Drag n Drop inside QgraphicsView doesn't work (PyQt)

Question

I created a custom class for some buttons. Those are a \"draggable\" buttons, which its name indicates, are buttons that you can drag and drop into each other (depending if is a

Answer 1

The solution appears to require that you subclass QGraphicsScene to explicitly pass the drop events to the QGraphicsItem at the drop coordinates. Further more, QGraphicsProxyWidget does not appear to pass drop events to the child widget. So again, you need to subclass QGraphicsProxyWidget and manually instantiate this class, add the widget and hen manually add the instance to the scene using scene.addItem().

Note: You are probably aware, but you the drag/drop isn't started unless you have first interacted with the widget (e.g. clicked on it). Presumably this could be fixed by also passing through the mouseMoveEvent from the scene to the proxy and then to the widget.

Note 2: I don't know why it takes so much effort to make this work. I do feel like I may be missing something. The documentation says:

QGraphicsProxyWidget supports all core features of QWidget, including tab focus, keyboard input, Drag & Drop, and popups

but I couldn't make it work without subclassing.

Relevant subclass implementations:

class MyScene(QGraphicsScene):
    def dragEnterEvent(self, e):
        e.acceptProposedAction()

    def dropEvent(self, e):
        # find item at these coordinates
        item = self.itemAt(e.scenePos())
        if item.setAcceptDrops == True: 
            # pass on event to item at the coordinates
            try:
               item.dropEvent(e)
            except RuntimeError: 
                pass #This will supress a Runtime Error generated when dropping into a widget with no MyProxy        

    def dragMoveEvent(self, e):
        e.acceptProposedAction()

class MyProxy(QGraphicsProxyWidget):    
    def dragEnterEvent(self, e):
        e.acceptProposedAction()

    def dropEvent(self, e):
        # pass drop event to child widget
        return self.widget().dropEvent(e)        

    def dragMoveEvent(self, e):
        e.acceptProposedAction()

Modified application code:

scene = MyScene()
...
my_proxy = MyProxy()
my_proxy.setWidget(button2)
my_proxy.setAcceptDrops(True)
scene.addItem(my_proxy)
...

Full working (well, it prints out "linked" when the drag drop succeeds...which is all you had written it to do previously) application:

from PyQt4 import QtGui, QtCore

class WiringGraphicsView(QtGui.QGraphicsView):
    #Initializer method
    def __init__(self, parent = None,  scene=None):
        QtGui.QGraphicsView.__init__(self, scene, parent)
    #Set Accept Drops property true
        self.setAcceptDrops(True)

    #This method creates a line between two widgets
    def paintWire(self, start_widget,  end_widget):
        #Size and Position of both widgets
        _start = start_widget.geometry()
        _end = end_widget.geometry()
        #Creates a Brush object with Red color 
        brush = QtGui.QBrush(QtGui.QColor(255, 0, 0) )
        #Creates Pen object with specified brush
        pen = QtGui.QPen(brush, 2)
        #Create a Line object between two widgets
        line = QtGui.QGraphicsLineItem(_start.x() + _start.width() / 2, _start.y() + _start.height() / 2, _end.x() + _end.width() / 2, _end.y() + _end.height() / 2)
        #Set the Pen for the Line
        line.setPen(pen)
        #Add this line item to the scene.
        self.scene().addItem( line )

class DragButton(QtGui.QPushButton):
    def __init__(self, parent):
         super(DragButton,  self).__init__(parent)
         self.allowDrag = True

    def setAllowDrag(self, allowDrag):
        if type(allowDrag) == bool:
           self.allowDrag = allowDrag
        else:
            raise TypeError("You have to set a boolean type")

    def mouseMoveEvent(self, e):
        if e.buttons() != QtCore.Qt.RightButton:
            return QtGui.QPushButton.mouseMoveEvent(self, e)

        if self.allowDrag == True:
            # write the relative cursor position to mime data
            mimeData = QtCore.QMimeData()
            # simple string with 'x,y'
            mimeData.setText('%d,%d' % (e.x(), e.y()))
            # print mimeData.text()

            # let's make it fancy. we'll show a "ghost" of the button as we drag
            # grab the button to a pixmap
            pixmap = QtGui.QPixmap.grabWidget(self)

            # below makes the pixmap half transparent
            painter = QtGui.QPainter(pixmap)
            painter.setCompositionMode(painter.CompositionMode_DestinationIn)
            painter.fillRect(pixmap.rect(), QtGui.QColor(0, 0, 0, 127))
            painter.end()

