DBus-spec

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Revision as of 23:17, 1 February 2007 by Mr Mirsal (talk | contribs) (Added New_Capabilities signal definition draft.)
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Specification for a Common, Desktop neutral, Media Player D-Bus interface

Please note this document is not specific to VLC and is a work in progress.

THIS IS A DRAFT

What follows has been largely stolen from http://bmpx.beep-media-player.org/site/MPRIS_Interfacing_Specification

This is an attempt to create a specification for media players to use, for YOUR applications to be able to interact with ANY Media Player YOU want to use on ANY desktop YOU want to use.

This is in the spirit of http://www.freedesktop.org and uses code and ideas from this project, most notably D-Bus : http://dbus.freedesktop.org and uses ideas from http://www.musicbrainz.org

Other players

How other "Media Players" interact with D-Bus

bmpx

http://bmpx.beep-media-player.org/site/MPRIS_Interfacing_Specification

banshee

http://cvs.gnome.org/viewcvs/*checkout*/banshee/src/Banshee.Base/DBusPlayer.cs

rhythmbox

http://cvs.gnome.org/viewcvs/rhythmbox/remote/dbus/rb-client.c?view=markup

muine

http://cvs.gnome.org/viewcvs/muine/DBusLib/Player.cs?view=markup

amarok

http://amarok.kde.org/wiki/DCOP_Functions (D-Bus will replace dcop in kde4 ?)

Definitions

"Media Player"

An application, either with GUI or GUI-less (or which allows for both modes of operation) which is capable of playing back audio/video streams. The means by which it does accomplish that (e.g. which audio backend to use, and which output method) is out of the scope of this document.

The "Media Player" must:

  • Be able to play back at least local storage file streams.
  • Be able to play back at least one stream at a time.

"Client"

The Client is an unspecified application that will interact with the "Media Player" using D-Bus protocol. For example:

  • A softphone, that pauses, or step the volume down, when a call is being received.
  • An applet that nicely integrates in your desktop environment, to control your "Media Player".
  • An instant messaging client which informs your buddies of what you're listening to.
  • A monitor that stores in a database what you are listening to, for statistics purpose.
  • A video editing software that calls an external "Media Player" to preview your work.

"Tracklist"

A list of media files which resides within the "Media Player", and whose implementation is opaque to the remote interface.

The "Tracklist" must:

  • Hold an ordered list of locations of the media files as URIs (e.g. file:// or http://). The implementation of this can be opaque, which means, it does not neccesarily need to store URIs, but upon remote request, it must be able to return an URI for a given media file.
  • Keep the list in an ordered fashion. The order can be changed at any time trough e.g. sorting algorithms, in which case the "Media Player" must send information about the reordering trough it's remote interface

The "Tracklist" can, but does not need to:

  • Hold "Metadata" about media files

"Metadata"

"Metadata" is data that the files carry within themselves as a means of self-identification (commonly known as "tags"), or data that has been retrieved about the files trough other means, e.g. an internet service that provides additional data about particular media files. "Metadata" is an array of dict entries, like ("length", 253), ("name", "Bolero"), ("video-codec", "DIVX"). The dict entry is in the form (string, variant) eg: {sv}.

Furthermore, "Metadata" can be split into two categories:

1) Technical metadata:

  • "URI":"s"
  • "length":"i" (length in seconds)
  • "video-codec":"s" (video codec as a fourcc)
  • "audio-codec":"s" (audio codec as a fourcc)
  • "video-bitrate":"i" video bitrate
  • "audio-bitrate":"i" audio bitrate
  • "audio-samplerate":"i" (audio samplerate)

2) Informational metadata

This list is informative, and can be extended up to your needs, for example: "age of the captain":"i"

Only Required fields are: "URI" and "length"

D-Bus

The Service

org.freedesktop.MediaPlayer

All "Media Players" must request this name and do not let other "Media Players" 'steal' this name. Using libdbus, that would be: 

DBusConnection *dbus_connection;
DBusError dbus_error;
dbus_bus_request_name( dbus_connection, "org.freedesktop.MediaPlayer", 0, &dbus_error );

To be able to request the name, and so, be able to communicate via D-Bus, no "Media Player" must be running. The "Media Player" may call the Quit method, and wait till the name is unregistered, to ensure this is the case.

