SDWebImage 源碼解讀（四）：Downloader

SDWebImageManager 是經過 imageLoader 來進行圖片下載的，其聲明以下：

/** * The image loader used by manager to load image. */
@property (strong, nonatomic, readonly, nonnull) id<SDImageLoader> imageLoader;
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能夠看到，imageLoader 是一個遵照了 SDImageLoader 協議的對象，在 SDWebImage 框架中默認使用的是 SDWebImageDownloader，接下來咱們就從這個類入手，看 SDWebImage 是如何實現圖片下載的。

SDWebImageDownloader

先看一下它的 initialize 方法：

+ (void)initialize {
    // Bind SDNetworkActivityIndicator if available (download it here: http://github.com/rs/SDNetworkActivityIndicator )
    // To use it, just add #import "SDNetworkActivityIndicator.h" in addition to the SDWebImage import
    if (NSClassFromString(@"SDNetworkActivityIndicator")) {

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
        id activityIndicator = [NSClassFromString(@"SDNetworkActivityIndicator") performSelector:NSSelectorFromString(@"sharedActivityIndicator")];
#pragma clang diagnostic pop

        // Remove observer in case it was previously added.
        [[NSNotificationCenter defaultCenter] removeObserver:activityIndicator name:SDWebImageDownloadStartNotification object:nil];
        [[NSNotificationCenter defaultCenter] removeObserver:activityIndicator name:SDWebImageDownloadStopNotification object:nil];

        [[NSNotificationCenter defaultCenter] addObserver:activityIndicator
                                                 selector:NSSelectorFromString(@"startActivity")
                                                     name:SDWebImageDownloadStartNotification object:nil];
        [[NSNotificationCenter defaultCenter] addObserver:activityIndicator
                                                 selector:NSSelectorFromString(@"stopActivity")
                                                     name:SDWebImageDownloadStopNotification object:nil];
    }
}
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在 initialize 方法中，SDWebImage 經過一種比較動態的方法使用了 SDNetworkActivityIndicator 類，如上所示。

接下來來看它的默認初始化方法：

- (instancetype)initWithConfig:(SDWebImageDownloaderConfig *)config {
    self = [super init];
    if (self) {
        if (!config) {
            config = SDWebImageDownloaderConfig.defaultDownloaderConfig;
        }
        _config = [config copy];
        [_config addObserver:self forKeyPath:NSStringFromSelector(@selector(maxConcurrentDownloads)) options:0 context:SDWebImageDownloaderContext]; // 監聽最大同時下載數
        _downloadQueue = [NSOperationQueue new]; // 初始化下載隊列
        _downloadQueue.maxConcurrentOperationCount = _config.maxConcurrentDownloads;
        _downloadQueue.name = @"com.hackemist.SDWebImageDownloader";
        _URLOperations = [NSMutableDictionary new];
        NSMutableDictionary<NSString *, NSString *> *headerDictionary = [NSMutableDictionary dictionary]; // 配置 HTTPHeader
        NSString *userAgent = nil; // 配置 userAgent
#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;
        }
        headerDictionary[@"Accept"] = @"image/*,*/*;q=0.8";
        _HTTPHeaders = headerDictionary;
        _HTTPHeadersLock = dispatch_semaphore_create(1); // 初始化 HTTPHeadersLock
        _operationsLock = dispatch_semaphore_create(1); // 初始化 operationsLock
        NSURLSessionConfiguration *sessionConfiguration = _config.sessionConfiguration; // 初始化 NSURLSession
        if (!sessionConfiguration) {
            sessionConfiguration = [NSURLSessionConfiguration defaultSessionConfiguration];
        }
        /** * Create the session for this task * We send nil as delegate queue so that the session creates a serial operation queue for performing all delegate * method calls and completion handler calls. */
        _session = [NSURLSession sessionWithConfiguration:sessionConfiguration
                                                 delegate:self
                                            delegateQueue:nil];
    }
    return self;
}
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初始化方法就是初始化了一些須要用到的屬性，接下來看一下 SDWebImageDownloaderConfig 的默認配置都包含一些什麼內容：

- (instancetype)init {
    self = [super init];
    if (self) {
        _maxConcurrentDownloads = 6; // 最大同時下載數 6
        _downloadTimeout = 15.0; // 超時時間 15s
        _executionOrder = SDWebImageDownloaderFIFOExecutionOrder; // 下載順序先進先出
    }
    return self;
}
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下載圖片

