This week I had an interesting question from a reader of my PyQt6 book, about how to handle dragging and dropping of widgets in a container showing the dragged widget as it is moved.
I'm interested in managing movement of a QWidget with mouse in a container. I've implemented the application with drag & drop, exchanging the position of buttons, but I want to show the motion of
QPushButton, like what you see in Qt Designer. Dragging a widget should show the widget itself, not just the mouse pointer.
First, we'll implement the simple case which drags widgets without showing anything extra. Then we can extend it to answer the question.
Drag & drop widgets
We'll start with this simple application which creates a window using QWidget and places a series of QPushButton widgets into it.
You can substitute QPushButton for any other widget you like, e.g. QLabel
from PyQt5.QtWidgets import QApplication, QHBoxLayout, QWidget, QPushButton
class Window(QWidget):
def __init__(self):
super().__init__()
self.blayout = QHBoxLayout()
for l in ['A', 'B', 'C', 'D']:
btn = QPushButton(l)
self.blayout.addWidget(btn)
self.setLayout(self.blayout)
app = QApplication([])
w = Window()
w.show()
app.exec_()
If you run this you should see something like this.
The series of QPushButton widgets in a horizontal layout.
Here we're creating a window, but the Window widget is subclassed from QWidget, meaning you can add this widget to any other layout. See later for an example of a generic object sorting widget.
QPushButton objects aren't usually draggable, so to handle the mouse movements and initiate a drag we need to implement a subclass. We can add the following to the top of the file.
from PyQt5.QtCore import Qt, QMimeData
from PyQt5.QtGui import QDrag
class DragButton(QPushButton):
def mouseMoveEvent(self, e):
if e.buttons() == Qt.LeftButton:
drag = QDrag(self)
mime = QMimeData()
drag.setMimeData(mime)
drag.exec_(Qt.MoveAction)
We implement a mouseMoveEvent which accepts the single e parameter of the event. We check to see if the left mouse button is pressed on this event -- as it would be when dragging -- and then initiate a drag. To start a drag, we create a QDrag object, passing in self to give us access later to the widget that was dragged. We also must pass in mime data. This is used for including information about what is dragged, particularly for passing data between applications. However, as here, it is fine to leave this empty.
Finally, we initiate a drag by calling drag.exec_(Qt.MoveAction). As with dialogs exec_() starts a new event loop, blocking the main loop until the drag is complete. The parameter Qt.MoveAction tells the drag handler what type of operation is happening, so it can show the appropriate icon tip to the user.
You can update the main window code to use our new DragButton class as follows.
class Window(QWidget):
def __init__(self):
super().__init__()
self.blayout = QHBoxLayout()
for l in ['A', 'B', 'C', 'D']:
btn = DragButton(l)
self.blayout.addWidget(btn)
self.setLayout(self.blayout)
If you run the code now, you can drag the buttons, but you'll notice the drag is forbidden.
Dragging of the widget starts but is forbidden.
What's happening? The mouse movement is being detected by our DragButton object and the drag started, but the main window does not accept drag & drop.
To fix this we need to enable drops on the window and implement dragEnterEvent to actually accept them.
class Window(QWidget):
def __init__(self):
super().__init__()
self.setAcceptDrops(True)
self.blayout = QHBoxLayout()
for l in ['A', 'B', 'C', 'D']:
btn = DragButton(l)
self.blayout.addWidget(btn)
self.setLayout(self.blayout)
def dragEnterEvent(self, e):
e.accept()
If you run this now, you'll see the drag is now accepted and you see the move icon. This indicates that the drag has started and been accepted by the window we're dragging onto. The icon shown is determined by the action we pass when calling drag.exec_().
Dragging of the widget starts and is accepted, showing a move icon.
Releasing the mouse button during a drag drop operation triggers a dropEvent on the widget you're currently hovering the mouse over (if it is configured to accept drops). In our case that's the window. To handle the move we need to implement the code to do this in our dropEvent method.
