一、库功能

主要提供了网络图片异步下载的功能。使ui展示大量网络图片的时候，能不堵塞主线程，保存界面流畅；同时，能有策略的异步下载图片，并正确的赋值到对应的控件上。sd库在异步下载的基础上，提供了磁盘缓存，提供了图片异步解压，尽可能的提高界面的体验。官方地址:https://github.com/SDWebImage/SDWebImage

二、整体需求

• 需求：imageview 把url作为入参，发起图片异步下载并在下载完毕后回调显示。

• 功能细分

  1. 图片完成下载后要正确的赋值给调用的控件，也即两者中要建立一个一对一的绑定关系。
  2. 图片异步下载的实现
     1. 包括单个下载的请求组装，状态控制，回调处理，图片预解压等。
     2. 请求并发控制。
  3. 图片缓存机制的实现
     1. 磁盘读写io的安全控制。
     2. 磁盘管理的安全策略设计。

三、 技术的实现

• 通过创建对应的id <SDWebImageOperation> operation,回调中对imageView赋值。

- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
                  placeholderImage:(nullable UIImage *)placeholder
                           options:(SDWebImageOptions)options
                      operationKey:(nullable NSString *)operationKey
             internalSetImageBlock:(nullable SDInternalSetImageBlock)setImageBlock
                          progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                         completed:(nullable SDExternalCompletionBlock)completedBlock
                           context:(nullable NSDictionary<NSString *, id> *)context {
  SDWebImageManager *manager = [context objectForKey:SDWebImageExternalCustomManagerKey];
        if (!manager) {
        ///通过单例创建全局的SDWebImageManager
            manager = [SDWebImageManager sharedManager];
        }
        
        ///每一个imageView的setImage操作对应的生成一个实现了SDWebImageOperation协议的operation. SDWebImageOperation中有一个cannel方法，用于cannel下载行为。
 id <SDWebImageOperation> operation = [manager loadImageWithURL:url options:options progress:combinedProgressBlock completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
            __strong __typeof (wself) sself = wself;
            if (!sself) { return; }

///省略
            
            UIImage *targetImage = nil;
            NSData *targetData = nil;
            if (image) {
                // case 2a: we got an image and the SDWebImageAvoidAutoSetImage is not set
                targetImage = image;
                targetData = data;
            } else if (options & SDWebImageDelayPlaceholder) {
                // case 2b: we got no image and the SDWebImageDelayPlaceholder flag is set
                targetImage = placeholder;
                targetData = nil;
            }
            
            dispatch_main_async_safe(^{
                if (group) {
                    dispatch_group_enter(group);
                }
                ///进行赋值
                [sself sd_setImage:targetImage imageData:targetData basedOnClassOrViaCustomSetImageBlock:setImageBlock cacheType:cacheType imageURL:imageURL];
                if (group) {
                    // compatible code for FLAnimatedImage, because we assume completedBlock called after image was set. This will be removed in 5.x
                    BOOL shouldUseGroup = [objc_getAssociatedObject(group, &SDWebImageInternalSetImageGroupKey) boolValue];
                    if (shouldUseGroup) {
                        dispatch_group_notify(group, dispatch_get_main_queue(), callCompletedBlockClojure);
                    } else {
                        callCompletedBlockClojure();
                    }
                } else {
                    callCompletedBlockClojure();
                }
            });
        }];
    }

///这个方法内部包含缓存获取和发起下载的相关逻辑
- (id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
                                     options:(SDWebImageOptions)options
                                    progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                   completed:(nullable SDInternalCompletionBlock)completedBlock {
    // Invoking this method without a completedBlock is pointless
    NSAssert(completedBlock != nil, @"If you mean to prefetch the image, use -[SDWebImagePrefetcher prefetchURLs] instead");

    // Very common mistake is to send the URL using NSString object instead of NSURL. For some strange reason, Xcode won't
    // throw any warning for this type mismatch. Here we failsafe this error by allowing URLs to be passed as NSString.
    if ([url isKindOfClass:NSString.class]) {
        url = [NSURL URLWithString:(NSString *)url];
    }

