分段录制的实现

分段录制也就是可以暂停之后恢复录制，并且录制结束之后是在同一个文件中。不管video还是audio都是有时间戳的frame，真是因为有时间戳播放器才能有序的进行播放了。因此在分段录制中，只要在暂停的时候记录一下当前的一个时间戳，然后在恢复之后计算一下这之间的时间差，然后在将这个frame写入之前修改一下这个frame的时间戳就行了。原理还是蛮简单的，因此实现也简单的多了。code说明一切，需要的咨询读一下代码。

- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection{
    
    @synchronized(self)
    {
        if(!self.isRecording||self.isPause){
            return;
        }
        
        BOOL isVideo = YES;
        if(captureOutput == self.audioOutput){
            isVideo = NO;
        }
        
        if(![self processSampleBuffer:sampleBuffer isVideo:isVideo]){
            
            if((sampleBuffer = [self processPartialRecord:sampleBuffer isVideo:isVideo]))
            {
                [self encodeFrame:sampleBuffer isVideo:isVideo];
                CFRelease(sampleBuffer);
            }
            
        }        
    }
    

}

- (CMSampleBufferRef)processPartialRecord:(CMSampleBufferRef)sampleBuffer isVideo:(BOOL)isVideo{
    if (_interrupted){
        if (isVideo){
            return nil;
        }
        _interrupted = NO;
        
        CMTime presentTimeStamp = CMSampleBufferGetPresentationTimeStamp(sampleBuffer);
        CMTime last = isVideo ? _lastVideo : _lastAudio;
        if (CMTIME_IS_VALID(last)){
            if (CMTIME_IS_VALID(_timeOffset)){
                presentTimeStamp = CMTimeSubtract(presentTimeStamp, _timeOffset);
            }
            CMTime offset = CMTimeSubtract(presentTimeStamp, last);
            [self logCMTime:offset];
            if (_timeOffset.value == 0){
                _timeOffset = offset;
            }
            else{
                _timeOffset = CMTimeAdd(_timeOffset, offset);
            }
        }
        _lastVideo.flags = 0;
        _lastAudio.flags = 0;
    }
    
    CFRetain(sampleBuffer);
    if (_timeOffset.value > 0){
        CFRelease(sampleBuffer);
        sampleBuffer = [self adjustTime:sampleBuffer by:_timeOffset];
    }
    
    CMTime presentTimeStamp = CMSampleBufferGetPresentationTimeStamp(sampleBuffer);
    CMTime duration = CMSampleBufferGetDuration(sampleBuffer);
    if (duration.value > 0){
        presentTimeStamp = CMTimeAdd(presentTimeStamp, duration);
    }
    
    if (isVideo){
        _lastVideo = presentTimeStamp;
    }
    else{
        _lastAudio = presentTimeStamp;
    }
    
    return sampleBuffer;
}

- (CMSampleBufferRef) adjustTime:(CMSampleBufferRef) sample by:(CMTime) offset
{
    CMItemCount count;
    CMSampleBufferGetSampleTimingInfoArray(sample, 0, nil, &count);
    CMSampleTimingInfo* pInfo = malloc(sizeof(CMSampleTimingInfo) * count);
    CMSampleBufferGetSampleTimingInfoArray(sample, count, pInfo, &count);
    
    for (CMItemCount i = 0; i < count; i++){
        pInfo[i].decodeTimeStamp = CMTimeSubtract(pInfo[i].decodeTimeStamp, offset);
        pInfo[i].presentationTimeStamp = CMTimeSubtract(pInfo[i].presentationTimeStamp, offset);
    }
    
    CMSampleBufferRef sout;
    CMSampleBufferCreateCopyWithNewTiming(nil, sample, count, pInfo, &sout);
    free(pInfo);
    return sout;
}





- (BOOL) encodeFrame:(CMSampleBufferRef) sampleBuffer isVideo:(BOOL)isVideo
{
    if (CMSampleBufferDataIsReady(sampleBuffer))
    {
        if (_writer.status == AVAssetWriterStatusUnknown){
            CMTime startTime = CMSampleBufferGetPresentationTimeStamp(sampleBuffer);
            [_writer startWriting];
            [_writer startSessionAtSourceTime:startTime];
        }
        if (_writer.status == AVAssetWriterStatusFailed){
            NSLog(@"error %@", _writer.error.localizedDescription);
            return NO;
        }
        
        if (isVideo){
            if (self.videoWriterInput.readyForMoreMediaData){
                [self.videoWriterInput appendSampleBuffer:sampleBuffer];
                return YES;
            }
        }else{
            if(self.audioWriterInput.readyForMoreMediaData){
                [self.audioWriterInput appendSampleBuffer:sampleBuffer];
                return YES;
            }
        }
    }
    return NO;
}