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POAVCaptureMovieFileOutput -- trimming the file while writing
 

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    <p>I am recording video using <code>AVCaptureMovieFileOutput</code>. Instead of retaining the captured video for the entire recording time, however, I would like only to retain the last 2 minutes of video. In essence, I would like to create a trailing buffer of video.</p> <p>I have tried to implement this by setting <code>movieFragmentInterval</code> equal to 15 seconds. As these 15 seconds buffered, the first 15 seconds of the MOV file would be trimmed off using this code:</p> <pre><code>//This would be called 7 seconds after the video stream started buffering. -(void)startTrimTimer { trimTimer = [NSTimer scheduledTimerWithTimeInterval:15 target:self selector:@selector(trimFlashbackBuffer) userInfo:nil repeats:YES]; } -(void)trimFlashbackBuffer { //make sure that there is enough video before trimming off 15 seconds if(trimReadyCount&lt;3){ trimReadyCount++; return; } AVURLAsset *videoAsset = [AVURLAsset URLAssetWithURL:[NSURL fileURLWithPath:[NSString stringWithFormat:@"%@/flashbackBuffer.MOV",tripDirectory]] options:nil]; AVAssetExportSession *exportSession = [[AVAssetExportSession alloc] initWithAsset:videoAsset presetName:AVAssetExportPresetHighestQuality]; exportSession.outputURL = [NSURL fileURLWithPath:[NSString stringWithFormat:@"%@/flashbackBuffer.MOV",tripDirectory]]; exportSession.outputFileType = AVFileTypeQuickTimeMovie; CMTimeRange timeRange = CMTimeRangeMake(CMTimeMake(15000, 1000), CMTimeMake(120000, 1000)); exportSession.timeRange = timeRange; [exportSession exportAsynchronouslyWithCompletionHandler:^{ switch (exportSession.status) { case AVAssetExportSessionStatusCompleted: // Custom method to import the Exported Video [self loadAssetFromFile:exportSession.outputURL]; break; case AVAssetExportSessionStatusFailed: // NSLog(@"Failed:%@",exportSession.error); break; case AVAssetExportSessionStatusCancelled: // NSLog(@"Canceled:%@",exportSession.error); break; default: break; } }]; } </code></pre> <p>However, I am receiving the following error every time <code>trimFlashbackBuffer</code> is called:</p> <pre><code>Failed:Error Domain=AVFoundationErrorDomain Code=-11823 "Cannot Save" UserInfo=0x12e710 {NSLocalizedRecoverySuggestion=Try saving again., NSLocalizedDescription=Cannot Save} </code></pre> <p>Is this because the file is already being written to by <code>AVCaptureMovieFileOutput</code>?</p> <p>How could I achieve the effect of a seamless trailing video buffer, if this method cannot work?</p> <p>Thanks!</p>
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