如何以与系统音量不同的音量播放 iPod 音乐库中的曲目？

Question

如何以与系统音量不同的音量播放 ipod 音乐库（如用户定义的播放列表等）中的音乐？

Answer 1

这适用于尝试以与系统音量不同的音量播放 ipod 音乐库中的音乐/播放列表的人。有几个帖子说[MPMusicPlayerController applicationMusicPlayer]可以做到这一点，但我发现每当我改变应用程序音乐播放器的音量时，系统音量也会改变。

还有一种使用 AVAudioPlayer 类播放音乐的复杂方法，但它需要您将音乐文件从 ipod 库复制到应用程序包，当您播放动态内容（例如用户生成的）时，这可能会变得很棘手播放列表。不过，该技术确实可以让您访问字节，如果您想对数据进行处理（如 DJ 应用程序），这是一种可行的方法。链接到该解决方案HERE。

我采用的解决方案使用 AVPlayer 类，那里有几篇关于如何做到这一点的好帖子。这篇文章基本上是我在 Stackoverflow 和其他地方找到的几种不同解决方案的组合。

我链接了以下框架：

AVFoundation
媒体播放器
音频工具箱
核心音频
核心媒体

（我不确定所有这些是否都很重要，但这就是我所拥有的。我也实现了一些 OpenAL 的东西，我没有在下面的代码中显示）

// 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

请原谅您看到的任何错误 - 请在 cmets 中告诉我，我会修复它们。否则，如果有人遇到与我相同的挑战，我希望这会有所帮助！

Answer 2

其实我通过loading iPod URL's from MPMusicPlayer 找到了一个非常简单的方法来做到这一点，然后通过 AVAudioPlayer 进行播放。

// 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.