            # make a QDrag
            drag = QtGui.QDrag(self)
            # put our MimeData
            drag.setMimeData(mimeData)
            # set its Pixmap
            drag.setPixmap(pixmap)
            # shift the Pixmap so that it coincides with the cursor position
            drag.setHotSpot(e.pos())

            # start the drag operation
            # exec_ will return the accepted action from dropEvent
            if drag.exec_(QtCore.Qt.LinkAction | QtCore.Qt.MoveAction) == QtCore.Qt.LinkAction:
                print 'linked'
            else:
                print 'moved'

        return QtGui.QPushButton.mouseMoveEvent(self, e)

    def mousePressEvent(self, e):

        if e.button() == QtCore.Qt.LeftButton:
            print 'press'
            #AQUI DEBO IMPLEMENTAR EL MENU CONTEXTUAL
        return QtGui.QPushButton.mousePressEvent(self, e)

    def dragEnterEvent(self, e):
        e.accept()
        return QtGui.QPushButton.dragEnterEvent(self, e)

    def dropEvent(self, e):
        # get the relative position from the mime data
        mime = e.mimeData().text()
        x, y = map(int, mime.split(','))
            # move
            # so move the dragged button (i.e. event.source())
        print e.pos()
        # e.source().move(e.pos()-QtCore.QPoint(x, y))
            # set the drop action as LinkAction
        e.setDropAction(QtCore.Qt.LinkAction)
        # tell the QDrag we accepted it
        e.accept()

        return QtGui.QPushButton.dropEvent(self, QDropEvent(QPoint(e.pos().x(), e.pos().y()), e.possibleActions(), e.mimeData(), e.buttons(), e.modifiers()))



from PyQt4.QtGui import *
from PyQt4.QtCore import *

class MyScene(QGraphicsScene):
    def dragEnterEvent(self, e):
        e.acceptProposedAction()

    def dropEvent(self, e):
    # find item at these coordinates
    item = self.itemAt(e.scenePos())
    if item.setAcceptDrops == True:
        # pass on event to item at the coordinates
        try:
           item.dropEvent(e)
        except RuntimeError: 
            pass #This will supress a Runtime Error generated when dropping into a widget with no ProxyWidget      

    def dragMoveEvent(self, e):
        e.acceptProposedAction()

class MyProxy(QGraphicsProxyWidget):    
    def dragEnterEvent(self, e):
        e.acceptProposedAction()

    def dropEvent(self, e):
        # pass drop event to child widget
        return self.widget().dropEvent(e)        

    def dragMoveEvent(self, e):
        e.acceptProposedAction()


app = QApplication([])

scene = MyScene()

menu = QMenu()

# put a button into the scene and move it
button1 = DragButton('Button 1')
button1.setText("aaa")
button1.setDefault(False)
button1.setAutoDefault(True)
#button1.setMouseTracking(True)
button1.setAllowDrag(True) #Allow Drag n Drop of DragButton
button1.setGeometry(QRect(50, 50, 51, 31)) #Set dimensions of it
#Set icon of button1
icon = QIcon()
icon.addPixmap(QPixmap(":/audio-input-line.png"), QIcon.Normal, QIcon.Off)
button1.setIcon(icon)
button1.setFlat(True)
button1.setMenu(menu)
#Create a QGraphicsProxyWidget adding the widget to scene
scene_button1 = scene.addWidget(button1)
#move the button on the scene
r1 = scene_button1.geometry()
r1.moveTo(-100, -50)

# put another button into the scene
button2 = DragButton('Button 2')
button2.setText("bbb")
#This button shoudn't be dragged, it is just for dropping.
button2.setAllowDrag(False)
button2.setAcceptDrops(True)
icon = QIcon()
icon.addPixmap(QPixmap(":/input_small.png"), QIcon.Normal, QIcon.Off)
button2.setIcon(icon)
#button2.setMouseTracking(True)
#button2.setGeometry(QRect(270, 150, 41, 31))

# Instantiate our own proxy which forwars drag/drop events to the child widget
my_proxy = MyProxy()
my_proxy.setWidget(button2)
my_proxy.setAcceptDrops(True)
scene.addItem(my_proxy)

# Create the view using the scene
view = WiringGraphicsView(None, scene)
view.resize(300, 200)
view.show()
#and paint a wire between those buttons
view.paintWire(button1, button2)
app.exec_()