每張圖片的下載都有一個(gè)Operation操作，操作的創(chuàng)建和加入已在方法

-downloadImageWithURL:options:progress:completed:
-addProgressCallback:completedBlock:forURL:createCallback:

中進(jìn)行。

操作的初始化方法為：

- (id)initWithRequest:(NSURLRequest *)request
            inSession:(NSURLSession *)session
              options:(SDWebImageDownloaderOptions)options
             progress:(SDWebImageDownloaderProgressBlock)progressBlock
            completed:(SDWebImageDownloaderCompletedBlock)completedBlock
            cancelled:(SDWebImageNoParamsBlock)cancelBlock;

SDWebImageDownloaderOperation遵守 SDWebImageOperation中的協(xié)議：

- (void)cancel;

代碼注釋如下：

/*
 * This file is part of the SDWebImage package.
 * (c) Olivier Poitrey <rs@dailymotion.com>
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

#import "SDWebImageDownloaderOperation.h"
#import "SDWebImageDecoder.h"
#import "UIImage+MultiFormat.h"
#import <ImageIO/ImageIO.h>
#import "SDWebImageManager.h"

NSString *const SDWebImageDownloadStartNotification = @"SDWebImageDownloadStartNotification";
NSString *const SDWebImageDownloadReceiveResponseNotification = @"SDWebImageDownloadReceiveResponseNotification";
NSString *const SDWebImageDownloadStopNotification = @"SDWebImageDownloadStopNotification";
NSString *const SDWebImageDownloadFinishNotification = @"SDWebImageDownloadFinishNotification";

@interface SDWebImageDownloaderOperation ()

@property (copy, nonatomic) SDWebImageDownloaderProgressBlock progressBlock;
@property (copy, nonatomic) SDWebImageDownloaderCompletedBlock completedBlock;
@property (copy, nonatomic) SDWebImageNoParamsBlock cancelBlock;

// 是否正在執(zhí)行
@property (assign, nonatomic, getter = isExecuting) BOOL executing;
// 是否已完成
@property (assign, nonatomic, getter = isFinished) BOOL finished;
// 圖片數(shù)據(jù)
@property (strong, nonatomic) NSMutableData *imageData;

// This is weak because it is injected by whoever manages this session. If this gets nil-ed out, we won't be able to run
// the task associated with this operation
// URL連接會(huì)話(huà)：因管理此會(huì)話(huà)的注入所以采用weak修飾。如不為nil，則無(wú)法運(yùn)行此操作關(guān)聯(lián)的任務(wù)
@property (weak, nonatomic) NSURLSession *unownedSession;
// This is set if we're using not using an injected NSURLSession. We're responsible of invalidating this one
// URL連接會(huì)話(huà)：strong修飾，不太理解這兩個(gè)區(qū)別？？？？？？
@property (strong, nonatomic) NSURLSession *ownedSession;
// URL連接會(huì)話(huà)任務(wù)：用于圖片下載數(shù)據(jù)傳輸。擴(kuò)展：http://www.cnblogs.com/biosli/p/iOS_Network_URL_Session.html
@property (strong, nonatomic, readwrite) NSURLSessionTask *dataTask;

@property (strong, atomic) NSThread *thread;

#if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0
// app退到后臺(tái)時(shí)，主線(xiàn)程會(huì)被暫停。但想在后臺(tái)完成一個(gè)長(zhǎng)期任務(wù)，須調(diào)用UIApplication的beginBackgroundTaskWithExpirationHandler:實(shí)例方法,來(lái)向 iOS 借點(diǎn)時(shí)間
// UIBackgroundTaskIdentifier相當(dāng)于一個(gè)借據(jù)，告訴iOS，程序?qū)⒁韪嗟臅r(shí)間來(lái)完成Long-Running Task任務(wù)
@property (assign, nonatomic) UIBackgroundTaskIdentifier backgroundTaskId;
#endif

@end

@implementation SDWebImageDownloaderOperation {
    size_t width, height; // 圖片的寬、高
    UIImageOrientation orientation; // 圖片方向
    BOOL responseFromCached; // 是否從緩存中響應(yīng)
}

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

- (id)initWithRequest:(NSURLRequest *)request
              options:(SDWebImageDownloaderOptions)options
             progress:(SDWebImageDownloaderProgressBlock)progressBlock
            completed:(SDWebImageDownloaderCompletedBlock)completedBlock
            cancelled:(SDWebImageNoParamsBlock)cancelBlock {

    return [self initWithRequest:request
                       inSession:nil
                         options:options
                        progress:progressBlock
                       completed:completedBlock
                       cancelled:cancelBlock];
}

