                        Drag-and-Drop Support in wxWidgets
                        ==================================

1. Overview
   --------

 a) What is it?

    We're calling drag-and-drop (or d&d for short) the OLE mechanism of data
transfer.

    OLE d&d allows application to transfer data of any type to the same or
another process.


 b) How is it done? (user's point of view)

    To start a d&d operation the user presses the mouse button 1 (left) and
drags the selected object to another window (which must be at least partially
visible on the screen) or to an icon on the taskbar in which case the 
corresponding window will be automatically restored. To finish the operation,
the user releases the button. Default d&d operation is "move", but several key
act as modifiers: keeping down the <Ctrl> key at the moment of drop does 
"copy", while <Shift> or <Alt> force the "move" (makes sense if default isn't
"move"). 


 c) How is it done? (programmer's point of view)

    There are several objects participating in a d&d operation. First of all,
there is the data object itself. Second, there is the drop source which is
responsible for creating the data object (if it doesn't exist yet) and starting
the d&d operation. Finally, the drop target receives the notification when
the data is dropped onto the associated window (see below) and is responsible
for pasting the data and returning the result code (copy, move or failure).
There is one class for each one of these roles in wxWindows d&d implementation,
please see their descriptions below for details.



2. Drop Target
   -----------

 a) Being a drop target

    ... is as easy as deriving your window class from wxDropTarget and 
associating it with a wxWindow object (or perhaps some wxWindow-derived class, 
such as wxFrame). The pure virtual function wxDropTarget::OnDrop() must be 
implemented in your application and will be called whenever the mouse button 
is released over the window in question. Other virtual functions that will be 
called in the process of the d&d operation are OnEnter and OnLeave.

@@ should OnDragOver() be user overridable also?

  You should associate wxDropTarget and wxWindow calling SetDropTarget:
    wxWindow *pWindow = GetTopWindow();
    pWindow->SetDropTarget(new MyDropTarget);

The object created passed to SetDropTarget becomes the propriety of wxWindow
and will be deleted with the window (or when you call SetDropTarget next
time). You can always break the association by calling SetDropTarget(nullptr).

  When some data is dragged over a window, the program must decide if it's
going to accept this data or not. The virtual function IsAcceptedData() is
called to do it. The default implementation takes care of OLE interface
pointer manipulations and only requires you to override GetCountFormats()
and GetFormat(n) functions to let it know what data formats you support.
If it's not flexible enough for your application (i.e. the set of supported
formats changes over time...), you should override IsAcceptedData(). In 99%
of cases the default implementation is OK and you only have to return count
of supported formats (CF_xxx constants or one of your custom formats which
must have been registered) and their values.

 b) OnDrop(long x, long y, const void *pData)

    (x, y) are drop point (client) coordinates, pData is the pointer to data
    (whatever it is).

    If 'true' is returned from OnDrop, the operation is considered to be
    successful and the corresponding code (MOVE or COPY depending on the
    keyboard control keys) is returned. Otherwise, the operation is cancelled.

    Please remember that returning 'true' here may mean 'move' and so the
    drop source will delete the corresponding data - which would lead to
    data loss if you didn't paste it properly.

 c) OnEnter()

    called when the mouse enters the window: you might use this function to
    give some additional visual feedback.

 d) OnLeave()

    called when the mouse leaves the window; might be a good place to clean
up things allocated in OnEnter.

 e) Simple wxDropTarget specializations

    Two (very simple) wxDropTarget-derived classes are provided for two most
common situations: d&d of text and file d&d. To use them you only need to
override one virtual function OnDropText in wxTextDropTarget's case and
OnDropFiles for wxFileDropTarget.

    The (x, y) are the same as for OnDrop() function. OnDropText's last
parameter points to a (always ANSI, not Unicode) text string, while
OnDropFiles() parameter is the array of file names just dropped (and the
count of them is passed in the 3rd parameter).

3. Data Object
   -----------

 a) Drag and drop and clipboard

    The effect of a d&d operation is the same as using the clipboard to
cut/copy and paste data and it would be nice to use the same code to implement
these two data transfer mechanisms. The wxDataObject allows you to do exactly
this. It encapsulates the data which can be passed either through the clipboard
or d&d.


 b) Data format

    There are several standard clipboard formats, such as text, bitmap or 
metafile picture. All of them are defined in wxDataObject::StdFormats 
enumeration. Of course, it's not always enough and you'll often need your
own format for data transfer. The simple helper class wxDataFormat may help
you: when you create an object of this class, it registers a new clipboard
data format identified by the string passed to its ctor.

    After your new format is registered, you may use it as any other one.

4. Drop Source
   -----------

 a) Starting the d&d operation
 
    In order to start the d&d operation you should call the DoDragDrop function
(typically in reply to a "mouse button press" message). NB: DoDragDrop() is a
blocking function which enters into its own message loop and may return after
an arbitrarily long time interval. During it, the QueryContinueDrag() is called
whenever the mouse or keyboard state changes. The default behaviour is quite
reasonable for 99% of cases: the drag operation is cancelled if the <Esc> key
is preessed and the drop is initiated if the mouse button is released.

 b) After the end of d&d

    The drop source behaviour depends on DoDragDrop() return code. If it 
returns wxDropSource::None or wxDropSource::Copy there is normally nothing to
do, but you shouldn't forget to delete your data if it returns the 
wxDropSource::Move code.

 c) DoDragDrop

 d) QueryContinueDrag


5. Remarks
   -------


@@@@ TODO: support tymed != TYMED_HGLOBAL;
           better support of CF_BMP, CF_METAFILE
           scrolling support!! (how?)
           sample demonstrating use of user-defined formats
           sample which really does something useful