圖片的下載是經過 SDImageLoader 的協議方法 requestImageWithURL:options:context:progress:completed: 開始的，其在 SDWebImageDownloader 類中的實現以下：

- (id<SDWebImageOperation>)requestImageWithURL:(NSURL *)url options:(SDWebImageOptions)options context:(SDWebImageContext *)context progress:(SDImageLoaderProgressBlock)progressBlock completed:(SDImageLoaderCompletedBlock)completedBlock {
    UIImage *cachedImage = context[SDWebImageContextLoaderCachedImage];
    
    SDWebImageDownloaderOptions downloaderOptions = 0;
    if (options & SDWebImageLowPriority) downloaderOptions |= SDWebImageDownloaderLowPriority; // 低優先級下載
    if (options & SDWebImageProgressiveLoad) downloaderOptions |= SDWebImageDownloaderProgressiveLoad; // 進度加載
    if (options & SDWebImageRefreshCached) downloaderOptions |= SDWebImageDownloaderUseNSURLCache; // 使用 NSURLCache
    if (options & SDWebImageContinueInBackground) downloaderOptions |= SDWebImageDownloaderContinueInBackground; // 後臺下載
    if (options & SDWebImageHandleCookies) downloaderOptions |= SDWebImageDownloaderHandleCookies; // Cookies
    if (options & SDWebImageAllowInvalidSSLCertificates) downloaderOptions |= SDWebImageDownloaderAllowInvalidSSLCertificates; // SSL
    if (options & SDWebImageHighPriority) downloaderOptions |= SDWebImageDownloaderHighPriority; // 高優先級
    if (options & SDWebImageScaleDownLargeImages) downloaderOptions |= SDWebImageDownloaderScaleDownLargeImages; // 下降圖片 Scale
    if (options & SDWebImageAvoidDecodeImage) downloaderOptions |= SDWebImageDownloaderAvoidDecodeImage; // 避免解碼圖片
    if (options & SDWebImageDecodeFirstFrameOnly) downloaderOptions |= SDWebImageDownloaderDecodeFirstFrameOnly; // 只解碼第一幀
    if (options & SDWebImagePreloadAllFrames) downloaderOptions |= SDWebImageDownloaderPreloadAllFrames; // 預先加載全部幀
    
    if (cachedImage && options & SDWebImageRefreshCached) { // 有圖片但須要刷新緩存
        // force progressive off if image already cached but forced refreshing
        downloaderOptions &= ~SDWebImageDownloaderProgressiveLoad;
        // ignore image read from NSURLCache if image if cached but force refreshing
        downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
    }
    
    return [self downloadImageWithURL:url options:downloaderOptions context:context progress:progressBlock completed:completedBlock];
}
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在經過 SDWebImageOptions 轉義到 SDWebImageDownloaderOptions 以後，經過 downloadImageWithURL:options:context:progress:completed:

- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
                                                   options:(SDWebImageDownloaderOptions)options
                                                   context:(nullable SDWebImageContext *)context
                                                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                                 completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock {
    // The URL will be used as the key to the callbacks dictionary so it cannot be nil. If it is nil immediately call the completed block with no image or data.
    if (url == nil) {
        if (completedBlock) {
            NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorInvalidURL userInfo:@{NSLocalizedDescriptionKey : @"Image url is nil"}];
            completedBlock(nil, nil, error, YES);
        }
        return nil;
    }
    
    SD_LOCK(self.operationsLock);
    NSOperation<SDWebImageDownloaderOperation> *operation = [self.URLOperations objectForKey:url]; // 從 URLOperations 中以 url 爲 key 取出 operation
    // There is a case that the operation may be marked as finished or cancelled, but not been removed from `self.URLOperations`.
    if (!operation || operation.isFinished || operation.isCancelled) { // 若是沒有取到或者已經成功或已經取消
        operation = [self createDownloaderOperationWithUrl:url options:options context:context]; // 生成新的 operation
        if (!operation) { // 生成失敗的話解鎖並報錯
            SD_UNLOCK(self.operationsLock);
            if (completedBlock) {
                NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorInvalidDownloadOperation userInfo:@{NSLocalizedDescriptionKey : @"Downloader operation is nil"}];
                completedBlock(nil, nil, error, YES);
            }
            return nil;
        }
        @weakify(self);
        operation.completionBlock = ^{
            @strongify(self);
            if (!self) {
                return;
            }
            SD_LOCK(self.operationsLock);
            [self.URLOperations removeObjectForKey:url]; // 執行成功以後，從 URLOperations 中移除
            SD_UNLOCK(self.operationsLock);
        };
        self.URLOperations[url] = operation;
        // Add operation to operation queue only after all configuration done according to Apple's doc.
        // `addOperation:` does not synchronously execute the `operation.completionBlock` so this will not cause deadlock.
        [self.downloadQueue addOperation:operation];
    }
    else if (!operation.isExecuting) { // 若是正在執行，則根據配置調整優先級
        if (options & SDWebImageDownloaderHighPriority) {
            operation.queuePriority = NSOperationQueuePriorityHigh;
        } else if (options & SDWebImageDownloaderLowPriority) {
            operation.queuePriority = NSOperationQueuePriorityLow;
        } else {
            operation.queuePriority = NSOperationQueuePriorityNormal;
        }
    }
    SD_UNLOCK(self.operationsLock);
    
