Desktop Abstraction Layer (DAL) Overview
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1. Overview

The DAL is an attempt to unify and standardize communication between different
desktop applications, regardless of there background: plain X apps, Gnome/GTK
based applications or KDE/Qt based ones should be able to talk to each other
easily, to enhance the whole desktop experience of the user.

This standardization process will be coordinated by a group of developers at
freedesktop.org. The development process will be open to comments, requires
etc from different sources.

The project consists of 2 parts: writing and maintaining the standard
interfaces applications should comply to, and libraries to ease the
implementation of the spec in existing and newly-written apps.

The technical implementation of DAL should be based on open, free standards,
not bound to a specific desktop environment. At the moment, DBUS [1] seems
best suited for this task.



2. Goals

Initially, DAL should be a system that allows applications to request
information from others, and signalize events.
Later on, there could be some more "activity", where one application can tell
another one "do this", or make requests to a group of applications. An example
of this is a VoIP/VideoChat application like GnomeMeeting that requests all
media player apps to pause playback. One problem to solve here: should this be
"pushed" or "pulled", i.e. should GnomeMeeting request all mediaplayers to
pause playback, or should it just emit a signal there's an incoming call, and
should mediaplayers be so intelligent to pause themselves?



3. Implementation

DAL should mainly consist of the well-known interfaces we define. These
interface define the namespace of the generated DBUS messages, and the
parameters carried with them.
There are 2 types of messages (TODO: maybe more?):
- Event signals.
        This type of messages are triggered by an application when something
        "inside" the application happens. Examples of this:
        "ApplicationStarted", "EmailReceived", "BuddyLoggedIn",
        "BuddyLoggedOut","NewTrackStarted",... Each of them with some specific 
        additional parameters.
- Information requests
        This is a type of message sent by an application on the bus,
        requesting information from applications that can give it. An example
        of this is "WhoIsPlayingMusic" (just a stupid sample). If a media
        player receives an information request like this, it can send out a
        signal "PlayingMusic (String path_to_file)". (TODO: find more
        samples).

The information given (DBUS Iter members) follows some strict rules. As an
example, we could state paths should be returned following the fdo File URI
spec [2].


4. Initial steps

- Get in touch with the XMMS2 and Evo2.1 hackers, because they're already
using DBUS for desktop-wide communication
- Figure out a new, final name for this
- Get some Wiki page on fdo
- Write some simple client library
- As a proof of concept, write a simple interface for something, and patch
some existing apps so they start using the DAL lib and start communicating

I'll keep working on my initial example for now: let xscreensaver send out
DBUS events (using DAL), even thought the xscreensaver maintainer doesn't like
this, complete the Gaim plugin (too hard to get inside the Gaim core for now),
maybe patch up some media player to pause playback when a screensaver is
started,...



[1] http://freedesktop.org/wiki/Software_2fdbus
[2] http://freedesktop.org/wiki/Standards_2ffile_2duri_2dspec