    // Prevents app crashing on argument type error like sending NSNull instead of NSURL
    if (![url isKindOfClass:NSURL.class]) {
        url = nil;
    }

    SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
    operation.manager = self;

    BOOL isFailedUrl = NO;
    if (url) {
        LOCK(self.failedURLsLock);
        isFailedUrl = [self.failedURLs containsObject:url];
        UNLOCK(self.failedURLsLock);
    }

    if (url.absoluteString.length == 0 || (!(options & SDWebImageRetryFailed) && isFailedUrl)) {
        [self callCompletionBlockForOperation:operation completion:completedBlock error:[NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorFileDoesNotExist userInfo:nil] url:url];
        return operation;
    }

    LOCK(self.runningOperationsLock);
    [self.runningOperations addObject:operation];
    UNLOCK(self.runningOperationsLock);
    ///根据规则生成缓存key
    NSString *key = [self cacheKeyForURL:url];
    
    SDImageCacheOptions cacheOptions = 0;
    if (options & SDWebImageQueryDataWhenInMemory) cacheOptions |= SDImageCacheQueryDataWhenInMemory;
    if (options & SDWebImageQueryDiskSync) cacheOptions |= SDImageCacheQueryDiskSync;
    if (options & SDWebImageScaleDownLargeImages) cacheOptions |= SDImageCacheScaleDownLargeImages;
    
    __weak SDWebImageCombinedOperation *weakOperation = operation;
    ///去获取缓存，并在内部根据option参数判断后续的缓存处理和下载行为
    operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key options:cacheOptions done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {
        __strong __typeof(weakOperation) strongOperation = weakOperation;
        if (!strongOperation || strongOperation.isCancelled) {
            [self safelyRemoveOperationFromRunning:strongOperation];
            return;
        }
        
        // Check whether we should download image from network
        BOOL shouldDownload = (!(options & SDWebImageFromCacheOnly))
            && (!cachedImage || options & SDWebImageRefreshCached)
            && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url]);
        if (shouldDownload) {
            if (cachedImage && options & SDWebImageRefreshCached) {
                // If image was found in the cache but SDWebImageRefreshCached is provided, notify about the cached image
                // AND try to re-download it in order to let a chance to NSURLCache to refresh it from server.
                [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
            }

            // download if no image or requested to refresh anyway, and download allowed by delegate
            SDWebImageDownloaderOptions downloaderOptions = 0;
            if (options & SDWebImageLowPriority) downloaderOptions |= SDWebImageDownloaderLowPriority;
            if (options & SDWebImageProgressiveDownload) downloaderOptions |= SDWebImageDownloaderProgressiveDownload;
            if (options & SDWebImageRefreshCached) downloaderOptions |= SDWebImageDownloaderUseNSURLCache;
            if (options & SDWebImageContinueInBackground) downloaderOptions |= SDWebImageDownloaderContinueInBackground;
            if (options & SDWebImageHandleCookies) downloaderOptions |= SDWebImageDownloaderHandleCookies;
            if (options & SDWebImageAllowInvalidSSLCertificates) downloaderOptions |= SDWebImageDownloaderAllowInvalidSSLCertificates;
            if (options & SDWebImageHighPriority) downloaderOptions |= SDWebImageDownloaderHighPriority;
            if (options & SDWebImageScaleDownLargeImages) downloaderOptions |= SDWebImageDownloaderScaleDownLargeImages;
            
            if (cachedImage && options & SDWebImageRefreshCached) {
                // force progressive off if image already cached but forced refreshing
                downloaderOptions &= ~SDWebImageDownloaderProgressiveDownload;
                // ignore image read from NSURLCache if image if cached but force refreshing
                downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
            }
            
