AVAssetExportSession fileLengthLimit ignored and estimatedOutputFileLength returns 0

Question

I am reencoding video using AVAssetExportSession and I want to try and keep the resulting file size below a limit. My final call looks like this:

        NSArray *compatiblePresets = [AVAssetExportSession exportPresetsCompatibleWithAsset:avAsset];
        if ([compatiblePresets containsObject:AVAssetExportPresetMediumQuality])
        {
            AVAssetExportSession *exportSession = [[AVAssetExportSession alloc]initWithAsset:mixComposition presetName:AVAssetExportPresetMediumQuality];
            exportSession.outputURL = outputURL;
            exportSession.fileLengthLimit = 600000;
            exportSession.outputFileType = AVFileTypeMPEG4;
            exportSession.shouldOptimizeForNetworkUse = YES;
            exportSession.videoComposition = mainCompositionInst;
            NSLog(@"bytes = %lli", exportSession.estimatedOutputFileLength);
            [exportSession exportAsynchronouslyWithCompletionHandler:^{
            switch ([exportSession status])
            {
                case AVAssetExportSessionStatusFailed:
                    NSLog(@"Export failed: %@ : %@", [[exportSession error] localizedDescription], [exportSession error]);
                    handler(nil);

                    break;
                case AVAssetExportSessionStatusCancelled:

                    NSLog(@"Export canceled");
                    handler(nil);

                    break;
                default:

                    handler(outputURL);

                    break;

            }
            }];
         }

however estimatedOutputFileLength always returns 0 and fileLengthLimit seems to be totally ignored. I wanted to use estimatedOutputFileLength to determine whether to use Medium or Low quality encoding.

Could this be an iOS bug? Has anyone got these 2 properties to work?

Answer 1

Adding a note for posterity, regarding the exstimatedOutputFileLength always returning 0, I had to add a timeRange to the export session.

exportSession.timeRange = CMTimeRangeMake(kCMTimeZero, [asset duration]);