Python library for the Kivy framework that enables drag-n-drop of widgets.
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README.md

KivyDnD

NOTE: Version 0.5 will bring a philosophical and technical change to the deepcopy of widgets during a copy-and-drag operation. See the below, and the Release Notes, for more information. The code now stored on Github includes these changes.

KivyDnD is a Python library for the Kivy framework that enables drag-n-drop of widgets within your Kivy app (within an app only, not with any other applications running on your device). Source code is here: https://github.com/GreyGnome/KivyDnD .

This work is an update of Pavel Kostelnik's master thesis code found on: http://kostasprogramming.blogspot.com/2012/10/kivy-framework-drag-n-drop-widget.html He did a great job and created a solid foundation. Pavel agreed to release the library under the Apache 2.0 license.

Additional coding by Edvardas Dlugauskas: Significant restructuring of the project, added setup.py functionality, general code cleanup. It's nice to have another pair of eyes on the project. Thanks, Edvardas!

Class Descriptions

DragNDropWidget

A widget subclassed from this class will be able to be picked up and moved around in the main window, then dropped onto other chosen widgets. Then, defined functions can be run in response.

DropDestination

Using DropDestination is optional. It adds two features to the library: drop groups, and the ability to fire events when the pointer enters and leaves the boundaries of DropDestination Widgets.

Drop groups allow control over which widget(s) can receive a drop from particular DragNDropWidget(s). Though the DragNDropWidget can define who its recipients will be (see droppable_zone_objects in the example below), a draggable widget in a drop group can only be dropped into widgets in its same drop group. You can have multiple drop groups, any DragNDropWidget can be a member of multiple drop groups, and any DropDestination can be a member of multiple drop groups.

Usage

Installation

Go to https://github.com/GreyGnome/KivyDnD/tree/master/dist. Grab the latest .tar.gz file (e.g., "kivydnd-0.5.0.tar.gz"). Then run:

sudo pip setup.py install kivydnd-0.5.0.tar.gz

You should find the examples in /usr/share

If installing on Windows, you don't need to install as Administrator. If you simply pip setup.py install kivydnd-0.5.0.tar.gz then the files will get installed on your C: drive by default, under (for example) *C:\Users\loginname\AppData\Local\Programs\Python\Python36* You'll find the examples under Share\kivydnd-examples, and the library itself under Lib\site-packages\kivydnd

Importing and Using

Import the dragndropwidget.py file.

Create a widget and subclass DragNDropWidget. Example: place in your python file:

from kivydnd.dragndropwidget import DragNDropWidget

class DraggableButton(Button, DragNDropWidget):
    def __init__(self, **kw):
        super(DraggableButton, self).__init__(**kw)

Alternatively, you could simply import the dragndropwidget file, in which case you'd need to be specific about your subclassing. That is:

# Here's the directory and module
import kivydnd.dragndropwidget

class DraggableButton(Button, kivydnd.dragndropwidget.DragNDropWidget):
    def __init__(self, **kw):
        super(DraggableButton, self).__init__(**kw)

Next, in your .kv file (or Kivy language code section), include a declaration of your DragNDropWidget which defines the drop_func and (if you want) failed_drop_func. Objects that are able to be dropped onto must have an id defined. The id will match one of the id's listed in droppable_zone_objects, which will determine which widgets you can drop this DragNDropWidget onto. Here we call the id id_of_a_widget, but it can be called anything you want, like flatulent_fuzzbombs or laughing_llamas_are_ludicrous. As long as the DragNDropWidget and widget(s) to be dropped upon have matching id's.

    DraggableButton:
        text: 'Button 1'
        droppable_zone_objects: [id_of_a_widget]
        drop_func: app.greet
        failed_drop_func: app.oops
        size_hint: None, None
        size: 100, 100 

See the example code below. There are also 4 or 5 examples delivered with the library, in increasing complexities. See them for more on how to use the library.

