SDWebimage源码分析这家伙其实就是完成了异步解压缩+缓存策略options的所有枚举类型typedefNS_OPTIONS(NSUInteger,SDWebImageOptions){　//失败后重试SDWebImageRetryFailed=1SDWebImage内部实现过程1.入口setImageWithURL:placeholderImage:options:会先把placeholderImage显示，然后SDWebImageManager根据URL开始处理图片进入SDWebImageManager-downloadWithURL:delegate:options:userInf