AVUtilities.m

#import "AVUtilities.h"
#import <AVFoundation/AVFoundation.h>

@implementation AVUtilities

+ (AVAsset *)assetByReversingAsset:(AVAsset *)asset outputURL:(NSURL *)outputURL {
    NSError *error;
    
    // Initialize the reader
    AVAssetReader *reader = [[AVAssetReader alloc] initWithAsset:asset error:&error];
    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];
    
    // read in the samples
    NSMutableArray *samples = [[NSMutableArray alloc] init];
    
    CMSampleBufferRef sample;
    while(sample = [readerOutput copyNextSampleBuffer]) {
        [samples addObject:(__bridge id)sample];
        CFRelease(sample);
    }

    // Initialize the writer
    AVAssetWriter *writer = [[AVAssetWriter alloc] initWithURL:outputURL
                                                      fileType:AVFileTypeMPEG4
                                                         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];
    
    // Initialize an input adaptor so that we can append PixelBuffer
    AVAssetWriterInputPixelBufferAdaptor *pixelBufferAdaptor = [[AVAssetWriterInputPixelBufferAdaptor alloc] initWithAssetWriterInput:writerInput sourcePixelBufferAttributes:nil];
    
    [writer addInput:writerInput];
    
    [writer startWriting];
    [writer startSessionAtSourceTime:CMSampleBufferGetPresentationTimeStamp((__bridge CMSampleBufferRef)samples[0])];
    
    // Append the frames to the output.
    // Notice we append the frames from the tail end, using the timing of the frames from the front.
    for(NSInteger i = 0; i < samples.count; i++) {
        // Get the presentation time for the frame
        CMTime presentationTime = CMSampleBufferGetPresentationTimeStamp((__bridge CMSampleBufferRef)samples[i]);
        
        // take the image/pixel buffer from tail end of the array
        CVPixelBufferRef imageBufferRef = CMSampleBufferGetImageBuffer((__bridge CMSampleBufferRef)samples[samples.count - i - 1]);

        while (!writerInput.readyForMoreMediaData) {
            [NSThread sleepForTimeInterval:0.1];
        }
        
        [pixelBufferAdaptor appendPixelBuffer:imageBufferRef withPresentationTime:presentationTime];
        
    }
    
    [writer finishWriting];
    
    return [AVAsset assetWithURL:outputURL];
}

@end