问题
I am using MPMusicPlayerController so my app can play music that the user has bought through iTunes. When I select a song and start to play, there is a lag before the sound starts. I am assuming that the song is being buffered from the cloud.
The problem is that I have not found a way to know when the buffering is complete and the audio actually starts.
To play the song I use:
_mediaController = [MPMusicPlayerController applicationMusicPlayer];
[_mediaController setQueueWithItemCollection:collection];
[_mediaController beginGeneratingPlaybackNotifications];
[_mediaController play];
As soon as I call "play", the playback state change notification is called and the playback state is "MPMusicPlaybackStatePlaying" even though the user can't hear any music. I've noticed that even though the mediaController is in the "playing" playback state, the _mediaController.currentPlaybackTime always equals 0 until the music can be heard at which time the currentPlaybackTime properly tracks with the music.
I thought I could use the [_mediaController prepareToPlay] method to preload the audio file, but when I use that, the MPMediaPlaybackIsPreparedToPlayDidChangeNotification notification is never called. So the mediaController is never marked as "prepared".
With all this being said, I have not found a way to prebuffer songs using the MPMusicPlayerController. I know this issue has been around for a while because there is an old question from a few years ago with essentially the same issue, but no answer. Does anyone know how to make this work?
回答1:
MPMediaPlaybackIsPreparedToPlayDidChangeNotification looks deprecated.
The MPMediaPlayerController getters and notifications are kind of garbage, and aren't "consistent" at all in the way where you set a value and expect the same value to come back when you grab it again.
I solved this by "buffering" the song first, as my app frequently will start in the middle of a song. So my buffering algo is - I'll play the song, then wait for the playback state changed notification, then pause it again and wait for another notifcation. This process will without a doubt trigger the MPMusicPlayerControllerNowPlayingItemDidChangeNotification, then finally the song is ready to play or be mutated (set currentTime or Rate). This seems to work very well.
The prepareToPlay completion handler is also garbage. It seems to fire before the song is actually ready to play, and the method actually seems to start playback :( , which is more than it leads on. This seems to be commonly reported as a "bug" in the apple dev forums.
In the prepareToPlay callback, setting currentPlaybackTime or Rate won't actually mutate the player - you need to wait for the additional MPMusicPlayerControllerNowPlayingItemDidChangeNotification sometime after playback starts for the first time on the song before you'll have any luck mutating any of the player properties.
currentPlaybackRate and Time also aren't very reliable, although they are more reliable once playback actually starts. I'll cache it to what the user sets it to until playback starts to solve the consistency problem. There is a lot more here to get the currentPlaybackRate or time consistently, let me know if you want a code sample, as grabbing those properties for reading will yield different results depending on the executing thread :(
Checking the playback state isn't reliable either unfortunately. Often I've found that the player reports MPMusicPlaybackStatePlaying when its fully paused and not playing - it stays like this indefinitely in certain situations. I recommend abstracting out any determinations if the MPMediaPlayerController is actually playing based on calls to play or pause and a subsequent follow-up of a MPMusicPlayerControllerPlaybackStateDidChangeNotification confirming the player has started that action.
I should mention I'm on iOS 12.1.3, as Apple seems to intermittently fix a small bug or add more API breakages as time goes on. Since its pretty broken now, any improvements may break any abstraction layer you build to fix it - so I'm testing each iOS release to make sure everything still "works".
Hope this helps a bit, its a real struggle.
来源：https://stackoverflow.com/questions/31520681/mpmusicplayercontroller-does-not-prepare-preload-correct