How to play tracks from iPod music library with di

How can I play music from the ipod music library (like user-defined playlists, etc.) at a different volume than the system volume?

回答1:

This is for anyone who is trying to play music / playlists from the ipod music library at a different volume than the system volume. There are several posts out there saying that the [MPMusicPlayerController applicationMusicPlayer] can do this, but I have found that anytime I change the volume of the applicationMusicPlayer, the system volume changes too.

There is a more involved method of playing music using the AVAudioPlayer class, but it requires you to copy music files from the ipod library to the application bundle, and that can get tricky when you're playing dynamic things, like user generated playlists. That technique does give you access to the bytes though, and is the way to go if you want to do processing on the data (like a DJ app). Link to that solution HERE.

The solution I went with uses the AVPlayer class, there are several good posts out there about how to do it. This post is basically a composite of several different solutions I found on Stackoverflow and elsewhere.

I have the following Frameworks linked:

AVFoundation
MediaPlayer
AudioToolbox
CoreAudio
CoreMedia

(I'm not sure if all of those are critical, but that's what I have. I have some OpenAL stuff implemented too that I don't show in the following code)

// Presumably in your SoundManage.m file (or whatever you call it) ...

#import <CoreAudio/CoreAudioTypes.h>
#import <AudioToolbox/AudioToolbox.h>

@interface SoundManager()

@property (retain, nonatomic) AVPlayer* audioPlayer;
@property (retain, nonatomic) AVPlayerItem* currentItem;

@property (retain, nonatomic) MPMediaItemCollection* currentPlaylist;
@property (retain, nonatomic) MPMediaItem* currentTrack;
@property (assign, nonatomic) MPMusicPlaybackState currentPlaybackState;

@end


@implementation SoundManager

@synthesize audioPlayer;
@synthesize currentItem = m_currentItem;

@synthesize currentPlaylist;
@synthesize currentTrack;
@synthesize currentPlaybackState;

- (id) init
{
    ...

    //Define an AVPlayer instance
    AVPlayer* tempPlayer = [[AVPlayer alloc] init];
    self.audioPlayer = tempPlayer;
    [tempPlayer release];

    ...

    //load the playlist you want to play
    MPMediaItemCollection* playlist = [self getPlaylistWithName: @"emo-pop-unicorn-blood-rage-mix-to-the-max"];
    if(playlist)
        [self loadPlaylist: playlist];

    ...

    //initialize the playback state
    self.currentPlaybackState = MPMusicPlaybackStateStopped;


    //start the music playing
    [self playMusic];

    ...

}


//Have a way to get a playlist reference (as an MPMediaItemCollection in this case)
- (MPMediaItemCollection*) getPlaylistWithName:(NSString *)playlistName
{
    MPMediaQuery* query = [[MPMediaQuery alloc] init];
    MPMediaPropertyPredicate* mediaTypePredicate = [MPMediaPropertyPredicate predicateWithValue: [NSNumber numberWithInteger: MPMediaTypeMusic]                                                                              forProperty:MPMediaItemPropertyMediaType];
    [query addFilterPredicate: mediaTypePredicate];
    [query setGroupingType: MPMediaGroupingPlaylist];

    NSArray* playlists = [query collections];
    [query release];

    for(MPMediaItemCollection* testPlaylist in playlists)
    {
        NSString* testPlaylistName = [testPlaylist valueForProperty: MPMediaPlaylistPropertyName];
        if([testPlaylistName isEqualToString: playlistName])
            return testPlaylist;
    }

    return nil;
}

//Override the setter on currentItem so that you can add/remove 
//the notification listener that will tell you when the song has completed
- (void) setCurrentItem:(AVPlayerItem *)currentItem
{
    if(m_currentItem)
    {
        [[NSNotificationCenter defaultCenter] removeObserver:self name:AVPlayerItemDidPlayToEndTimeNotification object:m_currentItem];
        [m_currentItem release];
    }

    if(currentItem)
        m_currentItem = [currentItem retain];
    else
        m_currentItem = nil;

    if(m_currentItem)
    {
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(handleMusicTrackFinished) name:AVPlayerItemDidPlayToEndTimeNotification object:m_currentItem];
    }
}

//handler that gets called when the name:AVPlayerItemDidPlayToEndTimeNotification notification fires
- (void) handleMusicTrackFinished
{
    [self skipSongForward];  //or something similar
}

//Have a way to load a playlist
- (void) loadPlaylist:(MPMediaItemCollection *)playlist
{
    self.currentPlaylist = playlist;
    self.currentTrack = [playlist.items objectAtIndex: 0];
}

