Drag and drop overview

It may be noted that data transfer to and from the clipboard is quite similar to data transfer with drag and drop and the code to implement these two types is almost the same.

In particular, both data transfer mechanisms store data in some kind of wx.DataObject and identify its format(s) using the wx.DataFormat class.

To be a drag source, i.e. to provide the data which may be dragged by the user elsewhere, you should implement the following steps:

  1. Preparation: First of all, a data object must be created and initialized with the data you wish to drag. For example:
     wx.TextDataObject my_data=new wx.TextDataObject("This text will be dragged.");
    Drag start: To start the dragging process (typically in response to a mouse click) you must call wx.DropSource.DoDragDrop like this:
     wx.DropSource dragSource=new wx.DropSource( this );
     dragSource.SetData( my_data );
     wx.DragResult result = dragSource.DoDragDrop( true );
  2. Dragging: The call to wx.DropSource.DoDragDrop() blocks the program until the user releases the mouse button (unless you override the GiveFeedback function to do something special). When the mouse moves in a window of a program which understands the same drag-and-drop protocol (any program under Windows or any program supporting the XDnD protocol under X Windows), the corresponding wx.DropTarget methods are called - see below.
  3. Processing the result: wx.DropSource.DoDragDrop() returns an effect code which is one of the values of wx.DragResult enum (explained here):
     switch (result)
        case wx.DragResult.DragCopy: copy the data; break;
        case wx.DragResult.DragMove: move the data; break;
        default:                     do nothing; break;
To be a drop target, i.e. to receive the data dropped by the user you should follow the instructions below:
  1. Initialization: For a window to be a drop target, it needs to have an associated wx.DropTarget object. Normally, you will call wxWindow.SetDropTarget during window creation associating your drop target with it. You must derive a class from wxDropTarget and override its pure virtual methods. Alternatively, you may derive from wx.TextDropTarget or wx.FileDropTarget and override their OnDropText() or OnDropFiles() method.
  2. Drop: When the user releases the mouse over a window, wxWidgets asks the associated wx.DropTarget object if it accepts the data. For this, a wx.DataObject must be associated with the drop target and this data object will be responsible for the format negotiation between the drag source and the drop target. If all goes well, then wx.DropTarget.OnData will get called and the wx.DataObject belonging to the drop target can get filled with data. The end: After processing the data, DoDragDrop() returns either wx.DragResult.DragCopy or wx.DragResult.DragMove depending on the state of the keys [Ctrl], [Shift] and [Alt] at the moment of the drop. There is currently no way for the drop target to change this return code.

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