            // `SDWebImageCombinedOperation` -> `SDWebImageDownloadToken` -> `downloadOperationCancelToken`, which is a `SDCallbacksDictionary` and retain the completed block below, so we need weak-strong again to avoid retain cycle
            __weak typeof(strongOperation) weakSubOperation = strongOperation;
            ///发起下载，并保存downloadToken值，以便能找到对应的dowanLoadOperation，去执行cannel操作等。这个会在cannel操作方法中很直观的看到。
            strongOperation.downloadToken = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) {
                __strong typeof(weakSubOperation) strongSubOperation = weakSubOperation;
                if (!strongSubOperation || strongSubOperation.isCancelled) {
                    // Do nothing if the operation was cancelled
                    // See #699 for more details
                    // if we would call the completedBlock, there could be a race condition between this block and another completedBlock for the same object, so if this one is called second, we will overwrite the new data
                } else if (error) {
                    [self callCompletionBlockForOperation:strongSubOperation completion:completedBlock error:error url:url];
                    BOOL shouldBlockFailedURL;
                    // Check whether we should block failed url
                    if ([self.delegate respondsToSelector:@selector(imageManager:shouldBlockFailedURL:withError:)]) {
                        shouldBlockFailedURL = [self.delegate imageManager:self shouldBlockFailedURL:url withError:error];
                    } else {
                        shouldBlockFailedURL = (   error.code != NSURLErrorNotConnectedToInternet
                                                && error.code != NSURLErrorCancelled
                                                && error.code != NSURLErrorTimedOut
                                                && error.code != NSURLErrorInternationalRoamingOff
                                                && error.code != NSURLErrorDataNotAllowed
                                                && error.code != NSURLErrorCannotFindHost
                                                && error.code != NSURLErrorCannotConnectToHost
                                                && error.code != NSURLErrorNetworkConnectionLost);
                    }
                    
                    if (shouldBlockFailedURL) {
                        LOCK(self.failedURLsLock);
                        [self.failedURLs addObject:url];
                        UNLOCK(self.failedURLsLock);
                    }
                }
                else {
                    if ((options & SDWebImageRetryFailed)) {
                        LOCK(self.failedURLsLock);
                        [self.failedURLs removeObject:url];
                        UNLOCK(self.failedURLsLock);
                    }
                    
                    BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);
                    
                    // We've done the scale process in SDWebImageDownloader with the shared manager, this is used for custom manager and avoid extra scale.
                    if (self != [SDWebImageManager sharedManager] && self.cacheKeyFilter && downloadedImage) {
                        downloadedImage = [self scaledImageForKey:key image:downloadedImage];
                    }

                    if (options & SDWebImageRefreshCached && cachedImage && !downloadedImage) {
                        // Image refresh hit the NSURLCache cache, do not call the completion block
                    } else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)]) {
                        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
                            @autoreleasepool {
                                UIImage *transformedImage = [self.delegate imageManager:self transformDownloadedImage:downloadedImage withURL:url];
                                
                                if (transformedImage && finished) {
                                    BOOL imageWasTransformed = ![transformedImage isEqual:downloadedImage];
                                    NSData *cacheData;
                                    // pass nil if the image was transformed, so we can recalculate the data from the image
                                    if (self.cacheSerializer) {
                                        cacheData = self.cacheSerializer(transformedImage, (imageWasTransformed ? nil : downloadedData), url);
                                    } else {
                                        cacheData = (imageWasTransformed ? nil : downloadedData);
                                    }
                                    [self.imageCache storeImage:transformedImage imageData:cacheData forKey:key toDisk:cacheOnDisk completion:nil];
                                }
                                
