@c -*-texinfo-*-
@c This is part of the XEmacs Lisp Reference Manual.
@c Copyright (C) 1998 Oliver Graf <ograf@fga.de>
@c Original reference is (c) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc.
@c See the file lispref.texi for copying conditions.
@setfilename ../../info/dragndrop.texi
@node Drag and Drop, Modes, Scrollbars, Top
@chapter Drag and Drop
@cindex drag and drop

@emph{WARNING}: the Drag'n'Drop API is still under development and the
interface may change! The current implementation is considered experimental.

  Drag'n'drop is a way to transfer information between multiple applications.
To do this several GUIs define their own protocols. Examples are OffiX, CDE,
Motif, KDE, MSWindows, GNOME, and many more. To catch all these protocols,
XEmacs provides a generic API.

One prime idea behind the API is to use a data interface that is
transparent for all systems. The author thinks that this is best
archived by using URL and MIME data, cause any internet enabled system
must support these for email already. XEmacs also already provides
powerful interfaces to support these types of data (tm and w3).

@menu
* Supported Protocols:: Which low-level protocols are supported.
* Drop Interface::      How XEmacs handles a drop from another application.
* Drag Interface::      Calls to initiate a drag from XEmacs.
@end menu

@node Supported Protocols
@section Supported Protocols

The current release of XEmacs only support a small set of Drag'n'drop
protocols. Some of these only support limited options available in the API.

@menu
* OffiX DND::           A generic X based protocol.
* CDE dt::              Common Desktop Environment used on suns.
* MSWindows OLE::       Mr. Gates way of live.
* Loose ends::          The other protocols.
@end menu

@node OffiX DND
@subsection OffiX DND
@cindex OffiX DND

@emph{WARNING}: If you compile in OffiX, you may not be able to use
multiple X displays successfully.  If the two servers are from
different vendors, the results may be unpredictable.

The OffiX Drag'n'Drop protocol is part of a X API/Widget library created by
Cesar Crusius. It is based on X-Atoms and ClientMessage events, and works with
any X platform supporting them.

OffiX is supported if 'offix is member of the variable dragdrop-protocols, or
the feature 'offix is defined.

Unfortunately it uses it's own data types. Examples are: File, Files,
Exe, Link, URL, MIME. The API tries to choose the right type for the data that
is dragged from XEmacs (well, not yet...).

XEmacs supports both MIME and URL drags and drops using this API. No application
interaction is possible while dragging is in progress.

For information about the OffiX project have a look at http://leb.net/~offix/

@node CDE dt
@subsection CDE dt
@cindex CDE dt

CDE stands for Common Desktop Environment. It is based on the Motif
widget library. It's drag'n'drop protocol is also an abstraction of the
Motif protocol (so it might be possible, that XEmacs will also support
the Motif protocol soon).

CDE has three different types: file, buffer, and text. XEmacs only uses
file and buffer drags. The API will disallow full URL drags, only file
method URLs are passed through.

Buffer drags are always converted to plain text.

@node MSWindows OLE
@subsection MSWindows OLE
@cindex MSWindows OLE

Only allows file drags and drops.

@node Loose ends
@subsection Loose ends

The following protocols will be supported soon: Xdnd, Motif, Xde (if I
get some specs), KDE OffiX (if KDE can find XEmacs windows).

In particular Xdnd will be one of the protocols that can benefit from
the XEmacs API, cause it also uses MIME types to encode dragged data.

@node Drop Interface
@section Drop Interface
@cindex drop
@cindex Drop API

For each activated low-level protocol, an internal routine will catch
incoming drops and convert them to a dragdrop-drop type
misc-user-event.

This misc-user-event has its function argument set to
@code{dragdrop-drop-dispatch} and the object contains the data of the drop
(converted to URL/MIME specific data). This function will search the variable
@code{experimental-dragdrop-drop-functions} for a function that can handle the
dropped data.

To modify the drop behavior, the user can modify the variable
@code{experimental-dragdrop-drop-functions}. Each element of this list
specifies a possible handler for dropped data. The first one that can handle
the data will return @code{t} and exit. Another possibility is to set a
extent-property with the same name. Extents are checked prior to the
variable.

The customization group @code{drag-n-drop} shows all variables of user
interest.

@node Drag Interface
@section Drag Interface
@cindex drag
@cindex Drag API

This describes the drag API (not implemented yet).