IOS开发笔记之浅读SDWebImage实现原理

SDWebImage内部实现过程(新版本在各方法前加上了sd_前缀，以区分UIImageView+AFNetworking中的方法)

• 1.启动入口 setImageWithURL:placeholderImage:options:会先把 placeholderImage(占位图片)显示，然后 SDWebImageManager 根据 URL 开始处理图片。

• 2.进入 SDWebImageManager-downloadWithURL:delegate:options:userInfo:，交给 SDImageCache 从缓存查找图片是否已经下载queryDiskCacheForKey:delegate:userInfo:

• 3.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。

• 4.SDWebImageManagerDelegate 回调webImageManager:didFinishWithImage:到 UIImageView+WebCache 等前端展示图片。

• 5.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。

• 6.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:。

• 7.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。

• 8.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:。

• 9.共享或重新生成一个下载器 SDWebImageDownloader 开始下载图片。

• 10.图片下载由 NSURLConnection 来做，实现相关 delegate 来判断图片下载中、下载完成和下载失败。

• 11.connection:didReceiveData:中利用 ImageIO 做了按图片下载进度加载效果。

• 12.connectionDidFinishLoading: 数据下载完成后交给 SDWebImageDecoder 做图片解码处理。

• 13.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。

• 14.在主线程notifyDelegateOnMainThreadWithInfo:宣告解码完成，imageDecoder:didFinishDecodingImage:userInfo:回调给 SDWebImageDownloader。

• 15.imageDownloader:didFinishWithImage:回调给 SDWebImageManager 告知图片下载完成。

• 16.通知所有的 downloadDelegates 下载完成，回调给需要的地方展示图片。

• 17.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。

• 18.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。

• 19.SDWI 也提供了 UIButton+WebCache 和MKAnnotationView+WebCache，方便使用。

• 20.SDWebImagePrefetcher 可以预先下载图片，方便后续使用。从上面流程可以看出，当你调用setImageWithURL:方法的时候，它会自动去给你干这么多事，当你需要在某一具体时刻做事情的时候，你可以覆盖这些方法。比如在下载某个图片的过程中要响应一个事件，就覆盖这个方法：

覆盖方法，指哪打哪，这个方法是下载imagePath2的时候响应

SDWebImageManager *manager = [SDWebImageManager sharedManager]; [manager downloadImageWithURL:imagePath2 options:SDWebImageRetryFailed progress:^(NSInteger receivedSize, NSInteger expectedSize) { NSLog(@"显示当前进度"); } completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) { NSLog(@"下载完成"); }];

当服务器更新了某一张图片资源时，客户端需要重新加载，那么就可以设置SDWebImageOption为SDWebImageRefreshCached；附上全部的SDWebImageOptions

typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
/**
 * By default, when a URL fail to be downloaded, the URL is blacklisted so the library won't keep trying.
 * This flag disable this blacklisting.
 */
SDWebImageRetryFailed = 1 << 0,

/**
 * By default, image downloads are started during UI interactions, this flags disable this feature,
 * leading to delayed download on UIScrollView deceleration for instance.
 */
SDWebImageLowPriority = 1 << 1,

/**
 * This flag disables on-disk caching
 */
SDWebImageCacheMemoryOnly = 1 << 2,

/**
 * This flag enables progressive download, the image is displayed progressively during download as a browser would do.
 * By default, the image is only displayed once completely downloaded.
 */
SDWebImageProgressiveDownload = 1 << 3,

/**
 * Even if the image is cached, respect the HTTP response cache control, and refresh the image from remote location if needed.
 * The disk caching will be handled by NSURLCache instead of SDWebImage leading to slight performance degradation.
 * This option helps deal with images changing behind the same request URL, e.g. Facebook graph api profile pics.
 * If a cached image is refreshed, the completion block is called once with the cached image and again with the final image.
 *
 * Use this flag only if you can't make your URLs static with embeded cache busting parameter.
 */
SDWebImageRefreshCached = 1 << 4,

/**
 * In iOS 4+, continue the download of the image if the app goes to background. This is achieved by asking the system for
 * extra time in background to let the request finish. If the background task expires the operation will be cancelled.
 */
SDWebImageContinueInBackground = 1 << 5,

/**
 * Handles cookies stored in NSHTTPCookieStore by setting
 * NSMutableURLRequest.HTTPShouldHandleCookies = YES;
 */
SDWebImageHandleCookies = 1 << 6,

/**
 * Enable to allow untrusted SSL ceriticates.
 * Useful for testing purposes. Use with caution in production.
 */
SDWebImageAllowInvalidSSLCertificates = 1 << 7,

/**
 * By default, image are loaded in the order they were queued. This flag move them to
 * the front of the queue and is loaded immediately instead of waiting for the current queue to be loaded (which 
 * could take a while).
 */
SDWebImageHighPriority = 1 << 8,

/**
 * By default, placeholder images are loaded while the image is loading. This flag will delay the loading
 * of the placeholder image until after the image has finished loading.
 */
SDWebImageDelayPlaceholder = 1 << 9,

/**
 * We usually don't call transformDownloadedImage delegate method on animated images,
 * as most transformation code would mangle it.
 * Use this flag to transform them anyway.
 */
SDWebImageTransformAnimatedImage = 1 << 10,
};