fixing return values and handling of invalid files

[wxWidgets.git] / docs / doxygen / overviews / dnd.h

/////////////////////////////////////////////////////////////////////////////
// Name:        dnd.h
// Purpose:     topic overview
// Author:      wxWidgets team
// RCS-ID:      $Id$
// Licence:     wxWindows license
/////////////////////////////////////////////////////////////////////////////

/**

@page overview_dnd Drag and Drop Overview

Classes: wxDataObject, wxTextDataObject, wxDropSource, wxDropTarget,
         wxTextDropTarget, wxFileDropTarget

Note that @c wxUSE_DRAG_AND_DROP must be defined in @c setup.h in order
to use drag and drop in wxWidgets.

See also: @ref overview_dataobject and @ref page_samples_dnd.

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 wxDataObject and identify its format(s)
using the wxDataFormat class.

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

@li @b Preparation: First of all, a data object must be created and
    initialized with the data you wish to drag. For example:

    @code
    wxTextDataObject my_data("This text will be dragged.");
    @endcode

@li <b>Drag start</b>: To start the dragging process (typically in response to a
    mouse click) you must call wxDropSource::DoDragDrop like this:

    @code
    wxDropSource dragSource( this );
    dragSource.SetData( my_data );
    wxDragResult result = dragSource.DoDragDrop( true );
    @endcode

@li @b Dragging: The call to DoDragDrop() blocks the program until the user releases
    the mouse button (unless you override the wxDropSource::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 wxDropTarget methods
    are called - see below.

@li <b>Processing the result</b>: DoDragDrop() returns an @e effect code which
    is one of the values of @c wxDragResult enum (explained in wxDropTarget page):

    @code
    switch (result)
    {
        case wxDragCopy:
            // copy the data
            break;
        case wxDragMove:
            // move the data
            break;
        default:
            // do nothing
            break;
    }
    @endcode


To be a @e drop target, i.e. to receive the data dropped by the user you should
follow the instructions below:

@li @b Initialization: For a window to be a drop target, it needs to have
    an associated wxDropTarget 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 wxTextDropTarget or wxFileDropTarget and override their OnDropText()
    or OnDropFiles() method.

@li @b Drop: When the user releases the mouse over a window, wxWidgets
    asks the associated wxDropTarget object if it accepts the data. For this,
    a wxDataObject 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 wxDropTarget::OnData will get called and the wxDataObject belonging
    to the drop target can get filled with data.

@li <b>The end</b>: After processing the data, DoDragDrop() returns either
    wxDragCopy or wxDragMove 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.

*/