How to download photos concurrently

[JianBinWu]

for (NSNumber *number in self.selectedIndexPathRows) {
        NSInteger row = [number integerValue];
        DJIMediaFile *mediaFile = self.mediaList[row];
        __block NSUInteger previousOffset = 0;
        __block NSMutableData *fileData = nil;
        [mediaFile fetchFileDataWithOffset:previousOffset updateQueue:dispatch_get_main_queue() updateBlock:^(NSData * _Nullable data, BOOL isComplete, NSError * _Nullable error) {
            if (error) {
                DMLog(@"error---%@", error);
            }
            if (fileData == nil) {
                fileData = [data mutableCopy];
            }
            else {
                [fileData appendData:data];
            }
            previousOffset += data.length;
            float progress = previousOffset * 100.0 / mediaFile.fileSizeInBytes;
            DMLog(@"%@-------Downloading: %0.1f%%", mediaFile.fileName, progress);
            if (previousOffset == mediaFile.fileSizeInBytes && isComplete) {
                [target showPhotoWithData:fileData];
                [target savePhotoWithData:fileData];
            }
        }];
    }

I want to use this code to download photos concurrently, But I always get error, How can I do to download photos concurrently.