如何从iPhone/iPod的音乐库中拷贝音乐到自己的App里

      最近做的app需要从iOS系统的音乐库里面拷贝选中的歌曲到APP的Documents目录下，在网上找了好久之后，终于找到两种方法，分别是可以导出成指定的格式和导出成Core Audio支持的caf格式，代码分别记录在下面
       1，导出成caf格式，这种导出方式，文件名必须以.caf作为后缀，使用其他后缀会导出失败

- (void) convertToCAF:(NSString *)filename (MPMediaItem *)song
{
    NSURL *assetURL = [song valueForProperty:MPMediaItemPropertyAssetURL];
	AVURLAsset *songAsset = [AVURLAsset URLAssetWithURL:assetURL options:nil];
    
	NSError *assetError = nil;
	AVAssetReader *assetReader = [[AVAssetReader assetReaderWithAsset:songAsset
                                                                error:&assetError]
								  retain];
	if (assetError) {
		NSLog (@"error: %@", assetError);
		return;
	}
	
	AVAssetReaderOutput *assetReaderOutput = [[AVAssetReaderAudioMixOutput
                                               assetReaderAudioMixOutputWithAudioTracks:songAsset.tracks
                                               audioSettings: nil]
											  retain];
	if (! [assetReader canAddOutput: assetReaderOutput]) {
		NSLog (@"can't add reader output... die!");
		return;
	}
	[assetReader addOutput: assetReaderOutput];
	
	NSArray *dirs = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
	NSString *documentsDirectoryPath = [dirs objectAtIndex:0];
	NSString *exportPath = [[documentsDirectoryPath stringByAppendingPathComponent:filename] retain];
	if ([[NSFileManager defaultManager] fileExistsAtPath:exportPath]) {
		[[NSFileManager defaultManager] removeItemAtPath:exportPath error:nil];
	}
	NSURL *exportURL = [NSURL fileURLWithPath:exportPath];
	AVAssetWriter *assetWriter = [[AVAssetWriter assetWriterWithURL:exportURL
                                                          fileType:AVFileTypeCoreAudioFormat
                                                              error:&assetError]
								  retain];
	if (assetError) {
		NSLog (@"error: %@", assetError);
		return;
	}
	AudioChannelLayout channelLayout;
	memset(&channelLayout, 0, sizeof(AudioChannelLayout));
	channelLayout.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo;
	NSDictionary *outputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
									[NSNumber numberWithInt:kAudioFormatLinearPCM], AVFormatIDKey,
									[NSNumber numberWithFloat:44100.0], AVSampleRateKey,
									[NSNumber numberWithInt:2], AVNumberOfChannelsKey,
									[NSData dataWithBytes:&channelLayout length:sizeof(AudioChannelLayout)], AVChannelLayoutKey,
									[NSNumber numberWithInt:16], AVLinearPCMBitDepthKey,
									[NSNumber numberWithBool:NO], AVLinearPCMIsNonInterleaved,
									[NSNumber numberWithBool:NO],AVLinearPCMIsFloatKey,
									[NSNumber numberWithBool:NO], AVLinearPCMIsBigEndianKey,
									nil];
	AVAssetWriterInput *assetWriterInput = [[AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeAudio
                                                                               outputSettings:outputSettings]
											retain];
	if ([assetWriter canAddInput:assetWriterInput]) {
		[assetWriter addInput:assetWriterInput];
	} else {
		NSLog (@"can't add asset writer input... die!");
		return;
	}
	
	assetWriterInput.expectsMediaDataInRealTime = NO;
    
	[assetWriter startWriting];
	[assetReader startReading];
    
	AVAssetTrack *soundTrack = [songAsset.tracks objectAtIndex:0];
	CMTime startTime = CMTimeMake (0, soundTrack.naturalTimeScale);
	[assetWriter startSessionAtSourceTime: startTime];
	
	__block UInt64 convertedByteCount = 0;
	
	dispatch_queue_t mediaInputQueue = dispatch_queue_create("mediaInputQueue", NULL);
	[assetWriterInput requestMediaDataWhenReadyOnQueue:mediaInputQueue
											usingBlock: ^
	 {
		 // NSLog (@"top of block");
		 while (assetWriterInput.readyForMoreMediaData) {
             CMSampleBufferRef nextBuffer = [assetReaderOutput copyNextSampleBuffer];
             if (nextBuffer) {
                 // append buffer
                 [assetWriterInput appendSampleBuffer: nextBuffer];
                 //				NSLog (@"appended a buffer (%d bytes)",
                 //					   CMSampleBufferGetTotalSampleSize (nextBuffer));
                 convertedByteCount += CMSampleBufferGetTotalSampleSize (nextBuffer);
      