The drop event contains the position the mouse was at when the button was released & the drop triggered. We can use this to determine where to move the widget to.
def dropEvent(self, e):
pos = e.pos()
widget = e.source()
for n in range(self.blayout.count()):
# Get the widget at each index in turn.
w = self.blayout.itemAt(n).widget()
if pos.x() < w.x():
# We didn't drag past this widget.
# insert to the left of it.
self.blayout.insertWidget(n-1, widget)
break
e.accept()
To determine where to place the widget, we iterate over all the widgets in the layout, until we find one who's x position is greater than that of the mouse pointer. If so then when insert the widget directly to the left of this widget and exit the loop.
The effect of this is that if you drag 1 pixel past the start of another widget, which might be a bit confusing. You adjust this the cut off to use the middle of the widget using if pos.x() < w.x() + w.size().width() // 2: -- that is x + half of the width.
def dropEvent(self, e):
pos = e.pos()
widget = e.source()
for n in range(self.blayout.count()):
# Get the widget at each index in turn.
w = self.blayout.itemAt(n).widget()
if pos.x() < w.x() + w.size().width() // 2:
# We didn't drag past this widget.
# insert to the left of it.
self.blayout.insertWidget(n-1, widget)
break
e.accept()
The complete working drag-drop code is shown below.
from PyQt5.QtWidgets import QApplication, QHBoxLayout, QWidget, QPushButton
from PyQt5.QtCore import Qt, QMimeData
from PyQt5.QtGui import QDrag
class DragButton(QPushButton):
def mouseMoveEvent(self, e):
if e.buttons() == Qt.LeftButton:
drag = QDrag(self)
mime = QMimeData()
drag.setMimeData(mime)
drag.exec_(Qt.MoveAction)
class Window(QWidget):
def __init__(self):
super().__init__()
self.setAcceptDrops(True)
self.blayout = QHBoxLayout()
for l in ['A', 'B', 'C', 'D']:
btn = DragButton(l)
self.blayout.addWidget(btn)
self.setLayout(self.blayout)
def dragEnterEvent(self, e):
e.accept()
def dropEvent(self, e):
pos = e.pos()
widget = e.source()
for n in range(self.blayout.count()):
# Get the widget at each index in turn.
w = self.blayout.itemAt(n).widget()
if pos.x() < w.x() + w.size().width() // 2:
# We didn't drag past this widget.
# insert to the left of it.
self.blayout.insertWidget(n-1, widget)
break
e.accept()
app = QApplication([])
w = Window()
w.show()
app.exec_()
Visual drag & drop
So now we have our working drag & drop implementation we can move on to showing the drag visually. What we want to achieve here is showing the button being dragged next to the mouse point as it is dragged.
Qt's QDrag handler natively provides a mechanism for showing dragged objects which we can use. We can update our DragButton class to pass a pixmap image to QDrag and this will be displayed under the mouse pointer as the drag occurs. To show the widget, we just need to get a QPixmap of the widget we're dragging.
from PyQt5.QtWidgets import QApplication, QHBoxLayout, QWidget, QPushButton
from PyQt5.QtCore import Qt, QMimeData
from PyQt5.QtGui import QDrag, QPixmap
class DragButton(QPushButton):
def mouseMoveEvent(self, e):
if e.buttons() == Qt.LeftButton:
drag = QDrag(self)
mime = QMimeData()
drag.setMimeData(mime)
pixmap = QPixmap(self.size())
self.render(pixmap)
drag.setPixmap(pixmap)
drag.exec_(Qt.MoveAction)
To create the pixmap we create a QPixmap object passing in the size of the widget this event is fired on with self.size(). This creates an empty pixmap which we can then pass into self.render to render -- or draw -- the current widget onto it. That's it. Then we set the resulting pixmap on the drag object.
If you run the code with this modification you'll see something like the following --
Dragging of the widget showing the dragged widget.
