如何让QGraphicsView的ScrollHandDrag支持鼠标中键？

你的问题出在递归调用mousePressEvent，导致左键事件被无限拦截，完全无法正常使用。下面提供两种基于Qt现代API的可行实现方式，无需老旧hack：

方法一：修改事件参数后交给父类处理 #

这种方式直接复用QGraphicsView自带的ScrollHandDrag逻辑，仅在事件传递前把中键事件伪装成左键事件，保留原事件的其他属性：

from PyQt5.QtWidgets import QGraphicsView, QGraphicsScene, QApplication
from PyQt5.QtGui import QPainter, QMouseEvent
from PyQt5.QtCore import Qt, QEvent
import sys

class InfiniteCanvas(QGraphicsView):
    def __init__(self, parent=None):
        super().__init__(parent)
        self.setAcceptDrops(True)
        self.setScene(QGraphicsScene(self))
        self.setRenderHint(QPainter.Antialiasing)
        self.setRenderHint(QPainter.SmoothPixmapTransform)
        self.setDragMode(QGraphicsView.DragMode.ScrollHandDrag)
        self.setHorizontalScrollBarPolicy(Qt.ScrollBarAlwaysOff)
        self.setVerticalScrollBarPolicy(Qt.ScrollBarAlwaysOff)

        screen = QApplication.primaryScreen()
        screen_size = screen.size()
        width = screen_size.width() * 0.7
        height = screen_size.height() * 0.7
        self.setGeometry(self.x(), self.y(), width, height)
        self.setStyleSheet("background-color: #2a2a2a;")

    def mousePressEvent(self, event):
        if event.button() == Qt.MidButton:
            # 构造新事件，将按钮替换为左键，确保父类识别拖动触发
            modified_event = QMouseEvent(
                QEvent.MouseButtonPress,
                event.pos(),
                Qt.LeftButton,
                Qt.LeftButton,
                event.modifiers()
            )
            super().mousePressEvent(modified_event)
        else:
            super().mousePressEvent(event)

    def mouseReleaseEvent(self, event):
        if event.button() == Qt.MidButton:
            # 模拟左键释放，结束拖动并恢复光标
            modified_event = QMouseEvent(
                QEvent.MouseButtonRelease,
                event.pos(),
                Qt.LeftButton,
                Qt.NoButton,
                event.modifiers()
            )
            super().mouseReleaseEvent(modified_event)
        else:
            super().mouseReleaseEvent(event)

    def dragEnterEvent(self, event):
        if event.mimeData().hasUrls():
            event.acceptProposedAction()

    def dragMoveEvent(self, event):
        if event.mimeData().hasUrls():
            event.acceptProposedAction()

if __name__ == "__main__":
    app = QApplication(sys.argv)
    view = InfiniteCanvas()
    view.show()
    sys.exit(app.exec())

关键说明 #

• 调用父类的super().mousePressEvent()而非自身递归，避免事件循环拦截
• 修改buttons()参数为Qt.LeftButton，确保内置ScrollHandDrag逻辑能触发
• 同步处理mouseReleaseEvent，保证拖动结束后状态正常恢复

方法二：手动实现中键拖动逻辑（不依赖ScrollHandDrag） #

如果不需要依赖内置DragMode，可完全自定义拖动逻辑，灵活性更高：

from PyQt5.QtWidgets import QGraphicsView, QGraphicsScene, QApplication
from PyQt5.QtGui import QPainter
from PyQt5.QtCore import Qt
import sys

class InfiniteCanvas(QGraphicsView):
    def __init__(self, parent=None):
        super().__init__(parent)
        self.setAcceptDrops(True)
        self.setScene(QGraphicsScene(self))
        self.setRenderHint(QPainter.Antialiasing)
        self.setRenderHint(QPainter.SmoothPixmapTransform)
        self.setDragMode(QGraphicsView.DragMode.NoDrag)
        self.setHorizontalScrollBarPolicy(Qt.ScrollBarAlwaysOff)
        self.setVerticalScrollBarPolicy(Qt.ScrollBarAlwaysOff)

        screen = QApplication.primaryScreen()
        screen_size = screen.size()
        width = screen_size.width() * 0.7
        height = screen_size.height() * 0.7
        self.setGeometry(self.x(), self.y(), width, height)
        self.setStyleSheet("background-color: #2a2a2a;")

        self._drag_start_pos = None

    def mousePressEvent(self, event):
        if event.button() == Qt.MidButton:
            self._drag_start_pos = event.pos()
            self.viewport().setCursor(Qt.ClosedHandCursor)
        else:
            super().mousePressEvent(event)

    def mouseMoveEvent(self, event):
        if self._drag_start_pos is not None and event.buttons() & Qt.MidButton:
            # 计算位移并滚动视图
            delta = event.pos() - self._drag_start_pos
            self._drag_start_pos = event.pos()
            self.horizontalScrollBar().setValue(self.horizontalScrollBar().value() - delta.x())
            self.verticalScrollBar().setValue(self.verticalScrollBar().value() - delta.y())
        else:
            super().mouseMoveEvent(event)

    def mouseReleaseEvent(self, event):
        if event.button() == Qt.MidButton:
            self._drag_start_pos = None
            self.viewport().setCursor(Qt.ArrowCursor)
        else:
            super().mouseReleaseEvent(event)

    def dragEnterEvent(self, event):
        if event.mimeData().hasUrls():
            event.acceptProposedAction()

    def dragMoveEvent(self, event):
        if event.mimeData().hasUrls():
            event.acceptProposedAction()

if __name__ == "__main__":
    app = QApplication(sys.argv)
    view = InfiniteCanvas()
    view.show()
    sys.exit(app.exec())

关键说明 #

• 记录中键按下时的初始位置_drag_start_pos
• 鼠标移动时计算位移，通过滚动条实现视图拖动
• 释放中键时重置状态，恢复光标样式

两种方法均不会影响左键的正常使用：方法一复用内置逻辑，代码更简洁；方法二完全自定义，适合需要扩展拖动行为的场景。

内容的提问来源于stack exchange，提问作者user21736604