                                [self callCompletionBlockForOperation:strongSubOperation completion:completedBlock image:transformedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                            }
                        });
                    } else {
                        if (downloadedImage && finished) {
                            if (self.cacheSerializer) {
                                dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
                                    @autoreleasepool {
                                        NSData *cacheData = self.cacheSerializer(downloadedImage, downloadedData, url);
                                        [self.imageCache storeImage:downloadedImage imageData:cacheData forKey:key toDisk:cacheOnDisk completion:nil];
                                    }
                                });
                            } else {
                                [self.imageCache storeImage:downloadedImage imageData:downloadedData forKey:key toDisk:cacheOnDisk completion:nil];
                            }
                        }
                        [self callCompletionBlockForOperation:strongSubOperation completion:completedBlock image:downloadedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                    }
                }

                if (finished) {
                    [self safelyRemoveOperationFromRunning:strongSubOperation];
                }
            }];
        } else if (cachedImage) {
            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
            [self safelyRemoveOperationFromRunning:strongOperation];
        } else {
            // Image not in cache and download disallowed by delegate
            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:nil data:nil error:nil cacheType:SDImageCacheTypeNone finished:YES url:url];
            [self safelyRemoveOperationFromRunning:strongOperation];
        }
    }];

    return operation;
}

SDWebImageOperation协议
@protocol SDWebImageOperation <NSObject>
- (void)cancel;
@end

///SDWebImageCombinedOperation的实现
@implementation SDWebImageCombinedOperation
- (void)cancel {
    @synchronized(self) {
        self.cancelled = YES;
        if (self.cacheOperation) {
            [self.cacheOperation cancel];
            self.cacheOperation = nil;
        }
        if (self.downloadToken) {
        ///根据downloadToke，对应的operation取消下载.[imageDownloader,是一个全局单例..sd里面单例模式用的很多]
            [self.manager.imageDownloader cancel:self.downloadToken];
        }
        [self.manager safelyRemoveOperationFromRunning:self];
    }
}

///相关单列
///SDWebImageManager 自身是一个单列的实现
@implementation SDWebImageManager
+ (nonnull instancetype)sharedManager {
    static dispatch_once_t once;
    static id instance;
    dispatch_once(&once, ^{
        instance = [self new];
    });
    return instance;
}

- (nonnull instancetype)init {
///其内部cache和downloader，分别是两个单例。
    SDImageCache *cache = [SDImageCache sharedImageCache];
    SDWebImageDownloader *downloader = [SDWebImageDownloader sharedDownloader];
    return [self initWithCache:cache downloader:downloader];
}

• 下载流程
  SDWebImageManager(单例) -> SDWebImageCombinedOperation<SDWebImageOperation>-> {SDImageCache(单例)->queryCache->if no ->SDWebImageDownloader(单例)->downloadImage}

• 图片异步下载的实现（包括单个下载的请求组装，状态控制，回调处理，图片预解压等）

///sd里面实现了一个SDWebImageDownloaderOperation的具体类。是命令模式的一种应用。

///这里实现了二个协议，第一个协议实际上是包含了urlsession的两个系统协议。后面贴上。这里比较重要的是，如果你看SDWebImageDownloader的实现文件，你会发现，也实现了<NSURLSessionTaskDelegate, NSURLSessionDataDelegate>协议。
////@interface SDWebImageDownloader () <NSURLSessionTaskDelegate, NSURLSessionDataDelegate>，并在SDWebImageDownloader中统一把事情回调到SDWebImageDownloaderOperation类中。这样能使类的职责更清晰，图片解压后续等处理能封装到具体的SDWebImageDownloaderOperation类中，更合适而更内聚。
例如
===========
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
    
    // Identify the operation that runs this task and pass it the delegate method
    NSOperation<SDWebImageDownloaderOperationInterface> *dataOperation = [self operationWithTask:task];
    if ([dataOperation respondsToSelector:@selector(URLSession:task:didCompleteWithError:)]) {
        [dataOperation URLSession:session task:task didCompleteWithError:error];
    }
}
===========
@interface SDWebImageDownloaderOperation : NSOperation <SDWebImageDownloaderOperationInterface, SDWebImageOperation>

/**
 * The request used by the operation's task.
 */
@property (strong, nonatomic, readonly, nullable) NSURLRequest *request;

/**
 * The operation's task
 */
@property (strong, nonatomic, readonly, nullable) NSURLSessionTask *dataTask;


