Windows Phone 8 : AudioPlayerAgent Sample (part 1)

The custom audio player agent for this section includes a simple playlist, which includes both locally stored and remote audio files. The class is called BackgroundAudioPlayerAgent, and it is located in the WPUnleashed.BackgroundAudio project, within the BackgroundAudio solution in the downloadable sample code.

A static list of audio tracks defines the playlist, shown in the following excerpt:

static readonly List<AudioTrack> playList
    = new List<AudioTrack>
        {
            new AudioTrack(new Uri("AudioFiles/Audio01.wma",
                                    UriKind.Relative),
                              "Ringtone 1",              // title
                              "Windows Phone",           // artist
                              "Windows Phone Ringtones", // album
                              null),
            /* ... */

            /* A remote URI. */
            new AudioTrack(new Uri("http://example.com/track.mp3",
                                   UriKind.Absolute),
                              "Episode 29",
                              "Windows Phone Radio",
                              "Windows Phone Radio Podcast",
                              null)
        };

If providing playback of local media files, the files must be copied to isolated storage before they can be played by the background audio player. The IIsolatedStorageUtility is a custom interface that abstracts access to isolated storage. It is used by the BackgroundAudioPlayerAgent to detect whether the media files have already been written to isolated storage; if not, it is used to copy the sample files. The downloadable sample code contains all the code for the IIsolatedStorageUtility and its default implementation IsolatedStorageUtility. See the following excerpt:

internal static void CopyAudioToIsolatedStorage(
    IIsolatedStorageUtility isolatedStorageUtility)
{
    ArgumentValidator.AssertNotNull(
        isolatedStorageUtility, "isolatedStorageUtility");
    /* If a file exists, do not copy. */
    if (isolatedStorageUtility.FileExists("AudioFiles/Audio01.wma"))
    {
        return;
    }

    string[] files = new[] { "Audio01.wma", "Audio02.wma", "Audio03.wma" };

    var sourceToDestinations = from file in files
                        let path = "AudioFiles/" + file
                        select new KeyValuePair<string, string>(path, path);

    isolatedStorageUtility.CopyApplicationResourcesToIsolatedStorage(
                                                     sourceToDestinations);
}

The custom BackgroundAudioPlayerAgent keeps track of the current track number using a static int field. This is made possible because, unlike its ScheduledTaskAgent counterpart, the process for an AudioPlayerAgent is not terminated when NotifyComplete is called.

The track number field is used to select the correct track to play when the user requests the previous or next track, as shown:

void PlayNextTrack(BackgroundAudioPlayer player)
{
    if (++trackNumber >= playList.Count)
    {
        trackNumber = 0;
    }

    PlayTrack(player);
}

void PlayPreviousTrack(BackgroundAudioPlayer player)
{
    if (--trackNumber < 0)
    {
        trackNumber = playList.Count - 1;
    }

    PlayTrack(player);
}

A PlayTrack method is used to toggle the play state of the BackgroundAudioPlayer, as shown:

void PlayTrack(BackgroundAudioPlayer player)
{
    if (player.PlayerState == PlayState.Paused)
    {
        player.Play();
    }
    else
    {
        player.Track = playList[trackNumber];
    }
}

During playback, when the BackgroundAudioPlayer.Track property is set, the track does not begin immediate playback, but rather the BackgroundAudioPlayer is placed into a TrackReady state, and the AudioPlayerAgent.OnPlayStateChanged method is called. You see how this fits together in a moment.