event.button() == QtCore.Qt.RightButton:

2. 完善clean_scene方法，彻底清理场景中的矩形项：

def clean_scene(self):
    # 遍历并删除所有矩形项
    for item in list(self.items()):
        if isinstance(item, QtWidgets.QGraphicsRectItem):
            self.removeItem(item)
    self.graphics_line = None

3. 修正mouseReleaseEvent的按钮判断逻辑：

if event.button() == QtCore.Qt.LeftButton:

修改后的完整代码 #

from PyQt5 import QtCore, QtWidgets, QtGui

class RectangleScene(QtWidgets.QGraphicsScene):
    def __init__(self, *args, **kwargs):
        super(RectangleScene, self).__init__(*args, **kwargs)
        self.graphics_line = None
        self.start_point = QtCore.QPointF()
        self.end_point = QtCore.QPointF()

    def clean_scene(self):
        # 清理所有矩形项
        for item in list(self.items()):
            if isinstance(item, QtWidgets.QGraphicsRectItem):
                self.removeItem(item)
        self.graphics_line = None

    def mousePressEvent(self, event):
        if event.button() == QtCore.Qt.LeftButton:
            self.clean_scene()
            self.start_point = event.scenePos()
            self.end_point = self.start_point
            self.graphics_line = QtWidgets.QGraphicsRectItem(QtCore.QRectF(self.start_point, self.end_point))
            self.addItem(self.graphics_line)
            self.update_path()
        elif event.button() == QtCore.Qt.RightButton:
            # 交给父类处理上下文菜单
            super().mousePressEvent(event)

    def mouseMoveEvent(self, event):
        if event.buttons() & QtCore.Qt.LeftButton:
            self.end_point = event.scenePos()
            self.update_path()
            super(RectangleScene, self).mouseMoveEvent(event)

    def mouseReleaseEvent(self, event):
        if event.button() == QtCore.Qt.LeftButton:
            self.end_point = event.scenePos()
            self.update_path()
            super(RectangleScene, self).mouseReleaseEvent(event)

    def update_path(self):
        if not self.start_point.isNull() and not self.end_point.isNull():
            # 归一化矩形，确保宽高为正
            rect = QtCore.QRectF(self.start_point, self.end_point).normalized()
            self.graphics_line.setRect(rect)
            self.graphics_line.setPen(QtGui.QPen(
                QtGui.QColor(242, 219, 7),
                3,
                QtCore.Qt.SolidLine,
                QtCore.Qt.FlatCap,
                QtCore.Qt.RoundJoin,
            ))

内容的提问来源于stack exchange，提问作者md.119