一、总览

本类继承自 NSOperation，每一张图片的下载对应一个 Operation。

二、头文件提供的属性及方法

SDWebImageDownloaderOperationInterface 协议，SDWebImageDownloaderOperation 遵守该协议，它一共有四个必须实现的方法，可以为自定义的下载操作统一接口。

/**
 Describes a downloader operation. If one wants to use a custom downloader op, it needs to inherit from `NSOperation` and conform to this protocol
 */
@protocol SDWebImageDownloaderOperationInterface<NSObject>

- (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;

@end

SDWebImageDownloaderOperation 继承自 NSOperation，使用 NSURLSession 实现图片的下载

@interface SDWebImageDownloaderOperation : NSOperation <SDWebImageDownloaderOperationInterface, SDWebImageOperation, NSURLSessionTaskDelegate, NSURLSessionDataDelegate>

/**
 * 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 `-connection:didReceiveAuthenticationChallenge:`.
 *
 * 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 connection.
 */
@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.
 */
- (BOOL)cancel:(nullable id)token;

@end

三、实现文件

1、私有属性

@interface SDWebImageDownloaderOperation ()

// 回调Block列表
@property (strong, nonatomic, nonnull) NSMutableArray<SDCallbacksDictionary *> *callbackBlocks;

// 自定义并行Operation需要管理的两个属性。默认是readonly的，我们这里通过声明改为可修改的。方便我们在后面操作。
@property (assign, nonatomic, getter = isExecuting) BOOL executing;
@property (assign, nonatomic, getter = isFinished) BOOL finished;

// 存储图片数据
@property (strong, nonatomic, nullable) NSMutableData *imageData;

// 通过SDWebImageDownloader传过来。所以这里是weak。因为他是通过SDWebImageDownloader管理的。
@property (weak, nonatomic, nullable) NSURLSession *unownedSession;

// 如果unownedSession是nil，我们需要手动创建一个并且管理他的生命周期和代理方法
@property (strong, nonatomic, nullable) NSURLSession *ownedSession;

// dataTask对象
@property (strong, nonatomic, readwrite, nullable) NSURLSessionTask *dataTask;

// 一个并行queue。用于控制数据的处理
@property (SDDispatchQueueSetterSementics, nonatomic, nullable) dispatch_queue_t barrierQueue;

#if SD_UIKIT

// 如果用户设置了后台继续加载选项。则通过backgroundTask来继续下载图片
@property (assign, nonatomic) UIBackgroundTaskIdentifier backgroundTaskId;
#endif

@end

2、实现

@implementation SDWebImageDownloaderOperation {
    size_t width, height;
#if SD_UIKIT || SD_WATCH
    UIImageOrientation orientation;
#endif
}

@synthesize executing = _executing;
@synthesize finished = _finished;

- (nonnull instancetype)init {
    return [self initWithRequest:nil inSession:nil options:0];
}

- (nonnull instancetype)initWithRequest:(nullable NSURLRequest *)request
                              inSession:(nullable NSURLSession *)session
                                options:(SDWebImageDownloaderOptions)options {
    if ((self = [super init])) {
        _request = [request copy];
        _shouldDecompressImages = YES;
        _options = options;
        _callbackBlocks = [NSMutableArray new];
        //默认情况下。_executing和finished都是NO
        _executing = NO;
        _finished = NO;
        _expectedSize = 0;
        _unownedSession = session;
        _barrierQueue = dispatch_queue_create("com.hackemist.SDWebImageDownloaderOperationBarrierQueue", DISPATCH_QUEUE_CONCURRENT);
    }
    return self;
}

- (void)dealloc {
    SDDispatchQueueRelease(_barrierQueue);
}

/**
 给Operation添加进度和回调Block

 @param progressBlock 进度Block
 @param completedBlock 回调Block
 @return 回调字典
 */
- (nullable id)addHandlersForProgress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                            completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock {
    SDCallbacksDictionary *callbacks = [NSMutableDictionary new];
    // 把Operation对应的回调和进度Block存入一个字典中
    if (progressBlock) callbacks[kProgressCallbackKey] = [progressBlock copy];
    if (completedBlock) callbacks[kCompletedCallbackKey] = [completedBlock copy];
    // 把完成和进度Block加入callbackBlocks中
    dispatch_barrier_async(self.barrierQueue, ^{
        [self.callbackBlocks addObject:callbacks];
    });
    return callbacks;
}

