如何覆盖 pyqt4 QGraphicsView.mousePressEvent？

Question

我正在尝试在 pyqt4 中调整照片查看器（我从本网站的另一个问题中获得了代码）。默认情况下，鼠标滚轮缩放，鼠标左键用于平移/拖动（不知道具体如何调用）图像。我想添加左键单击图像时获取像素信息的功能，所以我打算有一个按钮来从“平移”模式更改为“像素信息”模式。我的尝试如下所示，但它的行为不符合我的预期。

当我使用第一个 GUI 按钮加载图像并放大图像时，如果我不平移/拖动图像，则单击第二个 GUI 按钮会禁用平移/拖动，当我单击图像时，提供我点击的像素的像素信息。这正是我所期望的。

但是，如果我先放大然后平移/拖动图像，当我单击第二个 GUI 按钮时，它不再禁用平移/拖动并且不打印像素信息。

由于某种原因，我不明白（我希望有人能启发我）在缩放后拖动图像似乎锁定了平移/拖动方法并且不允许我更改它。谁能告诉我为什么。

from PyQt4 import QtCore, QtGui

class PhotoViewer(QtGui.QGraphicsView):
    def __init__(self, parent):
        super(PhotoViewer, self).__init__(parent)
        self._zoom = 0
        self._scene = QtGui.QGraphicsScene(self)
        self._photo = QtGui.QGraphicsPixmapItem()
        self._scene.addItem(self._photo)
        self.setScene(self._scene)
        self.setTransformationAnchor(QtGui.QGraphicsView.AnchorUnderMouse)
        self.setResizeAnchor(QtGui.QGraphicsView.AnchorUnderMouse)
        self.setVerticalScrollBarPolicy(QtCore.Qt.ScrollBarAlwaysOff)
        self.setHorizontalScrollBarPolicy(QtCore.Qt.ScrollBarAlwaysOff)
        self.setBackgroundBrush(QtGui.QBrush(QtGui.QColor(30, 30, 30)))
        self.setFrameShape(QtGui.QFrame.NoFrame)

    def fitInView(self):
        rect = QtCore.QRectF(self._photo.pixmap().rect())
        if not rect.isNull():
            unity = self.transform().mapRect(QtCore.QRectF(0, 0, 1, 1))
            self.scale(1 / unity.width(), 1 / unity.height())
            viewrect = self.viewport().rect()
            scenerect = self.transform().mapRect(rect)
            factor = min(viewrect.width() / scenerect.width(),
                         viewrect.height() / scenerect.height())
            self.scale(factor, factor)
            self.centerOn(rect.center())
            self._zoom = 0

    def setPhoto(self, pixmap=None):
        self._zoom = 0
        if pixmap and not pixmap.isNull():
            self.setDragMode(QtGui.QGraphicsView.ScrollHandDrag)
            self._photo.setPixmap(pixmap)
            self.fitInView()
        else:
            self.setDragMode(QtGui.QGraphicsView.NoDrag)
            self._photo.setPixmap(QtGui.QPixmap())

    def wheelEvent(self, event):
        if not self._photo.pixmap().isNull():
            if event.delta() > 0:
                factor = 1.25
                self._zoom += 1
            else:
                factor = 0.8
                self._zoom -= 1
            if self._zoom > 0:
                self.scale(factor, factor)
            elif self._zoom == 0:
                self.fitInView()
            else:
                self._zoom = 0

    def printPixInfo(self,event):
        print event.pos()

class Window(QtGui.QWidget):
    def __init__(self):
        super(Window, self).__init__()
        self.viewer = PhotoViewer(self)
        # 'Load image' button
        self.btnLoad = QtGui.QToolButton(self)
        self.btnLoad.setText('Load image')
        self.btnLoad.clicked.connect(self.loadImage)
        # Button to change from drag/pan to getting pixel info
        self.btnPixInfo = QtGui.QToolButton(self)
        self.btnPixInfo.setText('Enter pixel info mode')
        self.btnPixInfo.clicked.connect(self.pixInfo)
        # Arrange layout
        VBlayout = QtGui.QVBoxLayout(self)
        VBlayout.addWidget(self.viewer)
        HBlayout = QtGui.QHBoxLayout()
        HBlayout.setAlignment(QtCore.Qt.AlignLeft)
        HBlayout.addWidget(self.btnLoad)
        HBlayout.addWidget(self.btnPixInfo)
        VBlayout.addLayout(HBlayout)

    def loadImage(self):
        self.viewer.setPhoto(QtGui.QPixmap('pic.jpg'))

    def pixInfo(self):
        print 'Now in pixel info mode'
        self.viewer._photo.mousePressEvent = self.viewer.printPixInfo

if __name__ == '__main__':
    import sys
    app = QtGui.QApplication(sys.argv)
    window = Window()
    window.setGeometry(500, 300, 800, 600)
    window.show()
    sys.exit(app.exec_())

我还想知道，一旦我成功更改了鼠标单击方法，将鼠标左键单击返回到默认平移/拖动行为的语法是什么。

我是python新手，所以请尽量不要批评我。谢谢。

Answer 1

问题在于，通过完全覆盖 mousePressEvent，您将失去所有默认行为（例如，使用鼠标平移）。最好使用自定义信号来通知照片何时被点击。由于图形视图已经有一个用于更改拖动模式的 API，您可以添加一个用于切换模式的插槽：

class PhotoViewer(QtGui.QGraphicsView):
    photoClicked = QtCore.pyqtSignal(QtCore.QPoint)    
    ...

    def toggleDragMode(self):
        if self.dragMode() == QtGui.QGraphicsView.ScrollHandDrag:
            self.setDragMode(QtGui.QGraphicsView.NoDrag)
        elif not self._photo.pixmap().isNull():
            self.setDragMode(QtGui.QGraphicsView.ScrollHandDrag)

    def mousePressEvent(self, event):
        if self._photo.isUnderMouse():
            self.photoClicked.emit(QtCore.QPoint(event.pos()))  
        # keep the default behaviour
        super(PhotoViewer, self).mousePressEvent(event)

这两个新的 API 可用于为您提供所需的功能：

class Window(QtGui.QWidget):
    def __init__(self):
        ...
        # add a line-edit to display the position info
        self.editPixInfo = QtGui.QLineEdit(self)
        self.editPixInfo.setReadOnly(True)
        self.viewer.photoClicked.connect(self.photoClicked)
        ...
        HBlayout.addWidget(self.editPixInfo)

    def pixInfo(self):
        # switch modes
        self.viewer.toggleDragMode()

    def photoClicked(self, pos):
        # handle photo-click signals
        if self.viewer.dragMode() == QtGui.QGraphicsView.NoDrag:
            self.editPixInfo.setText('%d, %d' % (pos.x(), pos.y()))