使用SDWebImageDownloader隐藏和现实activityIndicator。
在每个button加载图片类：
__block UIActivityIndicatorView *activityIndicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
activityIndicator.center = mainImage.center;
activityIndicator.hidesWhenStopped = YES;
[activityIndicator startAnimating];
[imgView setImageWithURL:url placeholderImage:[UIImage imageNamed:@"placeholderLarge.png"] options:0 andResize:CGSizeMake(mainImage1.frame.size.width,imgview.frame.size.height) withContentMode:UIViewContentModeScaleAspectFit];
[imgView addSubview:activityIndicator];
没得到可以在哪移除activityIndicator。