//Play the beats, yo
- (void) playMusic
{
    //check the current playback state and exit early if we're already playing something
    if(self.currentPlaybackState == MPMusicPlaybackStatePlaying)
        return;

    if(self.currentPlaybackState == MPMusicPlaybackStatePaused)
    {
        [self.audioPlayer play];
    }
    else if(self.currentTrack)
    {
        //Get the system url of the current track, and use that to make an AVAsset object
        NSURL* url = [self.currentTrack valueForProperty:MPMediaItemPropertyAssetURL];
        AVAsset* asset = [AVURLAsset URLAssetWithURL:url options:nil];

        //Get the track object from the asset object - we'll need to trackID to tell the
        //AVPlayer that it needs to modify the volume of this track
        AVAssetTrack* track = [[asset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];

        //Build the AVPlayerItem - this is where you modify the volume, etc. Not the AVPlayer itself
        AVPlayerItem* playerItem = [[AVPlayerItem alloc] initWithAsset: asset]; //initWithURL:url];
        self.currentItem = playerItem;

        //Set up some audio mix parameters to tell the AVPlayer what to do with this AVPlayerItem
        AVMutableAudioMixInputParameters* audioParams = [AVMutableAudioMixInputParameters audioMixInputParameters];
        [audioParams setVolume: 0.5 atTime:kCMTimeZero];  //replace 0.5 with your volume
        [audioParams setTrackID: track.trackID];  //here's the track id

        //Set up the actual AVAudioMix object, which aggregates effects
        AVMutableAudioMix* audioMix = [AVMutableAudioMix audioMix];
        [audioMix setInputParameters: [NSArray arrayWithObject: audioParams]];

        //apply your AVAudioMix object to the AVPlayerItem
        [playerItem setAudioMix:audioMix];

        //refresh the AVPlayer object, and play the track
        [self.audioPlayer replaceCurrentItemWithPlayerItem: playerItem];
        [self.audioPlayer play];

    }

    self.currentPlaybackState = MPMusicPlaybackStatePlaying;
}

- (void) pauseMusic
{   
    if(self.currentPlaybackState == MPMusicPlaybackStatePaused)
        return;

    [self.audioPlayer pause];

    self.currentPlaybackState = MPMusicPlaybackStatePaused;
}

- (void) skipSongForward
{
    //adjust self.currentTrack to be the next object in self.currentPlaylist
    //start the new track in a manner similar to that used in -playMusic
}

- (void) skipSongBackward
{
    float currentTime = self.audioPlayer.currentItem.currentTime.value / self.audioPlayer.currentItem.currentTime.timescale;

    //if we're more than a second into the song, just skip back to the beginning of the current track
    if(currentTime > 1.0)
    {
        [self.audioPlayer seekToTime: CMTimeMake(0, 1)];
    }
    else
    {
        //otherwise, adjust self.currentTrack to be the previous object in self.currentPlaylist
        //start the new track in a manner similar to that used in -playMusic
    }
}

//Set volume mid-song - more or less the same process we used in -playMusic
- (void) setMusicVolume:(float)vol
{
    AVPlayerItem* item = self.audioPlayer.currentItem;

    AVAssetTrack* track = [[item.asset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];

    AVMutableAudioMixInputParameters* audioParams = [AVMutableAudioMixInputParameters audioMixInputParameters];
    [audioParams setVolume: vol atTime:kCMTimeZero];
    [audioParams setTrackID: track.trackID];

    AVMutableAudioMix* audioMix = [AVMutableAudioMix audioMix];
    [audioMix setInputParameters: [NSArray arrayWithObject: audioParams]];

    [item setAudioMix:audioMix];
}


@end

Please forgive any errors you see - let me know in the comments and I'll fix them. Otherwise, I hope this helps if anyone runs into the same challenge I did!

回答2:

Actually I found a really easy way to do this by loading iPod URL's from MPMusicPlayer, but then doing playback through AVAudioPlayer.

// Get-da iTunes player thing
MPMusicPlayerController* iTunes = [MPMusicPlayerController iPodMusicPlayer]; 

// whazzong
MPMediaItem *currentSong = [iTunes nowPlayingItem];

// whazzurl
NSURL *currentSongURL = [currentSong valueForProperty:MPMediaItemPropertyAssetURL];

info( "AVAudioPlayer playing %s", [currentSongURL.absoluteString UTF8String] ) ;

// mamme AVAudioPlayer
NSError *err;
avAudioPlayer = [[AVAudioPlayer alloc] initWithContentsOfURL:currentSongURL error:&err] ;
if( err!=nil )
{
  error( "AVAudioPlayer couldn't load %s", [currentSongURL.absoluteString UTF8String] ) ;
}
avAudioPlayer.numberOfLoops = -1; //infinite

// Play that t
[avAudioPlayer prepareToPlay] ;
[avAudioPlayer play];

[avAudioPlayer setVolume:0.5]; // set the AVAUDIO PLAYER's volume to only 50%. This
// does NOT affect system volume. You can adjust this music volume anywhere else too.