    id downloadOperationCancelToken = [operation addHandlersForProgress:progressBlock completed:completedBlock]; // 建立用於取消的 token
    
    SDWebImageDownloadToken *token = [[SDWebImageDownloadToken alloc] initWithDownloadOperation:operation]; // 建立 download token
    token.url = url;
    token.request = operation.request;
    token.downloadOperationCancelToken = downloadOperationCancelToken;
    token.downloader = self;
    
    return token;
}
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建立下載 operation

建立下載 operation 的方法以下：

- (nullable NSOperation<SDWebImageDownloaderOperation> *)createDownloaderOperationWithUrl:(nonnull NSURL *)url
                                                                                  options:(SDWebImageDownloaderOptions)options
                                                                                  context:(nullable SDWebImageContext *)context {
    NSTimeInterval timeoutInterval = self.config.downloadTimeout; // 設置請求超時時長
    if (timeoutInterval == 0.0) {
        timeoutInterval = 15.0;
    }
    
    // In order to prevent from potential duplicate caching (NSURLCache + SDImageCache) we disable the cache for image requests if told otherwise
    NSURLRequestCachePolicy cachePolicy = options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData; // 設置緩存策略
    NSMutableURLRequest *mutableRequest = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:cachePolicy timeoutInterval:timeoutInterval];
    mutableRequest.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies); // 設置是否須要使用 cookie
    mutableRequest.HTTPShouldUsePipelining = YES; // 支持 pipelining
    SD_LOCK(self.HTTPHeadersLock);
    mutableRequest.allHTTPHeaderFields = self.HTTPHeaders; // 設置 HTTPHeaders
    SD_UNLOCK(self.HTTPHeadersLock);
    id<SDWebImageDownloaderRequestModifier> requestModifier; // 配置 requestModifier，用於對請求作一些修改
    if ([context valueForKey:SDWebImageContextDownloadRequestModifier]) {
        requestModifier = [context valueForKey:SDWebImageContextDownloadRequestModifier];
    } else {
        requestModifier = self.requestModifier;
    }
    
    NSURLRequest *request;
    if (requestModifier) {
        NSURLRequest *modifiedRequest = [requestModifier modifiedRequestWithRequest:[mutableRequest copy]];
        // If modified request is nil, early return
        if (!modifiedRequest) {
            return nil;
        } else {
            request = [modifiedRequest copy];
        }
    } else {
        request = [mutableRequest copy];
    }
    Class operationClass = self.config.operationClass; // 有自定義 operationClass 用 operationClass，沒有就用默認的 SDWebImageDownloaderOperation
    if (operationClass && [operationClass isSubclassOfClass:[NSOperation class]] && [operationClass conformsToProtocol:@protocol(SDWebImageDownloaderOperation)]) {
        // Custom operation class
    } else {
        operationClass = [SDWebImageDownloaderOperation class];
    }
    NSOperation<SDWebImageDownloaderOperation> *operation = [[operationClass alloc] initWithRequest:request inSession:self.session options:options context:context];
    
    if ([operation respondsToSelector:@selector(setCredential:)]) { // 處理 urlCredential
        if (self.config.urlCredential) {
            operation.credential = self.config.urlCredential;
        } else if (self.config.username && self.config.password) {
            operation.credential = [NSURLCredential credentialWithUser:self.config.username password:self.config.password persistence:NSURLCredentialPersistenceForSession];
        }
    }
        
    if ([operation respondsToSelector:@selector(setMinimumProgressInterval:)]) { // 處理進度
        operation.minimumProgressInterval = MIN(MAX(self.config.minimumProgressInterval, 0), 1);
    }
    