@property (assign, nonatomic) BOOL shouldDecompressImages;

/**
 *  Was used to determine whether the URL connection should consult the credential storage for authenticating the connection.
 *  @deprecated Not used for a couple of versions
 */
@property (nonatomic, assign) BOOL shouldUseCredentialStorage __deprecated_msg("Property deprecated. Does nothing. Kept only for backwards compatibility");

/**
 * The credential used for authentication challenges in `-URLSession:task:didReceiveChallenge:completionHandler:`.
 *
 * This will be overridden by any shared credentials that exist for the username or password of the request URL, if present.
 */
@property (nonatomic, strong, nullable) NSURLCredential *credential;

/**
 * The SDWebImageDownloaderOptions for the receiver.
 */
@property (assign, nonatomic, readonly) SDWebImageDownloaderOptions options;

/**
 * The expected size of data.
 */
@property (assign, nonatomic) NSInteger expectedSize;

/**
 * The response returned by the operation's task.
 */
@property (strong, nonatomic, nullable) NSURLResponse *response;

/**
 *  Initializes a `SDWebImageDownloaderOperation` object
 *
 *  @see SDWebImageDownloaderOperation
 *
 *  @param request        the URL request
 *  @param session        the URL session in which this operation will run
 *  @param options        downloader options
 *
 *  @return the initialized instance
 */
- (nonnull instancetype)initWithRequest:(nullable NSURLRequest *)request
                              inSession:(nullable NSURLSession *)session
                                options:(SDWebImageDownloaderOptions)options NS_DESIGNATED_INITIALIZER;

/**
 *  Adds handlers for progress and completion. Returns a tokent that can be passed to -cancel: to cancel this set of
 *  callbacks.
 *
 *  @param progressBlock  the block executed when a new chunk of data arrives.
 *                        @note the progress block is executed on a background queue
 *  @param completedBlock the block executed when the download is done.
 *                        @note the completed block is executed on the main queue for success. If errors are found, there is a chance the block will be executed on a background queue
 *
 *  @return the token to use to cancel this set of handlers
 */
- (nullable id)addHandlersForProgress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                            completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock;

/**
 *  Cancels a set of callbacks. Once all callbacks are canceled, the operation is cancelled.
 *
 *  @param token the token representing a set of callbacks to cancel
 *
 *  @return YES if the operation was stopped because this was the last token to be canceled. NO otherwise.
 */
 ///取消操作
- (BOOL)cancel:(nullable id)token;

SDWebImageDownloaderOperationInterface继承了<NSURLSessionTaskDelegate, NSURLSessionDataDelegate>协议。
@protocol SDWebImageDownloaderOperationInterface <NSURLSessionTaskDelegate, NSURLSessionDataDelegate>
@required
- (nonnull instancetype)initWithRequest:(nullable NSURLRequest *)request
                              inSession:(nullable NSURLSession *)session
                                options:(SDWebImageDownloaderOptions)options;

- (nullable id)addHandlersForProgress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                            completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock;

- (BOOL)shouldDecompressImages;
- (void)setShouldDecompressImages:(BOOL)value;

- (nullable NSURLCredential *)credential;
- (void)setCredential:(nullable NSURLCredential *)value;

- (BOOL)cancel:(nullable id)token;

@optional
- (nullable NSURLSessionTask *)dataTask;

图片解压部分，使用了面相协议的方式把多个类型的图片解压方法分散到各个具体类中。贴近于工厂模式，创建多个产品。并使用统一的一套接口，使调用者只面向抽象接口调用，不面向具体的类接口调用，利于具体类的切换。
@protocol SDWebImageCoder <NSObject>
@required
#pragma mark - Decoding
- (BOOL)canDecodeFromData:(nullable NSData *)data;
- (nullable UIImage *)decodedImageWithData:(nullable NSData *)data;
- (nullable UIImage *)decompressedImageWithImage:(nullable UIImage *)image
                                            data:(NSData * _Nullable * _Nonnull)data
                                         options:(nullable NSDictionary<NSString*, NSObject*>*)optionsDict;
#pragma mark - Encoding
- (BOOL)canEncodeToFormat:(SDImageFormat)format;
- (nullable NSData *)encodedDataWithImage:(nullable UIImage *)image format:(SDImageFormat)format;
@end