I think this is a bad idea, it's not the place for a interop Spec to force single instance behaviour without a good reason (i run amarok all the time and use vlc to play videos or streams...). Why not suggest that mediaplayers register with the well known service name ("org.freedesktop.MediaPlayer") with the options QueueService and suggest the media player should register an application specific service in addition?
if done that way we gain the feature to query the dbus daemon for all clients queued for that dbus name, so enumerating all current musicplayers that support the Spec is not problem. Contacting the players is not problem to because every client on dbus has a unique name anyway. DBus signal do AFAIK carry the dbus service name of the sending client, so there is no trouble knowing which player send a signal. textshell

The Object Hierarchy

  • / : Media Player identification
  • /Player : Playback control
  • /TrackList : TrackList management

The interface

All methods must be accessed through the interface org.freedesktop.MediaPlayer

i.e. calling Quit with qdbus would be: 

$ qdbus org.freedesktop.MediaPlayer /Player org.freedesktop.MediaPlayer.Quit

The Methods

  • /

Get_Identity : Identify the "Media Player" as in "VLC 0.9.0", "bmpx 0.34.9", "Totem 2.16.2" ... 

  <method name="Get_Identity">
    <arg type="s" direction="out"/>
  </method>
  • /TrackList

Get_Metadata : Gives all meta data available for element at given position in the "TrackList" 

 <method name="Get_Metadata">
   <arg type="i" direction="in" />
   <arg type="a{sv}" direction="out" />
 </method>

Get_CurrentTrack : Position of Current URI in the "TrackList" 

  <method name="Get_CurrentTrack">
    <arg type="i" direction="out" />
  </method>

Get_Length : Number of elements in the "TrackList" 

  <method name="Get_Length">
    <arg type="i" direction="out" />
  </method>

Add_Track : Appends an URI in the "TrackList", play it immediately if the 2nd argument is TRUE 

  <method name="Add_Track">
    <arg type="s" />
    <arg type="b" />
  </method>

Remove_Track : Removes an URI from the "TrackList", given its position 

  <method name="Remove_Track">
    <arg type="i" />
  </method>
  • /Player

Next : Goes to the next element (what if we're at the end?) 

  <method name="Next">
  </method>

Prev : Goes to the previous element (what if we're at the beginning?) 

  <method name="Prev">
  </method>

Pause : If playing : pause. If paused : unpause. If stopped : start playing 

  <method name="Pause">
  </method>

Stop : Stop playing. 

  <method name="Stop">
  </method>

Play : If playing : rewind to the beginning of current track, else : start playing. 

  <method name="Play">
  </method>

Quit : Makes the "Media Player" exit. 

  <method name="Quit">
  </method>

Get_Status : Return the status of "Media Player": 0 = Playing, 1 = Paused, 2 = Stopped. 

  <method name="Get_Status">
  <arg type="i" direction="out"/>
  </method>

Set_Volume : Sets the volume (argument must be in [0;100]) 

  <method name="Set_Volume">
  <arg type="i"/>
  </method>

Get_Volume : Returns the current volume (must be in [0;100]) 

  <method name="Get_Volume">
  <arg type="i" direction="out"/>
  </method>

Set_Position : Sets the playing position (argument must be in [0;100]) 

  <method name="Set_Position">
  <arg type="i"/>
  </method>

Get_Position : Returns the playing position (must be in [0;100]) 

  <method name="Get_Position">
  <arg type="i" direction="out"/>
  </method>

The signals

New_Track : Signal is emitted when the "Media Player" plays another "Track". Argument of the signal is the metadata attached to the new "Track" 

  <signal name="New_Track">
    <arg type="a{sv}"/>
  </signal>

New_Status : Signal is emitted when the status of the "Media Player" change. The argument has the same meaning than the value returned by Get_Status. 

  <signal name="New_Status">
   <arg type="i"/>
  </signal>

New_Capabilities : [TODO] Signal is emitted when the "Media Player" changes capabilities, Flags are CAN_GO_NEXT CAN_GO_PREV CAN_PAUSE CAN_PLAY CAN_SEEK CAN_PROVIDE_METADATA PROVIDES_TIMING 

  <signal name="New_Capabilities">
    <arg type=TODO />
  </signal>


See Also

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