场景：相册视频导出，导出出现下面错误：

Error Domain=AVFoundationErrorDomain Code=-11800 "这项操作无法完成" UserInfo={NSLocalizedFailureReason=发生未知错误（-12769）, NSLocalizedDescription=这项操作无法完成, NSUnderlyingError=0x281c65ec0 {Error Domain=NSOSStatusErrorDomain Code=-12769 "(null)"}}

出现此错误，

1）、首先检查传入的输出路径是否有问题；

2）、presetName是否支持

3）、若其他配置参数都正确，还考虑是否为该视频编码有问题，可能是生成该视频时编码器出现了问题，可以通过下面方式让AVAssetExportSession重新编码来解决:

AVMutableComposition*mainComposition=[[AVMutableComposition alloc]init];

AVMutableCompositionTrack*compositionVideoTrack=[mainComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];

inttimeScale=100000;

Float64 seconds=CMTimeGetSeconds([avAsset duration])-0.001;

NSUInteger videoDurationI=(NSUInteger)(seconds*timeScale);

CMTime videoDuration=CMTimeMake(videoDurationI,timeScale);

CMTimeRange videoTimeRange=CMTimeRangeMake(kCMTimeZero,videoDuration);

NSArray<AVAssetTrack*>*videoTracks=[avAsset tracksWithMediaType:AVMediaTypeVideo];

AVAssetTrack*videoTrack=[videoTracks objectAtIndex:0];

[compositionVideoTrack insertTimeRange:videoTimeRange ofTrack:videoTrack atTime:kCMTimeZero error:nil];

 if ([compatiblePresets containsObject:AVAssetExportPresetMediumQuality]) {

        AVAssetExportSession *exportSession = [[AVAssetExportSession alloc]initWithAsset:mainComposition presetName:AVAssetExportPresetMediumQuality];

        NSString *mp4Path = [NSString stringWithFormat:@"%@/%d%d.mp4", [self _getAudioOrVideoPath], (int)[[NSDate date] timeIntervalSince1970], arc4random() % 100000];

        mp4Url = [NSURLfileURLWithPath:mp4Path];

        exportSession.outputURL= mp4Url;

        exportSession.shouldOptimizeForNetworkUse = YES;

        exportSession.outputFileType = AVFileTypeMPEG4;

        dispatch_semaphore_t wait = dispatch_semaphore_create(0l);

        [exportSessionexportAsynchronouslyWithCompletionHandler:^{

            switch([exportSessionstatus]) {

                case AVAssetExportSessionStatusFailed: {

                    NSLog(@"failed, error:%@.", exportSession.error);

                }break;

                case AVAssetExportSessionStatusCancelled: {

                    NSLog(@"cancelled.");

                }break;

                case AVAssetExportSessionStatusCompleted: {

                    NSLog(@"completed.");

                    [selfalertUploadVideo:mp4UrlwithImage:image];

                }break;

                default: {

                    NSLog(@"others.");

                }break;

            }

            dispatch_semaphore_signal(wait);

        }];

        longtimeout =dispatch_semaphore_wait(wait,DISPATCH_TIME_FOREVER);

        if(timeout) {

            NSLog(@"timeout.");

        }

        if(wait) {

            //dispatch_release(wait);

            wait =nil;

        }

    }