- (nullable NSArray<id> *)callbacksForKey:(NSString *)key {
    __block NSMutableArray<id> *callbacks = nil;
    dispatch_sync(self.barrierQueue, ^{
        // We need to remove [NSNull null] because there might not always be a progress block for each callback
        callbacks = [[self.callbackBlocks valueForKey:key] mutableCopy];
        [callbacks removeObjectIdenticalTo:[NSNull null]];
    });
    return [callbacks copy];    // strip mutability here
}

- (BOOL)cancel:(nullable id)token {
    __block BOOL shouldCancel = NO;
    dispatch_barrier_sync(self.barrierQueue, ^{
        [self.callbackBlocks removeObjectIdenticalTo:token];
        if (self.callbackBlocks.count == 0) {
            shouldCancel = YES;
        }
    });
    if (shouldCancel) {
        [self cancel];
    }
    return shouldCancel;
}

// 并行的Operation需要重写这个方法.在这个方法里面做具体的处理
- (void)start {
    @synchronized (self) {
        if (self.isCancelled) {
            self.finished = YES;
            [self reset];
            return;
        }

#if SD_UIKIT
        Class UIApplicationClass = NSClassFromString(@"UIApplication");
        BOOL hasApplication = UIApplicationClass && [UIApplicationClass respondsToSelector:@selector(sharedApplication)];
        // 如果用户设置了Background模式，则设置一个backgroundTask
        if (hasApplication && [self shouldContinueWhenAppEntersBackground]) {
            __weak __typeof__ (self) wself = self;
            UIApplication * app = [UIApplicationClass performSelector:@selector(sharedApplication)];
            self.backgroundTaskId = [app beginBackgroundTaskWithExpirationHandler:^{
                // background结束以后。做清理工作
                __strong __typeof (wself) sself = wself;
                if (sself) {
                    [sself cancel];
                    [app endBackgroundTask:sself.backgroundTaskId];
                    sself.backgroundTaskId = UIBackgroundTaskInvalid;
                }
            }];
        }
#endif
        NSURLSession *session = self.unownedSession;
        // 如果SDWebImageDownloader传入的session是nil，则自己手动初始化一个。
        if (!self.unownedSession) {
            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.
             */
            self.ownedSession = [NSURLSession sessionWithConfiguration:sessionConfig
                                                              delegate:self
                                                         delegateQueue:nil];
            session = self.ownedSession;
        }
        
        self.dataTask = [session dataTaskWithRequest:self.request];
        self.executing = YES;
    }
    // 发送请求
    [self.dataTask resume];

    if (self.dataTask) {
        // 第一次调用进度BLOCK
        for (SDWebImageDownloaderProgressBlock progressBlock in [self callbacksForKey:kProgressCallbackKey]) {
            progressBlock(0, NSURLResponseUnknownLength, self.request.URL);
        }
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStartNotification object:self];
        });
    } else {
        [self callCompletionBlocksWithError:[NSError errorWithDomain:NSURLErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Connection can't be initialized"}]];
    }

#if SD_UIKIT
    Class UIApplicationClass = NSClassFromString(@"UIApplication");
    if(!UIApplicationClass || ![UIApplicationClass respondsToSelector:@selector(sharedApplication)]) {
        return;
    }
    if (self.backgroundTaskId != UIBackgroundTaskInvalid) {
        UIApplication * app = [UIApplication performSelector:@selector(sharedApplication)];
        [app endBackgroundTask:self.backgroundTaskId];
        self.backgroundTaskId = UIBackgroundTaskInvalid;
    }
#endif
}

/**
 如果要取消一个Operation，就会调用这个方法。
 */
- (void)cancel {
    @synchronized (self) {
        [self cancelInternal];
    }
}

- (void)cancelInternal {
    if (self.isFinished) return;
    [super cancel];

    if (self.dataTask) {
        [self.dataTask cancel];
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:self];
        });

        // As we cancelled the connection, its callback won't be called and thus won't
        // maintain the isFinished and isExecuting flags.
        // 更新状态
        if (self.isExecuting) self.executing = NO;
        if (!self.isFinished) self.finished = YES;
    }

    [self reset];
}

- (void)done {
    self.finished = YES;
    self.executing = NO;
    [self reset];
}

- (void)reset {
    dispatch_barrier_async(self.barrierQueue, ^{
        [self.callbackBlocks removeAllObjects];
    });
    self.dataTask = nil;
    self.imageData = nil;
    if (self.ownedSession) {
        [self.ownedSession invalidateAndCancel];
        self.ownedSession = nil;
    }
}

