// 合成 我的视频大小都是一样的 所以没有对他们进行缩放裁剪等操作 

- (void)mergeAndExportVideos {

    if (videoArray.count==0) {

        return;

    }

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

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

　　/// 从mixComposition取出音视频 轨道

        AVMutableCompositionTrack *audioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];

        AVMutableCompositionTrack *videoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];

        CMTime totalDuration = kCMTimeZero;

        NSError *error = nil;

        for (int i = 0; i < [videoArray count]; i++) {

            AVAsset *asset = [AVAsset assetWithURL:[[NSURL alloc] initFileURLWithPath:videoArray[i]]];

            

            [audioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero,asset.duration)

                                ofTrack:[[asset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0]

                                 atTime:totalDuration

                                  error:nil];

            

            [videoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero,asset.duration)

                                ofTrack:[[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0]

                                 atTime:totalDuration

                                  error:&error];

            totalDuration = CMTimeAdd(totalDuration, asset.duration);

        }

　　/// 合成视频文件存储的路径

        NSString *path = [self getVideoMergeFilePathString];

        NSURL *mergeFileURL = [NSURL fileURLWithPath:path];

        AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPreset640x480];

        exporter.outputURL = mergeFileURL;

        exporter.outputFileType = AVFileTypeQuickTimeMovie;

        exporter.shouldOptimizeForNetworkUse = YES;

        [exporter exportAsynchronouslyWithCompletionHandler:^{

            for (NSString *path in videoArray) {

                [self delFromLocal:path];

            }

            [videoArray removeAllObjects];

//            [self savePhotoCmare:path];//写入相册

      

        }];

    });

}