ios – 反转音频文件Swift / Objective-C

有没有办法可以反转并导出.m4a音频文件？我找到了一个反转音频轨道 here的解决方案,但它似乎只是在处理.caf文件格式.如果唯一的方法是使用.caf,有没有办法首先将.m4a文件转换为.caf？

更新：
在another post我发现AVAssetReader可用于从音频文件中读取音频样本,但我不知道如何以相反的顺序写回样本.以下代码片段直接来自帖子.任何帮助,将不胜感激.谢谢

+ (void) reverseAudioTrack: (AVAsset *)audioAsset outputURL: (NSURL *)outputURL {
NSError *error;

AVAssetReader* reader = [[AVAssetReader alloc] initWithAsset:audioAsset error:&error];
if (error) {NSLog(@"%@",error.localizedDescription);}

AVAssetTrack* track = [[audioAsset tracksWithMediaType:AVMediaTypeAudio] objectATindex:0];

NSMutableDictionary* audioReadSetTings = [NSMutableDictionary Dictionary];
[audioReadSetTings SETVALue:[NSnumber numberWithInt:kAudioFormatLinearPCM]
                     forKey:AVFormatIDKey];

AVAssetReaderTrackOutput* readerOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:track outputSetTings:audioReadSetTings];
[reader addOutput:readerOutput];
[reader startReading];

CMSampleBufferRef sample; //= [readerOutput copyNextSampleBuffer];
NSMutableArray *samples = [[NSMutableArray alloc] init];

// Get all samples
while((sample = [readerOutput copyNextSampleBuffer])) {
    [samples addObject:(__bridge id)sample];
    CFRelease(samplE);
}

// Process samples in reverse
AudioChAnnelLayout acl;
bzero(&acl,sizeof(acl));
acl.mChAnnelLayoutTag = kAudioChAnnelLayoutTag_Stereo;

AVAssetWriter *writer = [[AVAssetWriter alloc] initWithURL:outputURL
                                                   fileType:AVFileTypeAppleM4A
                                                      error:&error];
if (error) {NSLog(@"%@",error.localizedDescription);}
NSDictionary *writerOutputSetTings = [ NSDictionary DictionaryWithObjectsAndKeys:
                                      [ NSnumber numberWithInt: kAudioFormatAppleLossless ],AVFormatIDKey,[ NSnumber numberWithInt: 16 ],AVEncoderBitDepthHintKey,[ NSnumber numberWithFloat: 44100.0 ],AVSampleRateKey,[ NSnumber numberWithInt: 1 ],AVnumberOfChAnnelsKey,[ NSData dataWithBytes: &acl length: sizeof( acl ) ],AVChAnnelLayoutKey,nil ];

AVAssetWriterInput *audioWriterInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeAudio outputSetTings:writerOutputSetTings];

[writer addInput:audioWriterInput];
[writer startWriTing];
[writer startSessionAtsourceTime:CMSampleBufferGetPresentationtimestamp((__bridge CMSampleBufferRef)samples[0]) ];

// (1) Would it work if I loop in reverse here?
for (NSInteger i = 0; i < samples.count; i++) {
    CMBlockBufferRef buffer = CMSampleBufferGetDataBuffer((__bridge CMSampleBufferRef)samples[i]);

    CMItemCount numSamplesInBuffer = CMSampleBufferGetNumSamples((__bridge CMSampleBufferRef)samples[i]);
    AudioBufferList audioBufferList;
    CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer((__bridge CMSampleBufferRef)samples[i],NULL,&audioBufferList,sizeof(audioBufferList),kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment,&buffer
                                                            );

    for (int bufferCount = 0; bufferCount < audioBufferList.mnumberBuffers; bufferCount++) {
        SInt16* samples = (SInt16 *)audioBufferList.mBuffers[bufferCount].mData;
        for (int i=0; i < numSamplesInBuffer; i++) {
            // amplitude for the sample is samples[i],assuming you have linear pcm to start with

            // (2) what should I be doing to write the samples into an audio file?
        }
    }
    CFRelease(buffer);
}

var outAudioFile:AudioFilEID?
var pcm = AudioStreamBasicDescription(mSampleRate: 44100.0,mFormatID: kAudioFormatLinearPCM,mFormatFlags: kAudioFormatFlagIsBigendian | kAudioFormatFlagIsSignedInteger,mBytesPerPacket: 2,mFramesPerPacket: 1,mBytesPerFrame: 2,mChAnnelsPerFrame: 1,mBitsPerChAnnel: 16,mReserved: 0)

var theErr = AudioFileCreateWithURL(desturl as CFURL!,kAudioFileAIFFType,&pcm,.eraseFile,&outAudioFilE)
if noErr == theErr,let outAudioFile = outAudioFile {
    var inAudioFile:AudioFilEID?
    theErr = AudioFiLeopenURL(sourceUrl as! CFURL,.readPermission,&inAudioFilE)

    if noErr == theErr,let inAudioFile = inAudioFile {

        var fileDataSize:UInt64 = 0
        var thePropertySize:UInt32 = UInt32(MemoryLayout<UInt64>.StridE)
        theErr = AudioFileGetProperty(inAudioFile,kAudioFilePropertyAudioDataByteCount,&thePropertySize,&fileDataSizE)

        if( noErr == theErr) {
            let dataSize:Int64 = Int64(fileDataSizE)
            let theData = UnsafeMutableRawPointer.allocate(bytes: Int(dataSizE),alignedTo: MemoryLayout<UInt8>.alignment)

            var readPoint:Int64 = Int64(dataSizE)
            var writePoint:Int64 = 0

            while( readPoint > 0 )
            {
                var bytesToRead = UInt32(2)

                AudioFileReadBytes( inAudioFile,false,readPoint,&bytesToRead,theData)
                AudioFileWriteBytes( outAudioFile,writePoint,theData)

                writePoint += 2
                readPoint -= 2
            }

            theData.deallocate(bytes: Int(dataSizE),alignedTo: MemoryLayout<UInt8>.alignment)

            AudioFileClose(inAudioFilE);
            AudioFileClose(outAudioFilE);
        }
    }
}