=====
实现协议的具体类
@interface SDWebImageGIFCoder : NSObject <SDWebImageCoder>
@interface SDWebImageCodersManager : NSObject<SDWebImageCoder>

• 请求并发控制。

- (nonnull instancetype)initWithSessionConfiguration:(nullable NSURLSessionConfiguration *)sessionConfiguration {
    if ((self = [super init])) {
        _operationClass = [SDWebImageDownloaderOperation class];
        _shouldDecompressImages = YES;
        _executionOrder = SDWebImageDownloaderFIFOExecutionOrder;
        _downloadQueue = [NSOperationQueue new];
        ///并发控制，利用了苹果封装的queue提供的功能
        _downloadQueue.maxConcurrentOperationCount = 6;
        _downloadQueue.name = @"com.hackemist.SDWebImageDownloader";
        _URLOperations = [NSMutableDictionary new];
        SDHTTPHeadersMutableDictionary *headerDictionary = [SDHTTPHeadersMutableDictionary dictionary];
        NSString *userAgent = nil;
        ///组装userAgent头
#if SD_UIKIT
        // User-Agent Header; see http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.43
        userAgent = [NSString stringWithFormat:@"%@/%@ (%@; iOS %@; Scale/%0.2f)", [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleExecutableKey] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleIdentifierKey], [[NSBundle mainBundle] infoDictionary][@"CFBundleShortVersionString"] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleVersionKey], [[UIDevice currentDevice] model], [[UIDevice currentDevice] systemVersion], [[UIScreen mainScreen] scale]];
#elif SD_WATCH
        // User-Agent Header; see http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.43
        userAgent = [NSString stringWithFormat:@"%@/%@ (%@; watchOS %@; Scale/%0.2f)", [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleExecutableKey] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleIdentifierKey], [[NSBundle mainBundle] infoDictionary][@"CFBundleShortVersionString"] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleVersionKey], [[WKInterfaceDevice currentDevice] model], [[WKInterfaceDevice currentDevice] systemVersion], [[WKInterfaceDevice currentDevice] screenScale]];
#elif SD_MAC
        userAgent = [NSString stringWithFormat:@"%@/%@ (Mac OS X %@)", [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleExecutableKey] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleIdentifierKey], [[NSBundle mainBundle] infoDictionary][@"CFBundleShortVersionString"] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleVersionKey], [[NSProcessInfo processInfo] operatingSystemVersionString]];
#endif
        if (userAgent) {
            if (![userAgent canBeConvertedToEncoding:NSASCIIStringEncoding]) {
                NSMutableString *mutableUserAgent = [userAgent mutableCopy];
                if (CFStringTransform((__bridge CFMutableStringRef)(mutableUserAgent), NULL, (__bridge CFStringRef)@"Any-Latin; Latin-ASCII; [:^ASCII:] Remove", false)) {
                    userAgent = mutableUserAgent;
                }
            }
            headerDictionary[@"User-Agent"] = userAgent;
        }
#ifdef SD_WEBP
        headerDictionary[@"Accept"] = @"image/webp,image/*;q=0.8";
#else
        headerDictionary[@"Accept"] = @"image/*;q=0.8";
#endif
        _HTTPHeaders = headerDictionary;
        _operationsLock = dispatch_semaphore_create(1);
        _headersLock = dispatch_semaphore_create(1);
        _downloadTimeout = 15.0;

        [self createNewSessionWithConfiguration:sessionConfiguration];
    }
    return self;
}

• 图片缓存机制的实现

- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns
                       diskCacheDirectory:(nonnull NSString *)directory {
    if ((self = [super init])) {
        NSString *fullNamespace = [@"com.hackemist.SDWebImageCache." stringByAppendingString:ns];
        
