NSString *urlStr；
NSUrl *url = [NSURL URLWithString:urlStr];
//缓存图片
SDWebImageManager *manager = [SDWebImageManager sharedManager];
if (![manager diskImageExistsForURL:url]) {
[manager downloadWithURL:url options:0 progress:^(NSInteger receivedSize, NSInteger expectedSize) {
} completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished) {
}];
}
//根据缓存地址读取图片
SDImageCache *imageCache = [SDImageCache sharedImageCache];
UIImage *image = [imageCache imageFromDiskCacheForKey:urlStr];