- (id)initWithRequest:(NSURLRequest *)request
            inSession:(NSURLSession *)session
              options:(SDWebImageDownloaderOptions)options
             progress:(SDWebImageDownloaderProgressBlock)progressBlock
            completed:(SDWebImageDownloaderCompletedBlock)completedBlock
            cancelled:(SDWebImageNoParamsBlock)cancelBlock {
    if ((self = [super init])) {
        _request = request;
        _shouldDecompressImages = YES;
        _options = options;
        _progressBlock = [progressBlock copy];
        _completedBlock = [completedBlock copy];
        _cancelBlock = [cancelBlock copy];
        _executing = NO;
        _finished = NO;
        _expectedSize = 0;
        _unownedSession = session;
        responseFromCached = YES; // Initially wrong until `- URLSession:dataTask:willCacheResponse:completionHandler: is called or not called
    }
    return self;
}

/**
 *  復(fù)寫(xiě)NSOperation的start方法：沒(méi)有簡(jiǎn)單的實(shí)現(xiàn)main方法，而是采用更加靈活的start方法，以便自己管理下載的狀態(tài)，創(chuàng)建了下載所使用的NSURLSession對(duì)象
 */
- (void)start {
    // @synchronized 的作用是創(chuàng)建一個(gè)互斥鎖，保證此時(shí)沒(méi)有其它線(xiàn)程對(duì)self對(duì)象進(jìn)行修改。這個(gè)是objective-c的一個(gè)鎖定令牌，防止self對(duì)象在同一時(shí)間內(nèi)被其它線(xiàn)程訪(fǎng)問(wèn)，起到線(xiàn)程的保護(hù)作用。
    @synchronized (self) {
        // 管理下載狀態(tài)，如果已取消，則重置當(dāng)前下載并設(shè)置完成狀態(tài)YES
        if (self.isCancelled) {
            self.finished = YES;
            [self reset];
            return;
        }

#if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0
        Class UIApplicationClass = NSClassFromString(@"UIApplication");
        BOOL hasApplication = UIApplicationClass && [UIApplicationClass respondsToSelector:@selector(sharedApplication)];
        // 是否允許后臺(tái)執(zhí)行
        if (hasApplication && [self shouldContinueWhenAppEntersBackground]) {
            __weak __typeof__ (self) wself = self;
            UIApplication * app = [UIApplicationClass performSelector:@selector(sharedApplication)];
            // 標(biāo)記一個(gè)可以在后臺(tái)長(zhǎng)時(shí)間運(yùn)行的后臺(tái)任務(wù)
            self.backgroundTaskId = [app beginBackgroundTaskWithExpirationHandler:^{
                __strong __typeof (wself) sself = wself;

                // 當(dāng)應(yīng)用程序留給后臺(tái)的時(shí)間快要結(jié)束時(shí)（該時(shí)間有限），這個(gè)block將執(zhí)行：進(jìn)行一些清理工作（主線(xiàn)程執(zhí)行），清理失敗會(huì)導(dǎo)致程序掛掉
                if (sself) {
                    [sself cancel];

                    // 標(biāo)記指定的后臺(tái)任務(wù)完成
                    [app endBackgroundTask:sself.backgroundTaskId];
                    // 銷(xiāo)毀后臺(tái)任務(wù)標(biāo)識(shí)符
                    sself.backgroundTaskId = UIBackgroundTaskInvalid;
                }
            }];
        }
#endif
        NSURLSession *session = self.unownedSession;
        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.
             */
            // delegateQueue設(shè)置為nil(設(shè)置delegate在哪個(gè)OperationQueue回調(diào))：以便創(chuàng)建一個(gè)用于執(zhí)行所有委托方法和完成處理程序調(diào)用的串行操作隊(duì)列
            self.ownedSession = [NSURLSession sessionWithConfiguration:sessionConfig
                                                              delegate:self
                                                         delegateQueue:nil];
            session = self.ownedSession;
        }
        
        // 創(chuàng)建數(shù)據(jù)任務(wù)
        self.dataTask = [session dataTaskWithRequest:self.request];
        self.executing = YES;
        self.thread = [NSThread currentThread];
    }
    
    // 啟動(dòng)任務(wù)
    [self.dataTask resume];

    if (self.dataTask) {
        if (self.progressBlock) {
            // 剛啟動(dòng)，還不知道圖片大小
            self.progressBlock(0, NSURLResponseUnknownLength);
        }
        // 在主線(xiàn)程發(fā)送下載開(kāi)始通知
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStartNotification object:self];
        });
    }
    else {
        if (self.completedBlock) {
            self.completedBlock(nil, nil, [NSError errorWithDomain:NSURLErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Connection can't be initialized"}], YES);
        }
    }

    // 確保后臺(tái)任務(wù)標(biāo)識(shí)符銷(xiāo)毀
#if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0
    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
}