        // Create IO serial queue
        ///串行队列,保证读写安全
        _ioQueue = dispatch_queue_create("com.hackemist.SDWebImageCache", DISPATCH_QUEUE_SERIAL);
        
        _config = [[SDImageCacheConfig alloc] init];
        
        // Init the memory cache
        ///内存缓存管理
        _memCache = [[SDMemoryCache alloc] initWithConfig:_config];
        _memCache.name = fullNamespace;

        // Init the disk cache
        if (directory != nil) {
            _diskCachePath = [directory stringByAppendingPathComponent:fullNamespace];
        } else {
            NSString *path = [self makeDiskCachePath:ns];
            _diskCachePath = path;
        }

        dispatch_sync(_ioQueue, ^{
            self.fileManager = [NSFileManager new];
        });
        
        
        #if SD_UIKIT
        // Subscribe to app events
        ////deleteOldFiles 与 backgroundDeleteOldFiles是内存和磁盘安全管理策略的执行方法
        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(deleteOldFiles)
                                                     name:UIApplicationWillTerminateNotification
                                                   object:nil];

        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(backgroundDeleteOldFiles)
                                                     name:UIApplicationDidEnterBackgroundNotification
                                                   object:nil];
#endif
    }

• 磁盘管理的安全策略

- (void)deleteOldFiles {
    [self deleteOldFilesWithCompletionBlock:nil];
}

- (void)deleteOldFilesWithCompletionBlock:(nullable SDWebImageNoParamsBlock)completionBlock {
    dispatch_async(self.ioQueue, ^{
        NSURL *diskCacheURL = [NSURL fileURLWithPath:self.diskCachePath isDirectory:YES];

        // Compute content date key to be used for tests
        NSURLResourceKey cacheContentDateKey = NSURLContentModificationDateKey;
        switch (self.config.diskCacheExpireType) {
            case SDImageCacheConfigExpireTypeAccessDate:
                cacheContentDateKey = NSURLContentAccessDateKey;
                break;

            case SDImageCacheConfigExpireTypeModificationDate:
                cacheContentDateKey = NSURLContentModificationDateKey;
                break;

            default:
                break;
        }
        
        NSArray<NSString *> *resourceKeys = @[NSURLIsDirectoryKey, cacheContentDateKey, NSURLTotalFileAllocatedSizeKey];

        // This enumerator prefetches useful properties for our cache files.
        NSDirectoryEnumerator *fileEnumerator = [self.fileManager enumeratorAtURL:diskCacheURL
                                                   includingPropertiesForKeys:resourceKeys
                                                                      options:NSDirectoryEnumerationSkipsHiddenFiles
                                                                 errorHandler:NULL];

        NSDate *expirationDate = [NSDate dateWithTimeIntervalSinceNow:-self.config.maxCacheAge];
        NSMutableDictionary<NSURL *, NSDictionary<NSString *, id> *> *cacheFiles = [NSMutableDictionary dictionary];
        NSUInteger currentCacheSize = 0;

        // Enumerate all of the files in the cache directory.  This loop has two purposes:
        //
        //  1. Removing files that are older than the expiration date.
        //  2. Storing file attributes for the size-based cleanup pass.
        NSMutableArray<NSURL *> *urlsToDelete = [[NSMutableArray alloc] init];
        for (NSURL *fileURL in fileEnumerator) {
            NSError *error;
            NSDictionary<NSString *, id> *resourceValues = [fileURL resourceValuesForKeys:resourceKeys error:&error];

            // Skip directories and errors.
            if (error || !resourceValues || [resourceValues[NSURLIsDirectoryKey] boolValue]) {
                continue;
            }

            // Remove files that are older than the expiration date;
            NSDate *modifiedDate = resourceValues[cacheContentDateKey];
            if ([[modifiedDate laterDate:expirationDate] isEqualToDate:expirationDate]) {
                [urlsToDelete addObject:fileURL];
                continue;
            }
            
