draggin.py

'''
An elementary breadboard for experimenting with Qt's Drag'n'Drop classes.
'''
from PyQt5.QtCore import (
    Qt,
    QMimeData,
    QPoint,
    QTime
)
from PyQt5.QtGui import QDrag
from PyQt5.QtWidgets import (
    QApplication,
    QHBoxLayout,
    QLabel,
    QMainWindow,
    QWidget
)

class SorcWidj(QLabel):
    '''A simple drag-source with ability
    to recognize the start of a drag motion
    and implement the drag.'''
    def __init__(self,text):
        super().__init__()
        self.setText(text)
        self.mouse_down = False # has a left-click happened yet?
        self.mouse_posn = QPoint() # if so, this was where...
        self.mouse_time = QTime() # ...and this was when.

    def mousePressEvent(self,event):
        if event.button() == Qt.LeftButton :
            self.mouse_down = True # we are left-clicked-upon
            self.mouse_posn = event.pos() # here and...
            self.mouse_time.start() # ...now
        event.ignore()
        super().mousePressEvent(event) # pass it on up

    def mouseReleaseEvent(self,event):
        # Mouse released in our rectangle, clear any drag info.
        self.mouse_down = False
        event.ignore()
        super().mouseReleaseEvent(event) # pass it on up

    def dragTargetChange(self, qob):
        # print signal from QDrag::targetChanged
        t = type(qob)
        print('target moved to',t)

    def doSomeDraggin(self):
        # Make a scaled pixmap of our widget to put under the cursor.
        thumb = self.grab().scaledToHeight(50)
        # Create the QDrag object and set its cursor over the pixmap
        dragster = QDrag(self)
        dragster.setPixmap(thumb)
        dragster.setHotSpot(QPoint(thumb.width()/2,thumb.height()/2))
        # Create some data to be dragged and load it in the dragster.
        md = QMimeData()
        md.setText(self.text())
        dragster.setMimeData(md)
        # Experiment: can we catch the target-change signal?
        dragster.targetChanged.connect(self.dragTargetChange)
        # Initiate the drag, which really is a form of modal dialog,
        # experimentally allowing any of the possible actions.
        # Result is supposed to be the action performed at the drop.
        act = dragster.exec_(Qt.MoveAction) # Qt.LinkAction Qt.CopyAction
        # Display the results of the drag.
        targ = dragster.target() # s.b. the widget that received the drop
        src = dragster.source() # s.b. this very widget
        print('action ',int(act),'target ',type(targ), 'source ',type(src))
        return

    def mouseMoveEvent(self,event):
        if self.mouse_down :
            # Mouse left-clicked and is now moving. Is this the start of a
            # drag? Note time since the click and approximate distance moved
            # since the click and test against the app's standard.
            t = self.mouse_time.elapsed()
            m = (event.pos() - self.mouse_posn).manhattanLength()
            if t >= QApplication.startDragTime() \
            or m >= QApplication.startDragDistance() :
                # Yes, a proper drag is indicated. Commence dragging.
                self.doSomeDraggin()
                event.accept()
                return
        # Move does not (yet) constitute a drag, ignore it.
        event.ignore()
        super().mouseMoveEvent(event)

class TargWidj(QLabel):
    '''A simple class that can detect an incoming drag
    and accept it, but only if it's a Copy of plain text.'''
    def __init__(self,text):
        super().__init__()
        self.setAcceptDrops(True)
        self.setText(text)
        self.setStyleSheet('')
    def dragEnterEvent(self, event):
        print('drag enter')
        dbg = event.proposedAction()
        if (event.proposedAction() & Qt.CopyAction) \
        and (event.mimeData().hasFormat("text/plain")) :
            self.setStyleSheet('background: green;')
            event.acceptProposedAction()
        else:
            self.setStyleSheet('background: red;')
    def dragMoveEvent(self, event):
        print('drag moving')
    def dragLeaveEvent(self, event):
        print('drag leaving')
        self.setStyleSheet('')
    def dropEvent(self, event):
        print('dropping')
        self.setText( event.mimeData().text() )
        self.setStyleSheet('')

def main():
    import sys
    app = QApplication(sys.argv)
    main = QMainWindow()
    central = QWidget()
    hbo = QHBoxLayout()
    hbo.addWidget( SorcWidj('Source Widget') )
    hbo.addWidget( TargWidj('Target') )
    central.setLayout(hbo)
    main.setCentralWidget( central )
    main.show()
    app.exec_()

if __name__ == '__main__' :
    main()