开始学习核心数据和异步调度.有一段代码可以从数据集中获取图像的url，并将其设置为核心数据模型，如下所示
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSString *urlString = [[[photoDictionary valueForKey:@"images"] objectAtIndex:0] valueForKey:@"url"];
NSData *imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString:urlString]];
dispatch_async(dispatch_get_main_queue(), ^{
[photoModel setValue:imageData forKey:@"photoImageData"];
有人能给我解释一下为什么外部调度用dispatch_get_global_queue，内部调度用dispatch_get_main_queue.