Dragging/Moving a QPushButton in PyQt

Question

I am really struggling to figure out a way to do this. Say I implement a button very simply in a widget window:

self.button = QPushButton("Drag Me", self)

I can move its initialization point around the parent widget's area using self.button.move(x,y), and I can get mouse events from mousePressEvent(self, e) via e.x() and e.y(), so that the button moves to wherever I click, but I just cannot seem to put all this together into a drag and drop framework.

Clarification: After reading on the 'true' meaning of Drag/Drop, that's not what I need. I just want to be able to move a widget around with my mouse, much similar to the way you move magnets on a fridge.

Answer 1

Here is an example of a moveable button that still supports the normal click signal properly:

from PyQt4 import QtCore, QtGui


class DragButton(QtGui.QPushButton):

    def mousePressEvent(self, event):
        self.__mousePressPos = None
        self.__mouseMovePos = None
        if event.button() == QtCore.Qt.LeftButton:
            self.__mousePressPos = event.globalPos()
            self.__mouseMovePos = event.globalPos()

        super(DragButton, self).mousePressEvent(event)

    def mouseMoveEvent(self, event):
        if event.buttons() == QtCore.Qt.LeftButton:
            # adjust offset from clicked point to origin of widget
            currPos = self.mapToGlobal(self.pos())
            globalPos = event.globalPos()
            diff = globalPos - self.__mouseMovePos
            newPos = self.mapFromGlobal(currPos + diff)
            self.move(newPos)

            self.__mouseMovePos = globalPos

        super(DragButton, self).mouseMoveEvent(event)

    def mouseReleaseEvent(self, event):
        if self.__mousePressPos is not None:
            moved = event.globalPos() - self.__mousePressPos 
            if moved.manhattanLength() > 3:
                event.ignore()
                return

        super(DragButton, self).mouseReleaseEvent(event)

def clicked():
    print "click as normal!"

if __name__ == "__main__":
    app = QtGui.QApplication([])
    w = QtGui.QWidget()
    w.resize(800,600)

    button = DragButton("Drag", w)
    button.clicked.connect(clicked)

    w.show()
    app.exec_()

In the mousePressEvent I record both the initial start position, and a position that will get updated throughout the drag.

In the mouseMoveEvent, I get the proper offset of the widget from where it was clicked to where the actual origin is, so that the move is accurate.

In the mouseReleaseEvent, I check to see if the overall move was greater than at least a tiny amount. If it was, then it was a drag and we ignore the normal event to not produce a "clicked" signal. Otherwise, we allow the normal event handler to produce the click.

Answer 2

It depends on what you are trying to do. If you are trying to do actual "Drag & Drop", you're going about it wrong. What you are doing is just moving the button around in its X,Y coordinate space within its parent. Its never actually invoking any Drag/Drop events, those are entirely different.

You should read through the drag & drop documentation here:

http://doc.qt.nokia.com/4.7-snapshot/dnd.html
http://qt-project.org/doc/qt-5.0/qdrag.html

Instead of moving the button within the mousePressEvent, you'll need to create a new QDrag object and execute it. You can make it look like your button by taking a snapshot of your button using the QPixmap::grabWidget method and assign it to the QDrag instance using the QDrag::setPixmap method.

Event if all you are trying to do is move the widget around in the parent space, I would recommend using this framework and just accepting the drop event for your the button. Then you don't trigger a bunch of unnecessary redraws.