Update (April 21, 2020):
Volume control support added.

This post shows steps to control playback status of Bluetooth audio which is streamed from a connected phone (or any Bluetooth capable media player). The goal is to enable below.

• Playback control (play, pause, next/previous song, and volume control)
• Music information display in real time (song title, artist name, and album name)

Prerequisites

• Raspberry Pi (e.g. Raspberry Pi 3 B+) *tested with Raspbian Stretch with Desktop (2019-04-08)
• 3.5 mm plug Speaker (e.g. Rumfo Mini Speaker)
• Smartphone

Steps
1. Enabling Bluetooth Audio (A2DP Sink)
1-1. If it’s not already done, please follow this post and enable Bluetooth Audio and connect a phone.

2. Implement Playback Control and Music Info Display [1]
2-1. Create a python script. This script prints playback status and music info when it’s updated. It also accept command input (i.e. “play”, “pause”, “next”, “prev”).

import dbus, dbus.mainloop.glib, sys
from gi.repository import GLib

def on_property_changed(interface, changed, invalidated):
    if interface != 'org.bluez.MediaPlayer1':
        return
    for prop, value in changed.items():
        if prop == 'Status':
            print('Playback Status: {}'.format(value))
        elif prop == 'Track':
            print('Music Info:')
            for key in ('Title', 'Artist', 'Album'):
                print('   {}: {}'.format(key, value.get(key, '')))

def on_playback_control(fd, condition):
    str = fd.readline()
    if str.startswith('play'):
        player_iface.Play()
    elif str.startswith('pause'):
        player_iface.Pause()
    elif str.startswith('next'):
        player_iface.Next()
    elif str.startswith('prev'):
        player_iface.Previous()
    elif str.startswith('vol'):
        vol = int(str.split()[1])
        if vol not in range(0, 128):
            print('Possible Values: 0-127')
            return True
        transport_prop_iface.Set(
                'org.bluez.MediaTransport1',
                'Volume',
                dbus.UInt16(vol))
    return True

if __name__ == '__main__':
    dbus.mainloop.glib.DBusGMainLoop(set_as_default=True)
    bus = dbus.SystemBus()
    obj = bus.get_object('org.bluez', "/")
    mgr = dbus.Interface(obj, 'org.freedesktop.DBus.ObjectManager')
    player_iface = None
    transport_prop_iface = None
    for path, ifaces in mgr.GetManagedObjects().items():
        if 'org.bluez.MediaPlayer1' in ifaces:
            player_iface = dbus.Interface(
                    bus.get_object('org.bluez', path),
                    'org.bluez.MediaPlayer1')
        elif 'org.bluez.MediaTransport1' in ifaces:
            transport_prop_iface = dbus.Interface(
                    bus.get_object('org.bluez', path),
                    'org.freedesktop.DBus.Properties')
    if not player_iface:
        sys.exit('Error: Media Player not found.')
    if not transport_prop_iface:
        sys.exit('Error: DBus.Properties iface not found.')

    bus.add_signal_receiver(
            on_property_changed,
            bus_name='org.bluez',
            signal_name='PropertiesChanged',
            dbus_interface='org.freedesktop.DBus.Properties')
    GLib.io_add_watch(sys.stdin, GLib.IO_IN, on_playback_control)
    GLib.MainLoop().run()

2-2. Run the script.

python media_control.py

3. Test: Playback Status and Music Info
3-1. Start music by operating a media player on the phone. The script should print the info like below:

$ python media_control.py 
Playback Status: playing
Music Info:
   Title: Basket Case
   Artist: Green Day
   Album: Dookie (Explicit)

4. Test: Playback/Volume Control
4-1. Enter commands from keyboard. You should be able to control the music. In the example below, line 1, 3, 5 and 10 are commands entered.

pause
Playback Status: paused
play
Playback Status: playing
next
Music Info:
   Title: Sloppy Seconds
   Artist: Watsky
   Album: Cardboard Castles
vol 100

What’s Next?
If you are interested in showing album cover art images, please check this out.

• Showing Album Cover Art Images for Bluetooth Audio

References
[1] bluez/doc/media-api.txt