                 
             } else {
                 // done!
                 [assetWriterInput markAsFinished];
                 [assetWriter finishWriting];
                 [assetReader cancelReading];
                 NSDictionary *outputFileAttributes = [[NSFileManager defaultManager]
                                                       attributesOfItemAtPath:exportPath
                                                       error:nil];
                 NSLog (@"done. file size is %lld",
					    [outputFileAttributes fileSize]);
                 
                 // release a lot of stuff
                 [assetReader release];
                 [assetReaderOutput release];
                 [assetWriter release];
                 [assetWriterInput release];
                 [exportPath release];
                 break;
             }
         }
         
	 }];
}

上面的代码来自  http://www.subfurther.com/blog/2010/12/13/from-ipod-library-to-pcm-samples-in-far-fewer-steps-than-were-previously-necessary/ ，删减了一些更新界面的代码。

      2，导出成mp3或者其他格式

- (void) convertToMp3: (MPMediaItem *)song
{
    NSURL *url = [song valueForProperty:MPMediaItemPropertyAssetURL];
    
    AVURLAsset *songAsset = [AVURLAsset URLAssetWithURL:url options:nil];
    
    NSFileManager *fileManager = [NSFileManager defaultManager];
    NSArray *dirs = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
	NSString *documentsDirectoryPath = [dirs objectAtIndex:0];
    
    NSLog (@"compatible presets for songAsset: %@",[AVAssetExportSession exportPresetsCompatibleWithAsset:songAsset]);
    
    NSArray *ar = [AVAssetExportSession exportPresetsCompatibleWithAsset: songAsset];
    NSLog(@"%@", ar);
    
    AVAssetExportSession *exporter = [[AVAssetExportSession alloc]
                                      initWithAsset: songAsset
                                      presetName: AVAssetExportPresetAppleM4A];
    
    NSLog (@"created exporter. supportedFileTypes: %@", exporter.supportedFileTypes);
    
    exporter.outputFileType = @"com.apple.m4a-audio";
    
    NSString *exportFile = [documentsDirectoryPath stringByAppendingPathComponent:[NSString stringWithFormat:@"%@.m4a",[song valueForProperty:MPMediaItemPropertyTitle]]];
    
    NSError *error1;
    
    if([fileManager fileExistsAtPath:exportFile])
    {
        [fileManager removeItemAtPath:exportFile error:&error1];
    }
    
    NSURL* exportURL = [[NSURL fileURLWithPath:exportFile] retain];
    
    exporter.outputURL = exportURL;
    
    // do the export
    [exporter exportAsynchronouslyWithCompletionHandler:^
     {
         NSData *data1 = [NSData dataWithContentsOfFile:exportFile];
         //NSLog(@"==================data1:%@",data1);
         
         
         int exportStatus = exporter.status;
         
         switch (exportStatus) {
                 
             case AVAssetExportSessionStatusFailed: {
                 
                 // log error to text view
                 NSError *exportError = exporter.error;
                 
                 NSLog (@"AVAssetExportSessionStatusFailed: %@", exportError);
                 
                 
                 
                 break;
             }
                 
             case AVAssetExportSessionStatusCompleted: {
                 
                 NSLog (@"AVAssetExportSessionStatusCompleted");
                 
                 
                 break;
             }
                 
             case AVAssetExportSessionStatusUnknown: {
                 NSLog (@"AVAssetExportSessionStatusUnknown");
                 break;
             }
             case AVAssetExportSessionStatusExporting: {
                 NSLog (@"AVAssetExportSessionStatusExporting");
                 break;
             }
                 
             case AVAssetExportSessionStatusCancelled: {
                 NSLog (@"AVAssetExportSessionStatusCancelled");
                 break;
             }
                 
             case AVAssetExportSessionStatusWaiting: {
                 NSLog (@"AVAssetExportSessionStatusWaiting");
                 break;
             }
                 
             default: 
             { NSLog (@"didn't get export status"); 
                 break;
             }
         }
         
     }];
}

以上的代码参考了 http://stackoverflow.com/questions/4746349/copy-ipod-music-library-audio-file-to-iphone-app-folder 这个帖子， 他原帖中创建AVAssetExportSession时用的是这样的创建方式

AVAssetExportSession *exporter = [[AVAssetExportSession alloc]
                                      initWithAsset: songAsset
                                      presetName: AVAssetExportPresetPassthrough];

但是这样的代码在我的APP上不能工作，所以把最后一个参数改成了AVAssetExportPresetAppleM4A，可以通过 [AVAssetExportSession exportPresetsCompatibleWithAsset:song]来获取歌曲支持哪些preset。