ios异步加载头像

    //绘制一张图片    _user_Image = [UIImage imageNamed:@"headImage.jpg"];        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^ {                NSString *user_Image_UrlString = @"http://photo.l99.com/bigger/31/1363231021567_5zu910.jpg";        NSURL *url = [NSURL URLWithString:[@"http://" stringByAppendingString:user_Image_UrlString ]];        NSError *error = nil;         NSData* data = [NSData dataWithContentsOfURL:url options:0 error:&error];        UIImage *imageStorage = [UIImage imageWithData:data];                //存入沙盒        NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,NSUserDomainMask, YES);        NSString *filePath = [[paths objectAtIndex:0] stringByAppendingString:@"/userImage.jpg"];   // 保存文件的名称        BOOL result = [UIImageJPEGRepresentation(imageStorage,1) writeToFile:filePath atomically:YES]; // 保存成功会返回YES                UIImage *image = [UIImage imageWithContentsOfFile:filePath];                dispatch_sync(dispatch_get_main_queue(), ^{                        if (image != nil) {                _user_Image = image;                [_tableView reloadData];            }        });    });