    if (options & SDWebImageDownloaderHighPriority) { // 設置下載優先級
        operation.queuePriority = NSOperationQueuePriorityHigh;
    } else if (options & SDWebImageDownloaderLowPriority) {
        operation.queuePriority = NSOperationQueuePriorityLow;
    }
    
    if (self.config.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) { // 若是下載順序是後進先出，經過依賴肯定下載順序
        // Emulate LIFO execution order by systematically adding new operations as last operation's dependency
        [self.lastAddedOperation addDependency:operation];
        self.lastAddedOperation = operation;
    }
    
    return operation;
}
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SDWebImageDownloaderOperation

SDWebImageDownloaderOperation 是一個 NSOperation 的子類，並遵照了 SDWebImageDownloaderOperation 協議，聲明以下：

/** The download operation class for SDWebImageDownloader. */
@interface SDWebImageDownloaderOperation : NSOperation <SDWebImageDownloaderOperation>

@end
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它實現了不少方法，在建立 download operation 的過程當中用到的以下：

- (nonnull instancetype)initWithRequest:(nullable NSURLRequest *)request
                              inSession:(nullable NSURLSession *)session
                                options:(SDWebImageDownloaderOptions)options
                                context:(nullable SDWebImageContext *)context {
    if ((self = [super init])) {
        _request = [request copy];
        _options = options;
        _context = [context copy];
        _callbackBlocks = [NSMutableArray new];
        _executing = NO;
        _finished = NO;
        _expectedSize = 0;
        _unownedSession = session;
        _callbacksLock = dispatch_semaphore_create(1);
        _coderQueue = dispatch_queue_create("com.hackemist.SDWebImageDownloaderOperationCoderQueue", DISPATCH_QUEUE_SERIAL);
#if SD_UIKIT
        _backgroundTaskId = UIBackgroundTaskInvalid;
#endif
    }
    return self;
}
// progressBlock 和 completedBlock 都放到字典裏，而後放到 callbackBlocks 數組裏
- (nullable id)addHandlersForProgress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                            completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock {
    SDCallbacksDictionary *callbacks = [NSMutableDictionary new];
    if (progressBlock) callbacks[kProgressCallbackKey] = [progressBlock copy];
    if (completedBlock) callbacks[kCompletedCallbackKey] = [completedBlock copy];
    SD_LOCK(self.callbacksLock);
    [self.callbackBlocks addObject:callbacks];
    SD_UNLOCK(self.callbacksLock);
    return callbacks;
}
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SDWebImageDownloadToken

SDWebImageDownloadToken 是一個模型類，遵照了 SDWebImageOperation 協議，SDWebImageOperation 協議聲明以下：

/// A protocol represents cancelable operation.
@protocol SDWebImageOperation <NSObject>

- (void)cancel;

@end

/// NSOperation conform to `SDWebImageOperation`.
@interface NSOperation (SDWebImageOperation) <SDWebImageOperation>

@end
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圖片下載

圖片真實的下載過程是封裝在 SDWebImageDownloaderOperation 裏的，從 start 方法開始 operation 的全部操做：

- (void)start {
    @synchronized (self) {
        if (self.isCancelled) { // 若是取消了，則 reset 並直接返回
            self.finished = YES;
            [self reset];
            return;
        }

#if SD_UIKIT
        Class UIApplicationClass = NSClassFromString(@"UIApplication"); // 由於 iOS 是支持後臺下載的，因此須要在 application 過時的時候 cancel 掉當前的操做
        BOOL hasApplication = UIApplicationClass && [UIApplicationClass respondsToSelector:@selector(sharedApplication)];
        if (hasApplication && [self shouldContinueWhenAppEntersBackground]) {
            __weak typeof(self) wself = self;
            UIApplication * app = [UIApplicationClass performSelector:@selector(sharedApplication)];
            self.backgroundTaskId = [app beginBackgroundTaskWithExpirationHandler:^{
                [wself cancel];
            }];
        }
#endif
        NSURLSession *session = self.unownedSession; 
        if (!session) { // 若是 session 不存在，則建立一個來用
            NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
            sessionConfig.timeoutIntervalForRequest = 15;
            
