遵循于： NSObject
声明处：SDWebImageDownloaderOperation.h
概述
描述一个下载操作。如果想使用一个自定义的下载操作，需要继承NSOpertaion并遵循这些协议。
任务
-
- initWithRequest:inSession:options:必须实现 -
- addHandlerForProgress:completed:必须实现 -
shouldDecompressImages:必须实现 -
setShouldDecompressImages:必须实现 -
credential必须实现 -
setCredential:必须实现
实例方法
addHandlersForProgress:completed:
- (nullable id)addHandlersForProgress:(nullable SDWebImageDownloaderProgressBlock)progressBlock completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock
credential
- (nullable NSURLCredential *)credential
initWithRequest:inSession:options:
- (nonnull instancetype)initWithRequest:(nullable NSURLRequest *)request inSession:(nullable NSURLSession *)session options:(SDWebImageDownloaderOptions)options
setCredential:
- (void)setCredential:(nullable NSURLCredential *)value
setShouldDecompressImages:
- (void)setShouldDecompressImages:(BOOL)value
shouldDecompressImages
- (BOOL)shouldDecompressImages
// END 呵呵哒，文档中都不需要解析怎么用，只要实现就好的。
