32. Drag and Drop

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 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).

32.1 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.

32.1.1 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.

32.1.2 MSWindows OLE

Only allows file drags and drops.

32.1.3 Loose ends

The following protocols will be supported soon: Xdnd, Motif, Xde (if I get some specs).

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.

32.2 Drop Interface

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 dragdrop-drop-dispatch and the object contains the data of the drop (converted to URL/MIME specific data). This function will search the variable experimental-dragdrop-drop-functions for a function that can handle the dropped data.

To modify the drop behavior, the user can modify the variable 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 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 drag-n-drop shows all variables of user interest.

This describes the drag API (not implemented yet).

This document was generated by Aidan Kehoe on December 27, 2016 using texi2html 1.82.