在iOS中，摄像头录制的视频是mov格式的，虽然mov兼容mp4，但是有些需求需要用到mp4格式的视频文件。

AVURLAsset *avAsset = [AVURLAsset URLAssetWithURL:[NSURL fileURLWithPath:path] options:nil];NSArray *compatiblePresets = [AVAssetExportSession exportPresetsCompatibleWithAsset:avAsset];if ([compatiblePresets containsObject:AVAssetExportPresetLowQuality]){AVAssetExportSession *exportSession = [[AVAssetExportSession alloc]initWithAsset:avAsset presetName:AVAssetExportPresetPassthrough];    NSString *exportPath = [NSString stringWithFormat:@"%@/%@.mp4",                            [NSHomeDirectory() stringByAppendingString:@"/tmp"],                            @"1"];    exportSession.outputURL = [NSURL fileURLWithPath:exportPath];    NSLog(@"%@", exportPath);    exportSession.outputFileType = AVFileTypeMPEG4;    [exportSession exportAsynchronouslyWithCompletionHandler:^{        switch ([exportSession status]) {            case AVAssetExportSessionStatusFailed:                NSLog(@"Export failed: %@", [[exportSession error] localizedDescription]);                break;            case AVAssetExportSessionStatusCancelled:                NSLog(@"Export canceled");                break;                case AVAssetExportSessionStatusCompleted:                NSLog(@"转换成功");                break;            default:                break;        }    }];}