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如何以不同于系统音量的音量播放iPod音乐库中的音乐（如用户定义的播放列表等）？如何使用AVPlayer以不同于系统音量的iPod音乐库播放音轨
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回答
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这适用于任何试图以不同于系统音量的音量播放iPod音乐库中的音乐/播放列表的人。在那里有几个帖子说[MPMusicPlayerController applicationMusicPlayer]可以做到这一点，但我发现，任何时候我改变applicationMusicPlayer的音量，系统音量也会改变。
使用AVAudioPlayer类还有一种更加复杂的播放音乐的方法，但它需要您将音乐文件从ipod库复制到应用程序包，并且在播放动态内容时会变得棘手，如用户生成播放列表。这种技术确实可以让你访问字节，如果你想对数据进行处理（如DJ应用程序），这种方法也是可行的。链接到该解决方案HERE。
我使用的解决方案使用AVPlayer类，有几个很好的职位关于如何做到这一点。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。
我有以下框架链接：
AVFoundation
MediaPlayer的
AudioToolbox
CoreAudio的
CoreMedia
（我不知道如果所有这些都是重要的，但是这就是我。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
请原谅你看到的任何错误 - 让我知道在评论中，我会解决它们。否则，如果有人遇到同样的挑战，我希望这会有所帮助！
2
其实我找到了一个非常简单的方法来做到这一点loading iPod URL's from MPMusicPlayer，但随后通过AVAudioPlayer进行回放。
// 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.