How can I override a pyqt4 QGraphicsView.mousePressEvent?

Question

I am trying to adapt a photo viewer in pyqt4 (I got the code from another question on this site). By default, the mouse wheel zooms, and the left mouse button is used to pan/drag (don't know exactly what to call this) the image. I want to add the capability to get pixel information when left-clicking on the image, so I intended to have a button to change from 'pan' mode to 'pixel info' mode. My attempt to do this is below but it is not behaving as I intended.

When I load an image with the first GUI button and zoom in on the image, if I don't pan/drag the image, then clicking the second GUI button disables the pan/drag and, when I click on the image, provides pixel info for pixel where I clicked. This is what I expected.

However if I zoom in and then pan/drag the image first, when I click on the second GUI button, it no longer disables the pan/drag and doesn't print pixel info.

For some reason I don't understand (and which I hope somebody can enlighten me with) dragging the image after zooming seems to lock in the pan/drag method and not allow me to change it. Can anyone tell me why.

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_())

I would also like to know, once I have successfully changed the mouse click method, what the syntax is for returning the mouse left-click back to the default pan/drag behaviour.

I am a python novice, so try not to criticise me too much please. Thanks.

Answer 1

The problem is that by completely overriding mousePressEvent you are losing all of the default behaviour (e.g. panning with mouse). It would be better to use a custom signal to notify when the photo has been clicked. And since the graphics-view already has an API for changing the drag-mode, you could add a slot for switching modes:

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)

These two new APIs can then be used to give you the functionality you want:

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()))