如何覆盖pyqt4 QGraphicsView.mousePressEvent？

Question

我正在尝试改编pyqt4中的照片浏览器（我从本网站的其他问题获得了代码）。默认情况下，鼠标滚轮放大，鼠标左键用于平移/拖动（不知道该怎么称呼它）图像。我想添加左键单击图像时获取像素信息的功能，因此我打算让按钮从“平移”模式更改为“像素信息”模式。我试图做到这一点在下面，但它不符合我的意图。

当我用第一个GUI按钮加载图像并放大图像时，如果我没有平移/拖动图像，然后单击第二个GUI按钮将禁用平移/拖动，并且当我单击图像，为点击的像素提供像素信息。这是我的预期。

但是，如果我放大，然后先平移/拖动图像，当我点击第二个GUI按钮时，它不再禁用平移/拖动并且不打印像素信息。

出于某种原因，我不明白（我希望有人可以启发我）在缩放后拖动图像似乎锁定在平移/拖动方法，不允许我改变它。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_()) 

我也想知道，一旦我已经成功地改变了鼠标点击的方法，什么语法是返回鼠标左键单击返回到默认的平移/阻力行为。

我是一个蟒蛇新手，所以尽量不要批评我太多请。谢谢。

Answer 1

问题是，通过完全覆盖mousePressEvent，您将失去所有的默认行为（例如，用鼠标平移）。最好使用自定义信号来通知照片被点击的时间。而且，由于图形视图已经改变了牵引式的API，你可以添加一个插槽用于切换模式：然后

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，可以用来给你的功能，你想：

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()))