Data corruption when reading realtime H.264 output from AVAssetWriter

Question

I\'m using some tricks to try to read the raw output of an AVAssetWriter while it is being written to disk. When I reassemble the individual files by concatenating them, the

Answer 1

I ended up abandoning the "read while it's written" approach in favor of a manual chunking approach where I call finishWriting every 5 seconds on a background thread. I was able to drop a negligible number of frames using a method originally described here:

- (void) segmentRecording:(NSTimer*)timer {
    AVAssetWriter *tempAssetWriter = self.assetWriter;
    AVAssetWriterInput *tempAudioEncoder = self.audioEncoder;
    AVAssetWriterInput *tempVideoEncoder = self.videoEncoder;
    self.assetWriter = queuedAssetWriter;
    self.audioEncoder = queuedAudioEncoder;
    self.videoEncoder = queuedVideoEncoder;
    //NSLog(@"Switching encoders");

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
        [tempAudioEncoder markAsFinished];
        [tempVideoEncoder markAsFinished];
        if (tempAssetWriter.status == AVAssetWriterStatusWriting) {
            if(![tempAssetWriter finishWriting]) {
                [self showError:[tempAssetWriter error]];
            }
        }
        if (self.readyToRecordAudio && self.readyToRecordVideo) {
            NSError *error = nil;
            self.queuedAssetWriter = [[AVAssetWriter alloc] initWithURL:[self newMovieURL] fileType:(NSString *)kUTTypeMPEG4 error:&error];
            if (error) {
                [self showError:error];
            }
            self.queuedVideoEncoder = [self setupVideoEncoderWithAssetWriter:self.queuedAssetWriter formatDescription:videoFormatDescription bitsPerSecond:videoBPS];
            self.queuedAudioEncoder = [self setupAudioEncoderWithAssetWriter:self.queuedAssetWriter formatDescription:audioFormatDescription bitsPerSecond:audioBPS];
            //NSLog(@"Encoder switch finished");

        }
    });
}

Full source code: https://github.com/chrisballinger/FFmpeg-iOS-Encoder/blob/master/AVSegmentingAppleEncoder.m