SDWebImage 和 AFNetWork 异步加载网络图片

SDWebImage是Github开源项目：https://github.com/rs/SDWebImage，它用于方便的请求、Cache网络图片，并且请求完毕后交由UIImageView显示。

Asynchronous image downloader with cache support with an UIImageView category.

SDWebImage作为UIImageView的一个Category提供的，所以使用起来非常简单：

// Here we use the new provided setImageWithURL: method to load the web image
[imageView setImageWithURL:[NSURL URLWithString:@ "http://www.domain.com/path/to/image.jpg" ]
               placeholderImage:[UIImage imageNamed:@ "placeholder.png" ]];

AFNetworking也提供了类似功能（UIImageView+AFNetworking）：

UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(0.0f, 0.0f, 100.0f, 100.0f)];
[imageView setImageWithURL:[NSURL URLWithString:@ "http://i.imgur.com/r4uwx.jpg" ] placeholderImage:[UIImage imageNamed:@ "placeholder-avatar" ]];