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：

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// 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）：

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1 2	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"]];