SDWebImage

在https://github.com/rs/SDWebImage这个网站上下载SDWebImage开源包，加到我们的工程中

导入MapKit.framework、ImageIO.framework两个框架

#import "UIImageView+WebCache.h"

[imageView setImageWithURL:[NSURL URLWithString:@"网络图片地址"] placeholderImage:[UIImage imageNamed:@"placeholder.jpg"] success:^(UIImage *image){} failure:^(NSError *error){} ];