Member Summary

DragNDropWidget

Member Name Type(default value) Description
Properties
droppable_zone_objects ListProperty([]) List of widgets that accept a drop of this widget.
bound_zone_objects ListProperty([]) List of widgets; this widget cannot be dragged outside of the limits given by the outside boundaries of all the widgets in this list. See "dndapp2.py" for an example.
drag_opacity NumericProperty(1.0) Opacity of this widget during a drag.
drop_args ListProperty([]) List of additional arguments given to drop_func. Note that this widget is always given as the first argument to drop_func (after self).
failed_drop_args ListProperty([]) List of additional arguments given to drop_func. Note that this widget is always given as the first argument to failed_drop_func (after self).
remove_on_drag BooleanProperty(True) Whether this widget should be removed upon a drag. Else a copy will be made.
drop_ok_do_animation BooleanProperty(True) Whether a fade animation should be performed at the end of a successful drag-n-drop.
drop_ok_animation_time NumericProperty(0.5) How long the fade should take place.
not_drop_ok_do_animation BooleanProperty(True) Whether a fade animation should take place after an unsuccessful drag. Note that by default no animation will take place if the widget is dropped back onto parent.
not_drop_ok_animation_time NumericProperty(0.2) How long the fade should take place.
motion_over_widget_args ListProperty([]) List of arguments given to motion_over_widget_func (after self).
motion_flee_widget_args ListProperty([]) List of arguments given to motion_over_widget_func (after self).
motion_outside_widget_args ListProperty([]) List of arguments given to motion_outside_widget_func (after self).
drag_start_args ListProperty([]) List of arguments given to drag_start_func (after self).
can_drop_into_parent BooleanProperty(False) Whether a drag-n-drop of the widget back onto its parent counts as a successful drop or not. If a widget's parent is a drop destination for this widget, a drag-n-drop will not be successful there unless this is set.
drop_group StringProperty(None) A StringProperty that you define, this is a name you assign to a group of widgets that can receive a drop from this widget. Can be used instead of, or in addition to, droppable_zone_objects. If used, Widgets in this drop group must subclass DropDestination. They must also be added to the 'drop_group' StringProperty in that object.
rebirth_failed_drop BooleanProperty(True) At the end of a failed drop, if True the widget is rebirthed into its original container.
close_on_fail BooleanProperty(False) At the end of a failed drop, if True the widget is closed- that is, deleted and all its references removed so that the garbage collector may return its memory to the system.
Methods arguments
drop_func self, drop_args The user-defined method or function that will be run at the end of a successful drop.
while_dragging_func self, MouseMotionEvent The user defined method or function that will be run as the widget is dragged.
failed_drop_func self, failed_drop_args The user-defined method or function that will be run at the end of a failed drop.
motion_over_widget_func self, motion_over_widget_args The user-defined method or function that will be run when the pointer enters the boundaries of this widget.
motion_flee_widget_func self, motion_flee_widget_args The user-defined method or function that will be run when the pointer leaves this widget, after previously having entered the widget.
motion_outside_widget_func self, motion_outside_widget_args The user-defined method or function that will be run as long as the pointer is outside this widget. Can be quite chatty.
drag_start_func self, drag_start_args, keyword args: "copy" The user-defined method or function that will be run at the beginning of a drag. If the widget is not removed on drag, then the copied object's drag_start_func is run. Regardless, the first argument to the method is the original "self" object because this function is called from the original widget. Therefore, the copy_of_self widget is given in keyword argument "copy"; pop it off the kwargs dict if you need it.
kivydnd_copy self The user-defined method or function that will be called at the start of a drag, if a widget's remove_on_drag Property had been set to False. For those widgets, this method should return an object which is assumed to be a new widget (perhaps, though not necessarily, of the same class). Copies of the DragnDropWidget's Properties are stored in this object by the library.

DropDestination

Properties Type(default value) Description
motion_over_widget_args ListProperty([]) List of arguments given to motion_over_widget_func (after self).
motion_flee_widget_args ListProperty([]) List of arguments given to motion_flee_widget_func (after self).
motion_outside_widget_args ListProperty([]) List of arguments given to motion_outside_widget_func (after self).
motion_inside_widget_args ListProperty([]) List of arguments given to motion_inside_widget_func (after self).
drop_group StringProperty(None) A StringProperty that you define, this is a name you assign to a group of widgets that can receive a drop from DragNDropWidget's in the same drop_group. Can be used instead of, or in addition to, droppable_zone_objects, which would be defined in a DragNDropWidget object.
Methods arguments
motion_over_widget_func self, self.motion_over_widget_args The user-defined method or function that will be called when your touch point crosses into this DropDestination object.
motion_flee_widget_func self, self.motion_flee_widget_args The user-defined method or function that will be called when your touch point leaves the boundaries of this DropDestination object.
motion_outside_widget_func self, self.motion_outside_widget_args The user-defined method or function that will be called when your touch point moves outside the boundaries of this DropDestination object. Can be quite chatty; be careful about adding this to too many widgets.
motion_inside_widget_func self, motion_inside_widget_args The user-defined method or function that will be called when your touch point moves inside the boundaries of this DropDestination object.

Example

Here's a complete, working example. For more examples check the distribution's examples folder. For more information, see API below.

