让AudioPlayerAgent支持动态播放列表（Windows Phone）
最近做的是有关背景音乐播放（调用系统播放器）的活儿，其中牵扯到Microsoft.Phone.BackgroundAudio.BackgroundAudioPlayer播放本地或远程音频时候的播放列表改动问题。如果希望播放系统Music + Videos中存储的音乐，那么直接在UI利用Microsoft.Xna.Framework.Media传递和播放音乐即可。但如果是BackgroundAudioPlayer和AudioPlayerAgent就不太好办。
可能有人会说这种问题还用得着单写一篇文章吗，AudioPlayerAgent当然支持动态列表了。但很可惜MSDN的官方How-To的例子就是硬编码的列表（还是本地音频），不然也不会在App Hub Forum上出现这样的问题（1、2）。
一般来说将播放列表传递给Agent有这几种方式：
- IsolatedStorageSettings
- 静态方法或属性（作为BackgroundAgent，实例化它没有意义）
- IsolatedStorageFile
实验之后就会发现，前两种方法并不好用：虽然在UI线程中设置了列表或者通过静态方法、属性传递了列表，但启动后的Agent拿不到这些数据。就算是单写一个存储播放列表数据的静态类也是一样。那么留给我们的方法只有通过文件传递。
在上面提到的那个帖子中，一位微软员工Mark Chamberlain给出的方案也是如此（全文请看原帖）：
This is how I did it:
In background audio agent code:
OnUserAction.Play/SkipNext/SkipPrevious: Check if BackgroundAudioPlayer.Instance.Track == null, if yes then load playlist from IsoStorage into m_playlist and start playing first track, if no then play current/next/previous song from m_playlist
In the album viewmodel code (each time a user selects an album a new viewmodel gets created for this app):
OnInitialization:…
Another developer who reviewed this suggested that instead of calling ..Instance.Close(), which also releases all the background audio resources, a better approach is to set BackgroundAudioPlayer.Instance.Track = null, which stops and clears, but keeps the background resources allocated.
基本思路即UI存好文件，Agent初始化时候载入。任何和播放相关操作前播放列表如果为null则再次载入。
具体到我手头的例子，文件格式我直接用的Xml序列化，写起来简单。由于需要和UI保持实时，那么每修改一次播放列表（添加、删除、改动）都要保存，同时在Agent一边，任何和播放相关操作前则不仅检查是否列表为null，而是直接全部重新载入。类似于：
protected override void OnUserAction(BackgroundAudioPlayer player, AudioTrack track, UserAction action, object param) {
var allTracks = TrackManager.GetAllTracks();
_playList = new List();
foreach ( TrackItem trackItem in allTracks ) {
_playList.Add(
new AudioTrack( null, trackItem.Title, trackItem.Album, null, null, trackItem.Url, EnabledPlayerControls.Pause ) );
}
//...
}
代码写出来非常愚蠢，但目前为止找不到更好的方法。
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