/**
 需要手动触发_finished的KVO。这个是自定义并发`NSOperation`必须实现的。

 @param finished 改变状态
 */
- (void)setFinished:(BOOL)finished {
    [self willChangeValueForKey:@"isFinished"];
    _finished = finished;
    [self didChangeValueForKey:@"isFinished"];
}
/**
 需要手动触发_executing的KVO。这个是自定义并发`NSOperation`必须实现的。
 
 @param executing 改变状态
 */
- (void)setExecuting:(BOOL)executing {
    [self willChangeValueForKey:@"isExecuting"];
    _executing = executing;
    [self didChangeValueForKey:@"isExecuting"];
}

/**
 返回YES，表明这个NSOperation对象是并发的

 @return 返回bool值
 */
- (BOOL)isConcurrent {
    return YES;
}

#pragma mark NSURLSessionDataDelegate

/*
 当一个task接收到返回信息。当所有信息都接收完毕以后，completionHandler会被调用。我们可以在这里取消一个网络请求或者把一个datatask转换为downloadtask。如果没有实现这个代理方法，我们也可以通过task的response属性获取到对应的数据。background模式的uploadtask不会调用这个方法。
 */
- (void)URLSession:(NSURLSession *)session
          dataTask:(NSURLSessionDataTask *)dataTask
didReceiveResponse:(NSURLResponse *)response
 completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler {
    
    //'304 Not Modified' is an exceptional one
    if (![response respondsToSelector:@selector(statusCode)] || (((NSHTTPURLResponse *)response).statusCode < 400 && ((NSHTTPURLResponse *)response).statusCode != 304)) {
        // 期望的总长度
        NSInteger expected = response.expectedContentLength > 0 ? (NSInteger)response.expectedContentLength : 0;
        self.expectedSize = expected;
        // 进度回调Block
        for (SDWebImageDownloaderProgressBlock progressBlock in [self callbacksForKey:kProgressCallbackKey]) {
            progressBlock(0, expected, self.request.URL);
        }
        
        self.imageData = [[NSMutableData alloc] initWithCapacity:expected];
        self.response = response;
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadReceiveResponseNotification object:self];
        });
    }
    else {
        NSUInteger code = ((NSHTTPURLResponse *)response).statusCode;
        
        //This is the case when server returns '304 Not Modified'. It means that remote image is not changed.
        //In case of 304 we need just cancel the operation and return cached image from the cache.
        /*
         如果返回304表示图片么有变化。在这种情况下，我们只需要取消operation并且返回缓存的图片就可以了。
         */
        if (code == 304) {
            [self cancelInternal];
        } else {
            [self.dataTask cancel];
        }
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:self];
        });
        
        [self callCompletionBlocksWithError:[NSError errorWithDomain:NSURLErrorDomain code:((NSHTTPURLResponse *)response).statusCode userInfo:nil]];

        [self done];
    }
    //这个表示允许继续加载
    if (completionHandler) {
        completionHandler(NSURLSessionResponseAllow);
    }
}
/*
 *会被多次调用。获取图片数据
 */
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data {
    
    [self.imageData appendData:data];

    if ((self.options & SDWebImageDownloaderProgressiveDownload) && self.expectedSize > 0) {
        // The following code is from http://www.cocoaintheshell.com/2011/05/progressive-images-download-imageio/
        // Thanks to the author @Nyx0uf

        // Get the total bytes downloaded
        // 获取已经下载的数据长度
        const NSInteger totalSize = self.imageData.length;

        // Update the data source, we must pass ALL the data, not just the new bytes
        CGImageSourceRef imageSource = CGImageSourceCreateWithData((__bridge CFDataRef)self.imageData, NULL);
        /*
         *width和height都是0的话表示还么有获取到图片的高度和宽度。我们可以通过数据来获取图片的宽度和高度
         *此时表示第一次收到图片数据
         */
        if (width + height == 0) {
            // 获取图片数据的属性
            CFDictionaryRef properties = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, NULL);
            if (properties) {
                NSInteger orientationValue = -1;
                // 获取高度值
                CFTypeRef val = CFDictionaryGetValue(properties, kCGImagePropertyPixelHeight);
                if (val) CFNumberGetValue(val, kCFNumberLongType, &height);
                // 获取宽度值
                val = CFDictionaryGetValue(properties, kCGImagePropertyPixelWidth);
                if (val) CFNumberGetValue(val, kCFNumberLongType, &width);
                // 获取图片的方向值
                val = CFDictionaryGetValue(properties, kCGImagePropertyOrientation);
                if (val) CFNumberGetValue(val, kCFNumberNSIntegerType, &orientationValue);
                CFRelease(properties);