Generic drag & drop container
We can take this a step further and implement a generic drag drop widget which allows us to sort arbitrary objects. In the code below we've created a new widget DragWidget which can be added to any window. You can add items -- instances of DragItem -- which you want to be sorted, as well as setting data on them. When items are re-ordered the new order is emitted as a signal orderChanged.
from PyQt5.QtWidgets import QApplication, QHBoxLayout, QWidget, QLabel, QMainWindow, QVBoxLayout
from PyQt5.QtCore import Qt, QMimeData, pyqtSignal
from PyQt5.QtGui import QDrag, QPixmap
class DragItem(QLabel):
def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)
self.setContentsMargins(25, 5, 25, 5)
self.setAlignment(Qt.AlignmentFlag.AlignCenter)
self.setStyleSheet("border: 1px solid black;")
# Store data separately from display label, but use label for default.
self.data = self.text()
def set_data(self, data):
self.data = data
def mouseMoveEvent(self, e):
if e.buttons() == Qt.LeftButton:
drag = QDrag(self)
mime = QMimeData()
drag.setMimeData(mime)
pixmap = QPixmap(self.size())
self.render(pixmap)
drag.setPixmap(pixmap)
drag.exec_(Qt.MoveAction)
class DragWidget(QWidget):
"""
Generic list sorting handler.
"""
orderChanged = pyqtSignal(list)
def __init__(self, *args, orientation=Qt.Orientation.Vertical, **kwargs):
super().__init__()
self.setAcceptDrops(True)
# Store the orientation for drag checks later.
self.orientation = orientation
if self.orientation == Qt.Orientation.Vertical:
self.blayout = QVBoxLayout()
else:
self.blayout = QHBoxLayout()
self.setLayout(self.blayout)
def dragEnterEvent(self, e):
e.accept()
def dropEvent(self, e):
pos = e.pos()
widget = e.source()
for n in range(self.blayout.count()):
# Get the widget at each index in turn.
w = self.blayout.itemAt(n).widget()
if self.orientation == Qt.Orientation.Vertical:
# Drag drop vertically.
drop_here = pos.y() < w.y() + w.size().height() // 2
else:
# Drag drop horizontally.
drop_here = pos.x() < w.x() + w.size().width() // 2
if drop_here:
# We didn't drag past this widget.
# insert to the left of it.
self.blayout.insertWidget(n-1, widget)
self.orderChanged.emit(self.get_item_data())
break
e.accept()
def add_item(self, item):
self.blayout.addWidget(item)
def get_item_data(self):
data = []
for n in range(self.blayout.count()):
# Get the widget at each index in turn.
w = self.blayout.itemAt(n).widget()
data.append(w.data)
return data
class MainWindow(QMainWindow):
def __init__(self):
super().__init__()
self.drag = DragWidget(orientation=Qt.Orientation.Vertical)
for n, l in enumerate(['A', 'B', 'C', 'D']):
item = DragItem(l)
item.set_data(n) # Store the data.
self.drag.add_item(item)
# Print out the changed order.
self.drag.orderChanged.connect(print)
container = QWidget()
layout = QVBoxLayout()
layout.addStretch(1)
layout.addWidget(self.drag)
layout.addStretch(1)
container.setLayout(layout)
self.setCentralWidget(container)
app = QApplication([])
w = MainWindow()
w.show()
app.exec_()
Generic drag-drop sorting in horizontal orientation.
You'll notice that when creating the item, you can set the label by passing it in as a parameter (just like for a normal QLabel which we've subclassed from). But you can also set a data value, which is the internal value of this item -- this is what will be emitted when the order changes, or if you call get_item_data yourself. This separates the visual representation from what is actually being sorted, meaning you can use this to sort anything not just strings.
In the example above we're passing in the enumerated index as the data, so dragging will output (via the print connected to orderChanged) something like:
[1, 0, 2, 3]
[1, 2, 0, 3]
[1, 0, 2, 3]
[1, 2, 0, 3]
If you remove the item.set_data(n) you'll see the labels emitted on changes.
['B', 'A', 'C', 'D']
['B', 'C', 'A', 'D']
We've also implemented orientation onto the DragWidget using the Qt built in flags Qt.Orientation.Vertical or Qt.Orientation.Horizontal. This setting this allows you sort items either vertically or horizontally -- the calculations are handled for both directions.
Generic drag-drop sorting in vertical orientation.
For more, see the complete PyQt6 tutorial.
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