            /** * Create the session for this task * We send nil as delegate queue so that the session creates a serial operation queue for performing all delegate * method calls and completion handler calls. */
            session = [NSURLSession sessionWithConfiguration:sessionConfig
                                                    delegate:self
                                               delegateQueue:nil];
            self.ownedSession = session;
        }
        
        if (self.options & SDWebImageDownloaderIgnoreCachedResponse) {
            // Grab the cached data for later check
            NSURLCache *URLCache = session.configuration.URLCache;
            if (!URLCache) {
                URLCache = [NSURLCache sharedURLCache];
            }
            NSCachedURLResponse *cachedResponse;
            // NSURLCache's `cachedResponseForRequest:` is not thread-safe, see https://developer.apple.com/documentation/foundation/nsurlcache#2317483
            @synchronized (URLCache) {
                cachedResponse = [URLCache cachedResponseForRequest:self.request];
            }
            if (cachedResponse) {
                self.cachedData = cachedResponse.data;
            }
        }
        
        self.dataTask = [session dataTaskWithRequest:self.request];
        self.executing = YES;
    }

    if (self.dataTask) {
        if (self.options & SDWebImageDownloaderHighPriority) { // 設置下載優先級
            self.dataTask.priority = NSURLSessionTaskPriorityHigh;
        } else if (self.options & SDWebImageDownloaderLowPriority) {
            self.dataTask.priority = NSURLSessionTaskPriorityLow;
        }
        [self.dataTask resume]; // 啓動下載
        for (SDWebImageDownloaderProgressBlock progressBlock in [self callbacksForKey:kProgressCallbackKey]) { // 初始化 porgressBlock
            progressBlock(0, NSURLResponseUnknownLength, self.request.URL);
        }
        __block typeof(self) strongSelf = self;
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStartNotification object:strongSelf];
        });
    } else { // 有問題就報錯
        [self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorInvalidDownloadOperation userInfo:@{NSLocalizedDescriptionKey : @"Task can't be initialized"}]];
        [self done];
    }
}
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對於圖片數據的操做都在代理方法裏實現：

URLSession:dataTask:didReceiveResponse:completionHandler:

- (void)URLSession:(NSURLSession *)session
          dataTask:(NSURLSessionDataTask *)dataTask
didReceiveResponse:(NSURLResponse *)response
 completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler {
    NSURLSessionResponseDisposition disposition = NSURLSessionResponseAllow;
    NSInteger expected = (NSInteger)response.expectedContentLength; // 取出圖片總大小
    expected = expected > 0 ? expected : 0;
    self.expectedSize = expected;
    self.response = response;
    NSInteger statusCode = [response respondsToSelector:@selector(statusCode)] ? ((NSHTTPURLResponse *)response).statusCode : 200;
    BOOL valid = statusCode >= 200 && statusCode < 400;
    if (!valid) { // 若是 statusCode 不在 200 到 400 之間，報錯
        self.responseError = [NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorInvalidDownloadStatusCode userInfo:@{SDWebImageErrorDownloadStatusCodeKey : @(statusCode)}];
    }
    //'304 Not Modified' is an exceptional one
    //URLSession current behavior will return 200 status code when the server respond 304 and URLCache hit. But this is not a standard behavior and we just add a check
    if (statusCode == 304 && !self.cachedData) { // 304 但無緩存，報錯
        valid = NO;
        self.responseError = [NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorCacheNotModified userInfo:nil];
    }
    
    if (valid) { // 沒問題更新 pregressBlock，有問題 cancel
        for (SDWebImageDownloaderProgressBlock progressBlock in [self callbacksForKey:kProgressCallbackKey]) {
            progressBlock(0, expected, self.request.URL);
        }
    } else {
        // Status code invalid and marked as cancelled. Do not call `[self.dataTask cancel]` which may mass up URLSession life cycle
        disposition = NSURLSessionResponseCancel;
    }
    __block typeof(self) strongSelf = self;
    dispatch_async(dispatch_get_main_queue(), ^{
        [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadReceiveResponseNotification object:strongSelf];
    });
    
    if (completionHandler) {
        completionHandler(disposition);
    }
}
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URLSession:dataTask:didReceiveData:

- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data {
    if (!self.imageData) { // 若是 imageData 不存在，則根據 expectedSize 建立一個，並拼接數據
        self.imageData = [[NSMutableData alloc] initWithCapacity:self.expectedSize];
    }
    [self.imageData appendData:data];
    
    self.receivedSize = self.imageData.length;
    if (self.expectedSize == 0) { 
        // Unknown expectedSize, immediately call progressBlock and return
        for (SDWebImageDownloaderProgressBlock progressBlock in [self callbacksForKey:kProgressCallbackKey]) {
            progressBlock(self.receivedSize, self.expectedSize, self.request.URL);
        }
        return;
    }
    