                // When we draw to Core Graphics, we lose orientation information,
                // which means the image below born of initWithCGIImage will be
                // oriented incorrectly sometimes. (Unlike the image born of initWithData
                // in didCompleteWithError.) So save it here and pass it on later.
#if SD_UIKIT || SD_WATCH
                orientation = [[self class] orientationFromPropertyValue:(orientationValue == -1 ? 1 : orientationValue)];
#endif
            }
        }
        /*
         * 这个表示已经收到部分图片数据并且还么有获取到所有的图片数据
         */
        if (width + height > 0 && totalSize < self.expectedSize) {
            // Create the image
            CGImageRef partialImageRef = CGImageSourceCreateImageAtIndex(imageSource, 0, NULL);

#if SD_UIKIT || SD_WATCH
            // Workaround for iOS anamorphic image
            if (partialImageRef) {
                const size_t partialHeight = CGImageGetHeight(partialImageRef);
                CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
                CGContextRef bmContext = CGBitmapContextCreate(NULL, width, height, 8, width * 4, colorSpace, kCGBitmapByteOrderDefault | kCGImageAlphaPremultipliedFirst);
                CGColorSpaceRelease(colorSpace);
                if (bmContext) {
                    CGContextDrawImage(bmContext, (CGRect){.origin.x = 0.0f, .origin.y = 0.0f, .size.width = width, .size.height = partialHeight}, partialImageRef);
                    CGImageRelease(partialImageRef);
                    partialImageRef = CGBitmapContextCreateImage(bmContext);
                    CGContextRelease(bmContext);
                }
                else {
                    CGImageRelease(partialImageRef);
                    partialImageRef = nil;
                }
            }
#endif

            if (partialImageRef) {
#if SD_UIKIT || SD_WATCH
                UIImage *image = [UIImage imageWithCGImage:partialImageRef scale:1 orientation:orientation];
#elif SD_MAC
                UIImage *image = [[UIImage alloc] initWithCGImage:partialImageRef size:NSZeroSize];
#endif
                // 获取图片url对应的key
                NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
                // 根据原始图片数据获取对应scale下面的图片
                UIImage *scaledImage = [self scaledImageForKey:key image:image];
                // 是否解压缩图片
                if (self.shouldDecompressImages) {
                    /*
                     *解压缩图片
                     */
                    image = [UIImage decodedImageWithImage:scaledImage];
                }
                else {
                    image = scaledImage;
                }
                CGImageRelease(partialImageRef);
                
                [self callCompletionBlocksWithImage:image imageData:nil error:nil finished:NO];
            }
        }

        CFRelease(imageSource);
    }

    for (SDWebImageDownloaderProgressBlock progressBlock in [self callbacksForKey:kProgressCallbackKey]) {
        progressBlock(self.imageData.length, self.expectedSize, self.request.URL);
    }
}

- (void)URLSession:(NSURLSession *)session
          dataTask:(NSURLSessionDataTask *)dataTask
 willCacheResponse:(NSCachedURLResponse *)proposedResponse
 completionHandler:(void (^)(NSCachedURLResponse *cachedResponse))completionHandler {
    //根据request的选项。决定是否缓存NSCachedURLResponse
    NSCachedURLResponse *cachedResponse = proposedResponse;

    if (self.request.cachePolicy == NSURLRequestReloadIgnoringLocalCacheData) {
        // Prevents caching of responses
        cachedResponse = nil;
    }
    if (completionHandler) {
        completionHandler(cachedResponse);
    }
}

#pragma mark NSURLSessionTaskDelegate

/*
 网络请求加载完成，在这里处理获得的数据
 */
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
    @synchronized(self) {
        self.dataTask = nil;
        //发送图片下载完成的通知
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:self];
            if (!error) {
                [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadFinishNotification object:self];
            }
        });
    }
    
    if (error) {
        [self callCompletionBlocksWithError:error];
    } else {
        if ([self callbacksForKey:kCompletedCallbackKey].count > 0) {
            /**
             *  If you specified to use `NSURLCache`, then the response you get here is what you need.
             *  if you specified to only use cached data via `SDWebImageDownloaderIgnoreCachedResponse`,
             *  the response data will be nil.
             *  So we don't need to check the cache option here, since the system will obey the cache option
             */
            if (self.imageData) {
                UIImage *image = [UIImage sd_imageWithData:self.imageData];
                //获取url对应的缓存Key
                NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
                
                image = [self scaledImageForKey:key image:image];
                
