SDWebImage內(nèi)部實(shí)現(xiàn)過(guò)程(新版本在各方法前加上了sd_前綴，以區(qū)分UIImageView+AFNetworking中的方法)

• 1.啟動(dòng)入口 setImageWithURL:placeholderImage:options:會(huì)先把 placeholderImage(占位圖片)顯示，然后 SDWebImageManager 根據(jù) URL 開(kāi)始處理圖片。

• 2.進(jìn)入 SDWebImageManager-downloadWithURL:delegate:options:userInfo:，交給 SDImageCache 從緩存查找圖片是否已經(jīng)下載queryDiskCacheForKey:delegate:userInfo:

• 3.先從內(nèi)存圖片緩存查找是否有圖片，如果內(nèi)存中已經(jīng)有圖片緩存，SDImageCacheDelegate 回調(diào)imageCache:didFindImage:forKey:userInfo:到 SDWebImageManager。

• 4.SDWebImageManagerDelegate 回調(diào)webImageManager:didFinishWithImage:到 UIImageView+WebCache 等前端展示圖片。

• 5.如果內(nèi)存緩存中沒(méi)有，生成 NSInvocationOperation 添加到隊(duì)列開(kāi)始從硬盤(pán)查找圖片是否已經(jīng)緩存。

• 6.根據(jù) URLKey 在硬盤(pán)緩存目錄下嘗試讀取圖片文件。這一步是在 NSOperation 進(jìn)行的操作，所以回主線程進(jìn)行結(jié)果回調(diào) notifyDelegate:。

• 7.如果上一操作從硬盤(pán)讀取到了圖片，將圖片添加到內(nèi)存緩存中（如果空閑內(nèi)存過(guò)小，會(huì)先清空內(nèi)存緩存）。SDImageCacheDelegate 回調(diào)imageCache:didFindImage:forKey:userInfo:。進(jìn)而回調(diào)展示圖片。

• 8.如果從硬盤(pán)緩存目錄讀取不到圖片，說(shuō)明所有緩存都不存在該圖片，需要下載圖片，回調(diào)imageCache:didNotFindImageForKey:userInfo:。

• 9.共享或重新生成一個(gè)下載器 SDWebImageDownloader 開(kāi)始下載圖片。

• 10.圖片下載由 NSURLConnection 來(lái)做，實(shí)現(xiàn)相關(guān) delegate 來(lái)判斷圖片下載中、下載完成和下載失敗。

• 11.connection:didReceiveData:中利用 ImageIO 做了按圖片下載進(jìn)度加載效果。

• 12.connectionDidFinishLoading: 數(shù)據(jù)下載完成后交給 SDWebImageDecoder 做圖片解碼處理。

• 13.圖片解碼處理在一個(gè) NSOperationQueue 完成，不會(huì)拖慢主線程 UI。如果有需要對(duì)下載的圖片進(jìn)行二次處理，最好也在這里完成，效率會(huì)好很多。

• 14.在主線程notifyDelegateOnMainThreadWithInfo:宣告解碼完成，imageDecoder:didFinishDecodingImage:userInfo:回調(diào)給 SDWebImageDownloader。

• 15.imageDownloader:didFinishWithImage:回調(diào)給 SDWebImageManager 告知圖片下載完成。

• 16.通知所有的 downloadDelegates 下載完成，回調(diào)給需要的地方展示圖片。

• 17.將圖片保存到 SDImageCache 中，內(nèi)存緩存和硬盤(pán)緩存同時(shí)保存。寫(xiě)文件到硬盤(pán)也在以單獨(dú) NSInvocationOperation 完成，避免拖慢主線程。

• 18.SDImageCache 在初始化的時(shí)候會(huì)注冊(cè)一些消息通知，在內(nèi)存警告或退到后臺(tái)的時(shí)候清理內(nèi)存圖片緩存，應(yīng)用結(jié)束的時(shí)候清理過(guò)期圖片。

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

• 20.SDWebImagePrefetcher 可以預(yù)先下載圖片，方便后續(xù)使用。從上面流程可以看出，當(dāng)你調(diào)用setImageWithURL:方法的時(shí)候，它會(huì)自動(dòng)去給你干這么多事，當(dāng)你需要在某一具體時(shí)刻做事情的時(shí)候，你可以覆蓋這些方法。比如在下載某個(gè)圖片的過(guò)程中要響應(yīng)一個(gè)事件，就覆蓋這個(gè)方法：

覆蓋方法，指哪打哪，這個(gè)方法是下載imagePath2的時(shí)候響應(yīng)

SDWebImageManager *manager = [SDWebImageManager sharedManager]; [manager downloadImageWithURL:imagePath2 options:SDWebImageRetryFailed progress:^(NSInteger receivedSize, NSInteger expectedSize) { NSLog(@"顯示當(dāng)前進(jìn)度"); } completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) { NSLog(@"下載完成"); }];

當(dāng)服務(wù)器更新了某一張圖片資源時(shí)，客戶端需要重新加載，那么就可以設(shè)置SDWebImageOption為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,
};