    // Get the finish status
    BOOL finished = (self.receivedSize >= self.expectedSize);
    // Get the current progress
    double currentProgress = (double)self.receivedSize / (double)self.expectedSize;
    double previousProgress = self.previousProgress;
    double progressInterval = currentProgress - previousProgress;
    // Check if we need callback progress
    if (!finished && (progressInterval < self.minimumProgressInterval)) {
        return;
    }
    self.previousProgress = currentProgress;

    if (self.options & SDWebImageDownloaderProgressiveLoad) { // 是否支持漸進式加載
        // Get the image data
        NSData *imageData = [self.imageData copy];
        
        // progressive decode the image in coder queue
        dispatch_async(self.coderQueue, ^{
            @autoreleasepool {
                UIImage *image = SDImageLoaderDecodeProgressiveImageData(imageData, self.request.URL, finished, self, [[self class] imageOptionsFromDownloaderOptions:self.options], self.context);
                if (image) {
                    // We do not keep the progressive decoding image even when `finished`=YES. Because they are for view rendering but not take full function from downloader options. And some coders implementation may not keep consistent between progressive decoding and normal decoding.
                    
                    [self callCompletionBlocksWithImage:image imageData:nil error:nil finished:NO];
                }
            }
        });
    }
    
    for (SDWebImageDownloaderProgressBlock progressBlock in [self callbacksForKey:kProgressCallbackKey]) {
        progressBlock(self.receivedSize, self.expectedSize, self.request.URL);
    }
}
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URLSession:dataTask:willCacheResponse:completionHandler:

- (void)URLSession:(NSURLSession *)session
          dataTask:(NSURLSessionDataTask *)dataTask
 willCacheResponse:(NSCachedURLResponse *)proposedResponse
 completionHandler:(void (^)(NSCachedURLResponse *cachedResponse))completionHandler {
    // 強制取消緩存
    NSCachedURLResponse *cachedResponse = proposedResponse;

    if (!(self.options & SDWebImageDownloaderUseNSURLCache)) {
        // Prevents caching of responses
        cachedResponse = nil;
    }
    if (completionHandler) {
        completionHandler(cachedResponse);
    }
}
複製代碼

URLSession:task:didCompleteWithError:

- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
    @synchronized(self) {
        self.dataTask = nil;
        __block typeof(self) strongSelf = self;
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:strongSelf];
            if (!error) {
                [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadFinishNotification object:strongSelf];
            }
        });
    }
    
    // make sure to call `[self done]` to mark operation as finished
    if (error) {
        // custom error instead of URLSession error
        if (self.responseError) {
            error = self.responseError;
        }
        [self callCompletionBlocksWithError:error];
        [self done];
    } else {
        if ([self callbacksForKey:kCompletedCallbackKey].count > 0) { // 若是存儲了回調 block
            NSData *imageData = [self.imageData copy];
            self.imageData = nil;
            if (imageData) {
                /** if you specified to only use cached data via `SDWebImageDownloaderIgnoreCachedResponse`, * then we should check if the cached data is equal to image data */
                if (self.options & SDWebImageDownloaderIgnoreCachedResponse && [self.cachedData isEqualToData:imageData]) { // 若是忽略緩存則報錯
                    self.responseError = [NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorCacheNotModified userInfo:nil];
                    // call completion block with not modified error
                    [self callCompletionBlocksWithError:self.responseError];
                    [self done];
                } else { // 不然解碼圖片並返回
                    // decode the image in coder queue
                    dispatch_async(self.coderQueue, ^{
                        @autoreleasepool {
                            UIImage *image = SDImageLoaderDecodeImageData(imageData, self.request.URL, [[self class] imageOptionsFromDownloaderOptions:self.options], self.context);
                            CGSize imageSize = image.size;
                            if (imageSize.width == 0 || imageSize.height == 0) {
                                [self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorBadImageData userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0 pixels"}]];
                            } else {
                                [self callCompletionBlocksWithImage:image imageData:imageData error:nil finished:YES];
                            }
                            [self done];
                        }
                    });
                }
            } else {
                [self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorBadImageData userInfo:@{NSLocalizedDescriptionKey : @"Image data is nil"}]];
                [self done];
            }
        } else {
            [self done];
        }
    }
}
複製代碼

關於下載部分的全部代碼解析基本就是這些了。