SDWebImage源码探究（二）

一、入口

• 常用的类 #import "UIImageView+WebCache.h"

• /**
   * Set the imageView `image` with an `url`.
   *
   * The download is asynchronous and cached.
   *
   * @param url The url for the image.
   */
  - (void)sd_setImageWithURL:(nullable NSURL *)url;

  上面这个方法，会调用下面其他的几个方法，分别如下，其中最下面的这个方法是参数最齐全的。

• /**
   * Set the imageView `image` with an `url` and a placeholder.
   *
   * The download is asynchronous and cached.
   *
   * @param url         The url for the image.
   * @param placeholder The image to be set initially, until the image request finishes.
   * @see sd_setImageWithURL:placeholderImage:options:
   */
  - (void)sd_setImageWithURL:(nullable NSURL *)url
            placeholderImage:(nullable UIImage *)placeholder;

  /**
   * Set the imageView `image` with an `url`, placeholder and custom options.
   *
   * The download is asynchronous and cached.
   *
   * @param url         The url for the image.
   * @param placeholder The image to be set initially, until the image request finishes.
   * @param options     The options to use when downloading the image. @see SDWebImageOptions for the possible values.
   */
  - (void)sd_setImageWithURL:(nullable NSURL *)url
            placeholderImage:(nullable UIImage *)placeholder
                     options:(SDWebImageOptions)options;

  /**
   * Set the imageView `image` with an `url`.
   *
   * The download is asynchronous and cached.
   *
   * @param url            The url for the image.
   * @param completedBlock A block called when operation has been completed. This block has no return value
   *                       and takes the requested UIImage as first parameter. In case of error the image parameter
   *                       is nil and the second parameter may contain an NSError. The third parameter is a Boolean
   *                       indicating if the image was retrieved from the local cache or from the network.
   *                       The fourth parameter is the original image url.
   */
  - (void)sd_setImageWithURL:(nullable NSURL *)url
                   completed:(nullable SDExternalCompletionBlock)completedBlock;

  /**
   * Set the imageView `image` with an `url`, placeholder.
   *
   * The download is asynchronous and cached.
   *
   * @param url            The url for the image.
   * @param placeholder    The image to be set initially, until the image request finishes.
   * @param completedBlock A block called when operation has been completed. This block has no return value
   *                       and takes the requested UIImage as first parameter. In case of error the image parameter
   *                       is nil and the second parameter may contain an NSError. The third parameter is a Boolean
   *                       indicating if the image was retrieved from the local cache or from the network.
   *                       The fourth parameter is the original image url.
   */
  - (void)sd_setImageWithURL:(nullable NSURL *)url
            placeholderImage:(nullable UIImage *)placeholder
                   completed:(nullable SDExternalCompletionBlock)completedBlock;

  /**
   * Set the imageView `image` with an `url`, placeholder and custom options.
   *
   * The download is asynchronous and cached.
   *
   * @param url            The url for the image.
   * @param placeholder    The image to be set initially, until the image request finishes.
   * @param options        The options to use when downloading the image. @see SDWebImageOptions for the possible values.
   * @param completedBlock A block called when operation has been completed. This block has no return value
   *                       and takes the requested UIImage as first parameter. In case of error the image parameter
   *                       is nil and the second parameter may contain an NSError. The third parameter is a Boolean
   *                       indicating if the image was retrieved from the local cache or from the network.
   *                       The fourth parameter is the original image url.
   */
  - (void)sd_setImageWithURL:(nullable NSURL *)url
            placeholderImage:(nullable UIImage *)placeholder
                     options:(SDWebImageOptions)options
                   completed:(nullable SDExternalCompletionBlock)completedBlock;

  /**
   * Set the imageView `image` with an `url`, placeholder and custom options.
   *
   * The download is asynchronous and cached.
   *
   * @param url            The url for the image.
   * @param placeholder    The image to be set initially, until the image request finishes.
   * @param options        The options to use when downloading the image. @see SDWebImageOptions for the possible values.
   * @param progressBlock  A block called while image is downloading
   *                       @note the progress block is executed on a background queue
   * @param completedBlock A block called when operation has been completed. This block has no return value
   *                       and takes the requested UIImage as first parameter. In case of error the image parameter
   *                       is nil and the second parameter may contain an NSError. The third parameter is a Boolean
   *                       indicating if the image was retrieved from the local cache or from the network.
   *                       The fourth parameter is the original image url.
   */
  - (void)sd_setImageWithURL:(nullable NSURL *)url
            placeholderImage:(nullable UIImage *)placeholder
                     options:(SDWebImageOptions)options
                    progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                   completed:(nullable SDExternalCompletionBlock)completedBlock;

  入口就这些，下面继续研究

二、原理

• 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SDWebImageManagerdownloadWithURL:delegate:options:userInfo:，交给 SDImageCache 从缓存查找图片是否已经下载 queryDiskCacheForKey:delegate:userInfo:
• 先从内存图片缓存查找是否有图片，如果内存中已经有图片缓存，SDImageCacheDelegate 回调imageCache:didFindImage:forKey:userInfo: 到 SDWebImageManager。
• SDWebImageManagerDelegate 回调webImageManager:didFinishWithImage:到 UIImageView+WebCache 等前端展示图片。
• 如果内存缓存中没有，生成 NSInvocationOperation ，添加到队列开始从硬盘查找图片是否已经缓存。
• 根据 URLKey 在硬盘缓存目录下尝试读取图片文件。这一步是在 NSOperation 进行的操作，所以回主线程进行结果回调 notifyDelegate:。

• 如果上一操作从硬盘读取到了图片，将图片添加到内存缓存中
  （如果空闲内存过小，会先清空内存缓存）。 SDImageCacheDelegate 回调 imageCache:didFindImage:forKey:userInfo:。进而回调展示图片。

• 如果从硬盘缓存目录读取不到图片，说明所有缓存都不存在该图片，需要下载图片，回调 imageCache:didNotFindImageForKey:userInfo:。
• 共享或重新生成一个下载器 SDWebImageDownloader 开始下载图片。
• 图片下载由NSURLSession(NSURLConnection已经被剔除了) 来做，实现相关 delegate 来判断图片下载中、下载完成和下载失败。
• session:didReceiveData: 中利用 ImageIO做了按图片下载进度加载效果。
• sessionDidFinishLoading:数据下载完成后交给SDWebImageDecoder 做图片解码处理。
• 图片解码处理在一个NSOperationQueue完成，不会拖慢主线程 UI。如果有需要对下载的图片进行二次处理，最好也在这里完成，效率会好很多。
• 在主线程 notifyDelegateOnMainThreadWithInfo: 宣告解码完成，imageDecoder:didFinishDecodingImage:userInfo:回调给 SDWebImageDownloader。
• imageDownloader:didFinishWithImage:回调给 SDWebImageManager 告知图片下载完成。
• 通知所有的 downloadDelegates下载完成，回调给需要的地方展示图片
• 将图片保存到 SDImageCache 中，内存缓存和硬盘缓存同时保存。

• 写文件到硬盘也在以单独 NSInvocationOperation 完成，
  避免拖慢主线程。

• SDImageCache 在初始化的时候会注册一些消息通知，在内存警告或退到后台的时候清理内存图片缓存，应用结束的时候清理过期图片。
• DWI 也提供了 UIButton+WebCache 和 MKAnnotationView+WebCache，方便使用。
• SDWebImagePrefetcher 可以预先下载图片，方便后续使用。

总的流程图如下：

未完、待续、、、