# File: DnDExample1.py
#       Simplest example of the DragNDropWidget Kivy library.
#
from __future__ import print_function
#
from kivy.app import App
from kivy.lang import Builder
from kivy.uix.button import Button
from kivy.uix.label import Label
from dragndropwidget import DragNDropWidget
#
#
kv = '''
FloatLayout:
    BoxLayout:
        id: from_box
        canvas:
            Color:
                rgb: 1, 0.2, 0.2
            Rectangle:
                pos: self.pos
                size: self.size
        size_hint: 0.8, 0.25
        pos_hint: {'x': 0.1, 'y': 0.4}
        # Here's our DragNDropWidget. See below for the class declaration.
        DraggableButton:
            text: 'Button 1'
            droppable_zone_objects: [upper_to_box]
            drop_func: app.greet
            failed_drop_func: app.oops
            size_hint: None, None
            size: 100, 100 
    Label:
        id: upper_to_box
        text: 'drop here'
        canvas.before:
            Color:
                rgb: 0.4, 0.4, 1
            Rectangle:
                pos: self.pos
                size: self.size
        size_hint: 0.8, 0.2
        pos_hint: {'x': 0.1, 'y': 0.8}
'''
#
#
class  DraggableButton(Button, DragNDropWidget):
    def __init__(self, **kw):
        super(DraggableButton, self).__init__(**kw)
#
#
class DnDExample1(App):
    def __init__(self, **kw):
        super(DnDExample1, self).__init__(**kw)

    def build(self):
        return Builder.load_string(kv)

    def greet(self, calling_widget):
        print ("App says: Nicely Dropped!!")

    def oops(self, the_widget=None, parent=None, kv_root=None):
        print("App says: Ooops! Can't drop there!")


if __name__ == '__main__':
    DnDExample1().run()

API

Classes:

  • DragNDropWidget
    • Your draggable widget must subclass this class.
  • DropDestination
    • This is designed to be optional. This is an object that receives a DragNDropWidget. Use this if you want to:
      • Have the destination perform actions while an item is being dragged or dropped, or
      • Organize widgets into "drop groups", meaning that certain widgets are restricted to being dropped only onto other widgets in a group.

DragNDropWidget

Methods

The following are all ObjectProperties (the methods) or ListProperties (the args). Each Method has a ListProperty that accompanies it, named with an "args" suffix rather than "func", as shown below. For example, failed_drop_func has failed_drop_args. The one exception is while_dragging_func; there is no while_dragging_args.

Note that the first argument given to each function after self is the widget that called it. This is built in to the library. It's necessary because if you create a widget in a KV language block, whenever it's dragged self is the object you created in the KV language.. Normally that's fine, unless you're dragging a copy. Then you want a reference to the copy, not the original.

  • drop_func
    • If defined in your DragNDropWidget subclass, this function is called on a successful drop.
    • If defined in the object being dropped onto, a function by this name will be called when a droppable object is dropped onto it.
    • Argument ListProperty: drop_args. Your function will be called with the following arguments: self, the calling widget, and then the arguments in the ListProperty.
  • while_dragging_func
    • If set, this is called continuously as the widget is being dragged.
  • failed_drop_func
    • If defined in your DragNDropWidget subclass, this function will be called if you drag but fail to drop it onto any widget that has been configured to receive a drop from this widget.
    • Argument ListProperty: failed_drop_args
  • drag_start_func
    • If defined in your DragNDropWidget subclass, this function is called when the widget senses that a drag has begun.

Event Generation

Events

If you assign a method to any of these on_motion_... Properties, on_motion events are bound to the kivy.core.Window (the main Window that encloses your Kivy program). 'on_motion events' are dispatched with each move of the pointer:

  • on_motion_over when the pointer crosses from outside the widget to inside the widget
  • on_motion_flee when the pointer cross out from inside the widget
  • on_motion_outside if the pointer is moved anywhere outside the bounds of the widget. This can be quite chatty, as it's called for all DragNDropWidgets that are bound to 'on_motion' events.

Event Methods Called

  • motion_over_widget_func
    • If defined in your DragNDropWidget subclass, this method will be called whenever the pointer enters into the widget. It is called once, upon entry.
  • motion_flee_widget_func
    • If defined in your DragNDropWidget subclass, this method will be called whenever the pointer leaves the widget. It is called once, upon crossing out of the widget.
  • motion_outside_widget_func
    • If defined in your DragNDropWidget subclass, this method will be called for all pointer motions that are not inside each and every widget that defines this method. Of course, it will not be called on the widget you are currently inside, if you are currently inside one.

Called After a Drop

At the end of a drag and drop, the on_drag_finish() method is called. Its job is to find any possible drag recipients, decide if there was a successful drop and who was dropped onto, and then call the appropriate user-defined functions. It may then call the ending animation. Finally, it performs cleanup. The order of methods called from on_drag_finish() is as follows:

  • If there was at least one successful drop:

    • Call on_successful_drop(). This is called once even if we successfully drop on one or more recipients.
      • If the drop_ok_do_animation Property is True (the default), we want an end-of-drop Animation. Then:

        • Start the Animation.
        • On complete, call post_successful_animation()
      • Call self.drop_func(), if defined.

      • For each successful drop recipient, call found_drop_recipient.drop_func(self), if defined.