            // Store a reference to this file and account for its total size.
            NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
            currentCacheSize += totalAllocatedSize.unsignedIntegerValue;
            cacheFiles[fileURL] = resourceValues;
        }
        
        for (NSURL *fileURL in urlsToDelete) {
            [self.fileManager removeItemAtURL:fileURL error:nil];
        }

        // If our remaining disk cache exceeds a configured maximum size, perform a second
        // size-based cleanup pass.  We delete the oldest files first.
        if (self.config.maxCacheSize > 0 && currentCacheSize > self.config.maxCacheSize) {
            // Target half of our maximum cache size for this cleanup pass.
            const NSUInteger desiredCacheSize = self.config.maxCacheSize / 2;

            // Sort the remaining cache files by their last modification time or last access time (oldest first).
            NSArray<NSURL *> *sortedFiles = [cacheFiles keysSortedByValueWithOptions:NSSortConcurrent
                                                                     usingComparator:^NSComparisonResult(id obj1, id obj2) {
                                                                         return [obj1[cacheContentDateKey] compare:obj2[cacheContentDateKey]];
                                                                     }];

            // Delete files until we fall below our desired cache size.
            for (NSURL *fileURL in sortedFiles) {
                if ([self.fileManager removeItemAtURL:fileURL error:nil]) {
                    NSDictionary<NSString *, id> *resourceValues = cacheFiles[fileURL];
                    NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
                    currentCacheSize -= totalAllocatedSize.unsignedIntegerValue;

                    if (currentCacheSize < desiredCacheSize) {
                        break;
                    }
                }
            }
        }
        if (completionBlock) {
            dispatch_async(dispatch_get_main_queue(), ^{
                completionBlock();
            });
        }
    });
}

• 其他线程安全的处理，锁的应用

#define LOCK(lock) dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER);
#define UNLOCK(lock) dispatch_semaphore_signal(lock);
///多线程中读写安全，通过锁保证，主要是数组和字典的读写操作安全；并包含其操作的整个代码段都要通过加锁保证其原子性。
例子
  LOCK(self.weakCacheLock);
        // Do the real copy of the key and only let NSMapTable manage the key's lifetime
        // Fixes issue #2507 https://github.com/SDWebImage/SDWebImage/issues/2507
        [self.weakCache setObject:obj forKey:[[key mutableCopy] copy]];
        UNLOCK(self.weakCacheLock);
=====
  LOCK(self.headersLock);
    if (value) {
        self.HTTPHeaders[field] = value;
    } else {
        [self.HTTPHeaders removeObjectForKey:field];
    }
    UNLOCK(self.headersLock);
    
======
    LOCK(self.callbacksLock);
    NSMutableArray<id> *callbacks = [[self.callbackBlocks valueForKey:key] mutableCopy];
    UNLOCK(self.callbacksLock);

///外观模式(通过face层去调用，内部调用更高级的子系统接口。)
提供多个外部接口，内部通过调用一个更高级的子接口来现实。
@interface UIImageView (WebCache)
- (void)sd_setImageWithURL:(nullable NSURL *)url NS_REFINED_FOR_SWIFT;
- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder NS_REFINED_FOR_SWIFT;
          
=======
- (void)sd_setImageWithURL:(nullable NSURL *)url {
///多级调用
    [self sd_setImageWithURL:url placeholderImage:nil options:0 progress:nil completed:nil];
}

- (void)sd_setImageWithURL:(nullable NSURL *)url placeholderImage:(nullable UIImage *)placeholder {
    [self sd_setImageWithURL:url placeholderImage:placeholder options:0 progress:nil completed:nil];
}

四、使用的设计模式

单例模式、命令模式、工厂模式、外观模式

五、利用语言特性的相关技巧

• runtime关联对象
• 信号量作为锁的使用
• 实现NSOperation子类，控制任务并发

六、SDWebImage官方类图

SDWebImageClassDiagram.png

七、官方时序图

SDWebImageSequenceDiagram.png