"tkinter.dnd" --- 拖放支援
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**原始碼：**Lib/tkinter/dnd.py

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備註:

  這是一個實驗性的功能，並將在被 Tk DND 取代時棄用。

The "tkinter.dnd" module provides drag-and-drop support for objects
within a single application, within the same window or between
windows. To enable an object to be dragged, you must create an event
binding for it that starts the drag-and-drop process. Typically, you
bind a ButtonPress event to a callback function that you write (see
Bindings and events). The function should call "dnd_start()", where
*source* is the object to be dragged, and *event* is the event that
invoked the call (the argument to your callback function).

目標物件的選擇過程如下：

1. Top-down search of the area under the mouse for a target widget:

   * the target widget should have a callable *dnd_accept* attribute;

   * if *dnd_accept* is not present or returns "None", the search
     moves to the parent widget;

   * if no target widget is found, the target object is "None".

2. Call to "<old_target>.dnd_leave(source, event)".

3. Call to "<new_target>.dnd_enter(source, event)".

4. Call to "<target>.dnd_commit(source, event)" to notify of the drop.

5. Call to "<source>.dnd_end(target, event)" to signal the end of
   drag-and-drop.

class tkinter.dnd.DndHandler(source, event)

   *DndHandler* 類別處理拖放事件，它會追蹤事件元件的根上的 Motion 和
   ButtonRelease 事件。

   cancel(event=None)

      取消拖放過程。

   finish(event, commit=0)

      執行拖放結束函式。

   on_motion(event)

      Inspect area below mouse for target objects while a drag is
      performed.

   on_release(event)

      當釋放模式被觸發時，發出拖動結束的訊號。

tkinter.dnd.dnd_start(source, event)

   Factory function for the drag-and-drop process. Return the
   "DndHandler" instance managing the drag, or "None" if a drag could
   not be started.

也參考: Bindings and events
