在iOS中使用AVFoundation反转视频时丢失音频

Question

使用avfoundation制作反向视频，一切正常。但是一旦完成写作，就会缺少音频。即使处理视频也要花费时间（最多20秒）。

我尝试了以下代码来使视频反向播放，

// Initialize the reader
NSError *error;
// check input path
if (![[NSFileManager defaultManager] fileExistsAtPath:inputPath]) {
    [self.delegate didFinishReverse:NO withError:nil];
    return;
}

[[NSFileManager defaultManager] removeItemAtPath:outputPath error:nil];

NSURL *inputUrl = [NSURL fileURLWithPath:inputPath isDirectory:NO];
AVURLAsset *asset = [AVURLAsset URLAssetWithURL:inputUrl options:nil];;
AVAssetReader *reader = [[AVAssetReader alloc] initWithAsset:asset error:nil];
AVAssetTrack *videoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] lastObject];

NSDictionary *readerOutputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                      [NSNumber numberWithInt:kCVPixelFormatType_420YpCbCr8BiPlanarFullRange], kCVPixelBufferPixelFormatTypeKey,
                                      nil];
AVAssetReaderTrackOutput* readerOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:videoTrack
                                                                                    outputSettings:readerOutputSettings];
[reader addOutput:readerOutput];
[reader startReading];

NSMutableArray *samples = [[NSMutableArray alloc] init];

CMSampleBufferRef sample;
while((sample = [readerOutput copyNextSampleBuffer])) {
    [samples addObject:(__bridge id)sample];
    CFRelease(sample);
}

NSURL *outputURL = [NSURL fileURLWithPath:outputPath isDirectory:NO];


AVAssetWriter *writer = [[AVAssetWriter alloc] initWithURL:outputURL
                                                  fileType:AVFileTypeQuickTimeMovie
                                                     error:&error];


NSDictionary *videoCompressionProps = [NSDictionary dictionaryWithObjectsAndKeys:
                                       @(videoTrack.estimatedDataRate), AVVideoAverageBitRateKey,
                                       nil];

NSDictionary *writerOutputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                      AVVideoCodecH264, AVVideoCodecKey,
                                      [NSNumber numberWithInt:videoTrack.naturalSize.width], AVVideoWidthKey,
                                      [NSNumber numberWithInt:videoTrack.naturalSize.height], AVVideoHeightKey,
                                      videoCompressionProps, AVVideoCompressionPropertiesKey,
                                      nil];

AVAssetWriterInput *writerInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeVideo
                                                                 outputSettings:writerOutputSettings
                                                               sourceFormatHint:(__bridge CMFormatDescriptionRef)[videoTrack.formatDescriptions lastObject]];

[writerInput setExpectsMediaDataInRealTime:NO];
[writer addInput:writerInput];

AudioChannelLayout acl;
bzero( &acl, sizeof(acl));
acl.mChannelLayoutTag = kAudioChannelLayoutTag_Mono;
NSDictionary* audioOutputSettings  = [NSDictionary dictionaryWithObjectsAndKeys:
                                      [ NSNumber numberWithInt: kAudioFormatMPEG4AAC], AVFormatIDKey,
                                      [ NSNumber numberWithInt: 1 ], AVNumberOfChannelsKey,
                                      [ NSNumber numberWithFloat: 44100.0 ], AVSampleRateKey,
                                      [ NSData dataWithBytes: &acl length: sizeof( acl ) ], AVChannelLayoutKey,
                                      [ NSNumber numberWithInt: 64000 ], AVEncoderBitRateKey,
                                      nil];

AVAssetWriterInput *WriterAudioInput  = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeAudio outputSettings:audioOutputSettings];
WriterAudioInput.expectsMediaDataInRealTime = YES;
if([writer canAddInput:WriterAudioInput]) {
    [writer addInput:WriterAudioInput];
}

 AVAssetWriterInputPixelBufferAdaptor *pixelBufferAdaptor = [[AVAssetWriterInputPixelBufferAdaptor alloc] initWithAssetWriterInput:writerInput sourcePixelBufferAttributes:nil];

[writer startWriting];
[writer startSessionAtSourceTime:CMSampleBufferGetPresentationTimeStamp((__bridge CMSampleBufferRef)samples[0])];

for(NSInteger i = 0; i < samples.count; i++) {

    CMTime presentationTime = CMSampleBufferGetPresentationTimeStamp((__bridge CMSampleBufferRef)samples[i]);

    CVPixelBufferRef imageBufferRef = CMSampleBufferGetImageBuffer((__bridge CMSampleBufferRef)samples[samples.count - i - 1]);

    while (!writerInput.readyForMoreMediaData) {
        [NSThread sleepForTimeInterval:0.1];
    }
    [pixelBufferAdaptor appendPixelBuffer:imageBufferRef
                     withPresentationTime:presentationTime];


}

[writerInput markAsFinished];
[writer finishWritingWithCompletionHandler:^(){
    [self.delegate didFinishReverse:YES withError:nil];

}];

效果很好。我尝试通过搜索google和stackover流进行尝试，但没有得到确切的解决方案。

感谢您的帮助。