                // Do not force decoding animated GIFs
                if (!image.images) {
                    //是否加压缩图片数据
                    if (self.shouldDecompressImages) {
                        //如果设置了SDWebImageDownloaderScaleDownLargeImages。则返回处理过的图片
                        if (self.options & SDWebImageDownloaderScaleDownLargeImages) {
#if SD_UIKIT || SD_WATCH
                            image = [UIImage decodedAndScaledDownImageWithImage:image];
                            [self.imageData setData:UIImagePNGRepresentation(image)];
#endif
                        } else {
                            image = [UIImage decodedImageWithImage:image];
                        }
                    }
                }
                //构建回调Block
                if (CGSizeEqualToSize(image.size, CGSizeZero)) {
                    [self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0 pixels"}]];
                } else {
                    [self callCompletionBlocksWithImage:image imageData:self.imageData error:nil finished:YES];
                }
            } else {
                [self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Image data is nil"}]];
            }
        }
    }
    [self done];
}
/*
 验证HTTPS的证书
 */
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential *credential))completionHandler {
    
    NSURLSessionAuthChallengeDisposition disposition = NSURLSessionAuthChallengePerformDefaultHandling;
    __block NSURLCredential *credential = nil;
    //使用可信任证书机构的证书
    if ([challenge.protectionSpace.authenticationMethod isEqualToString:NSURLAuthenticationMethodServerTrust]) {
        //如果SDWebImageDownloaderAllowInvalidSSLCertificates属性设置了，则不验证SSL证书。直接信任
        if (!(self.options & SDWebImageDownloaderAllowInvalidSSLCertificates)) {
            disposition = NSURLSessionAuthChallengePerformDefaultHandling;
        } else {
            credential = [NSURLCredential credentialForTrust:challenge.protectionSpace.serverTrust];
            disposition = NSURLSessionAuthChallengeUseCredential;
        }
    } else {
        //使用自己生成的证书
        if (challenge.previousFailureCount == 0) {
            if (self.credential) {
                credential = self.credential;
                disposition = NSURLSessionAuthChallengeUseCredential;
            } else {
                disposition = NSURLSessionAuthChallengeCancelAuthenticationChallenge;
            }
        } else {
            disposition = NSURLSessionAuthChallengeCancelAuthenticationChallenge;
        }
    }
    //验证证书
    if (completionHandler) {
        completionHandler(disposition, credential);
    }
}

#pragma mark Helper methods

#if SD_UIKIT || SD_WATCH

/**
 把整数转换为对应的枚举值

 @param value 整数值
 @return 枚举值
 */
+ (UIImageOrientation)orientationFromPropertyValue:(NSInteger)value {
    switch (value) {
        case 1:
            return UIImageOrientationUp;
        case 3:
            return UIImageOrientationDown;
        case 8:
            return UIImageOrientationLeft;
        case 6:
            return UIImageOrientationRight;
        case 2:
            return UIImageOrientationUpMirrored;
        case 4:
            return UIImageOrientationDownMirrored;
        case 5:
            return UIImageOrientationLeftMirrored;
        case 7:
            return UIImageOrientationRightMirrored;
        default:
            return UIImageOrientationUp;
    }
}
#endif

/**
* 通过image对象获取对应scale模式下的图像
 */
- (nullable UIImage *)scaledImageForKey:(nullable NSString *)key image:(nullable UIImage *)image {
    return SDScaledImageForKey(key, image);
}

- (BOOL)shouldContinueWhenAppEntersBackground {
    return self.options & SDWebImageDownloaderContinueInBackground;
}

- (void)callCompletionBlocksWithError:(nullable NSError *)error {
    [self callCompletionBlocksWithImage:nil imageData:nil error:error finished:YES];
}


/**
 处理回调

 @param image UIImage数据
 @param imageData Image的data数据
 @param error 错误
 @param finished 是否完成的标记位
 */
- (void)callCompletionBlocksWithImage:(nullable UIImage *)image
                            imageData:(nullable NSData *)imageData
                                error:(nullable NSError *)error
                             finished:(BOOL)finished {
    //获取key对应的回调Block数组
    NSArray<id> *completionBlocks = [self callbacksForKey:kCompletedCallbackKey];
    dispatch_main_async_safe(^{
        //调用回调
        for (SDWebImageDownloaderCompletedBlock completedBlock in completionBlocks) {
            completedBlock(image, imageData, error, finished);
        }
    });
}

@end