      • Once the Animation completes, we enter post_successful_animation():

        • We are un rooted, which simply means removed from the root window.
        • We are set to our old capacity.
        • For each successful drop recipient, call found_drop_recipient.post_drop_func(self), if defined.
        • Call set_drag_finish_state.
      • If the drop_ok_do_animation Property to False, we do not want an animation:

      • Do not start an Animation.

      • Everything else is as if we had an Animation.

      • post_successful_animation() is where you want to put behaviors such as adding a dragged widget to a new parent. You can override this method in a subclass of DragNDropWidget.

  • If there was not any successful drop:

    • Call on_unsuccessful_drop(). This is called once even no matter how many widgets we were unsuccessful in dropping onto.
      • If the not_drop_ok_do_animation Property is True (the default), animate the failed drop. Then:
        • Start the Animation.
        • On complete, call post_unsuccessful_animation()
      • Call self.failed_drop_func(), if defined.
      • If the not_drop_ok_do_animation Property is False, call post_unsuccessful_animation.
      • post_unsuccessful_animation()
        • If self.remove_on_drag is True (the default),
          • If self.rebirth_failed_drop is True (the default),
            • Call self.reborn(), which removes the widget from the root Window and re-adds it to the original parent.
          • else,
            • If self.close_on_fail is True:
              • unroot and destroy the widget
        • else,
          • Calls self.un_root_and_close(), which removes the widget from the root Window and destroys it (because it is a copy of the original DragNDropWidget).
        • Call self.set_drag_finish_state()

############## TODO: test app_relative_layout.py ########################################

Properties:

  • droppable_zone_objects
    • ListProperty: IDs of objects that you can drop widgets of this class onto (called a "drop destination"). You can use this and/or drop_group to specify drop destinations.
  • bound_zone_objects
    • ListProperty: a list of objects that create a boxed-in boundary where the widget cannot be dragged past.
  • drag_opacity
    • NumericProperty a real number between 0.0 and 1.0 that defines the opacity of the widget while it is dragged.
  • remove_on_drag
    • BooleanProperty. If True, the widget is removed from the parent widget (perhaps a layout of some sort) and added to the destination widget. It will be re-added to the parent, in its old position, if the object is not dragged elsewhere. If False, a copy of the widget is made and that copy is added to the destination. If the object is not dragged elsewhere the copy is destroyed.
  • drop_ok_animation_time
    • NumericProperty When it's dropped the object is faded away. This is the duration of that fade. Defaults to 0.7s.
  • not_drop_ok_animation_time
    • NumericProperty If the object is not dragged elsewhere, this is the duration of the fade animation. Defaults to 0.7s.
  • can_drop_into_parent
    • BooleanProperty If the object is allowed to drop back onto its parent, just as if it was dropped onto a non-parent widget. Note that the parent must be designated as a drop destination or member of a drop_group. If not, the widget will not be droppable onto the parent no matter the setting of this property.
  • drop_group
    • StringProperty This is a string of your choosing. DragNDropWidgets of this drop_group will be able to be dropped onto DropDestination widgets that are also members of this drop_group. If you use drop_group then droppable_zone_objects are not recommended.

Detailed Usage:

TODO: Create examples.

In addition to the simple usage given above, the module has additional capabilities. In the simplest case you would set droppable_zone_objects and those are the objects you can drop into. However, you can assign DragNDropWidget's to a drop_group, which is a Kivy StringProperty. Accordingly you must subclass a drop destination to the DropDestination class, and then assign it to the same drop_group. Your dragged widget, then, will only be droppable onto widgets in that drop_group. You can make as many drop groups as you can manage.

There are a number of Kivy Properties available.

Additional Capabilities and Notes:

At the end of a drop, the widget is left with no parent. It is up to you to decide what to do with the widget. So if you want to add the dragged widget onto the place on which it was dropped, you should do so in drop_func() of the recipient.


Known Issues

You can have a number of widgets stacked on top of each other as drop recipients, but it's undefined which widget will get chosen first.

Support

This software is delivered without a warranty, and not even a guarantee that it will work as advertised. If you encounter a bug, please

  • Send me a fix. This is best. If you can't,
  • Create a small- wery small- app that demonstrates the anomalous behavior you observe.

I will get to it when I can, but realize that I've got a day job and this class is being used for an app that I'm building which takes the lion's share of my time.

Bugs

Besides the ones I don't know about:

  • There is a Kivy bug that when removing the leftmost of the children of a widget, if you try to re-insert the child back into its former place at the beginning of the child list, it is not actually displayed on the screen. So rather than re-adding a dragged child whose drag was not completed, I am removing all widgets and re-adding them in order, as a workaround.
    • The bug holds true for any Layout widgets.

License

Distributed under the Apache 2.0 License. See the LICENSE file for more information.