使用SDWebImage 的一个方法完美解决
[[SDWebImageDownloader sharedDownloader]downloadImageWithURL:[NSURL URLWithString:string] options:SDWebImageDownloaderUseNSURLCache progress:^(NSInteger receivedSize, NSInteger expectedSize) {
} completed:^(UIImage *image, NSData *data, NSError *error, BOOL finished) {
//这边就能拿到图片了
nslog(@"%f",image.heigh);
}];