- (void)cancel {
    @synchronized (self) {
        if (self.thread) {
            [self performSelector:@selector(cancelInternalAndStop) onThread:self.thread withObject:nil waitUntilDone:NO];
        }
        else {
            [self cancelInternal];
        }
    }
}

- (void)cancelInternalAndStop {
    if (self.isFinished) return;
    [self cancelInternal];
}

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

    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.
        if (self.isExecuting) self.executing = NO;
        if (!self.isFinished) self.finished = YES;
    }

    [self reset];
}

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

/**
 *  重置當(dāng)前下載
 */
- (void)reset {
    self.cancelBlock = nil;
    self.completedBlock = nil;
    self.progressBlock = nil;
    self.dataTask = nil;
    self.imageData = nil;
    self.thread = nil;
    if (self.ownedSession) {
        [self.ownedSession invalidateAndCancel];
        self.ownedSession = nil;
    }
}

- (void)setFinished:(BOOL)finished {
    [self willChangeValueForKey:@"isFinished"];
    _finished = finished;
    [self didChangeValueForKey:@"isFinished"];
}

- (void)setExecuting:(BOOL)executing {
    [self willChangeValueForKey:@"isExecuting"];
    _executing = executing;
    [self didChangeValueForKey:@"isExecuting"];
}

- (BOOL)isConcurrent {
    return YES;
}

#pragma mark -- NSURLSessionDataDelegate
// 告訴delegate已經(jīng)接收到服務(wù)器的初始應(yīng)答, 接下來(lái)準(zhǔn)備數(shù)據(jù)任務(wù)的操作
- (void)URLSession:(NSURLSession *)session
          dataTask:(NSURLSessionDataTask *)dataTask
didReceiveResponse:(NSURLResponse *)response
 completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler {
    
    //'304 Not Modified' is an exceptional one
    // HTTP狀態(tài)碼：200+正常成功的；300+重定向；400+請(qǐng)求錯(cuò)誤;500+一般時(shí)服務(wù)端的問(wèn)題；304 — 未修改，未按預(yù)期修改文檔。  
    if (![response respondsToSelector:@selector(statusCode)] || ([((NSHTTPURLResponse *)response) statusCode] < 400 && [((NSHTTPURLResponse *)response) statusCode] != 304)) {
        // 預(yù)期文件的長(zhǎng)度，對(duì)于下載來(lái)說(shuō)就是文件的大小
        NSInteger expected = response.expectedContentLength > 0 ? (NSInteger)response.expectedContentLength : 0;
        self.expectedSize = expected;
        if (self.progressBlock) {
            self.progressBlock(0, expected);
        }
        
        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.
        // 返回304狀態(tài)碼時(shí)，說(shuō)明服務(wù)器上的圖片沒(méi)有改變。僅需要取消操作，返回緩存中的圖片
        if (code == 304) {
            [self cancelInternal];
        } else {
            [self.dataTask cancel];
        }
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:self];
        });
        
        if (self.completedBlock) {
            self.completedBlock(nil, nil, [NSError errorWithDomain:NSURLErrorDomain code:[((NSHTTPURLResponse *)response) statusCode] userInfo:nil], YES);
        }
        [self done];
    }
    
    if (completionHandler) {
        completionHandler(NSURLSessionResponseAllow);
    }
}

// 告訴delegate已經(jīng)接收到部分?jǐn)?shù)據(jù)
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data {
    // 一張圖片的數(shù)據(jù)是追加到imageData后
    [self.imageData appendData:data];

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

        // 已下載圖片數(shù)據(jù)的大小（字節(jié)）：Get the total bytes downloaded
        const NSInteger totalSize = self.imageData.length;

        // 更新數(shù)據(jù)源，須傳遞所有的數(shù)據(jù)，而不只是新的字節(jié)：Update the data source, we must pass ALL the data, not just the new bytes
        CGImageSourceRef imageSource = CGImageSourceCreateWithData((__bridge CFDataRef)self.imageData, NULL);

        // 首次獲得圖片的數(shù)據(jù)時(shí)，從圖片的數(shù)據(jù)源中獲取圖片相關(guān)信息：方向、高、寬
        if (width + height == 0) {
            CFDictionaryRef properties = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, NULL);
            if (properties) {
                NSInteger orientationValue = -1;
                
                // 高
                CFTypeRef val = CFDictionaryGetValue(properties, kCGImagePropertyPixelHeight);
                if (val) CFNumberGetValue(val, kCFNumberLongType, &height);
                
                // 寬
                val = CFDictionaryGetValue(properties, kCGImagePropertyPixelWidth);
                if (val) CFNumberGetValue(val, kCFNumberLongType, &width);
                
                // 方向
                val = CFDictionaryGetValue(properties, kCGImagePropertyOrientation);
                if (val) CFNumberGetValue(val, kCFNumberNSIntegerType, &orientationValue);
                
                // CF相關(guān)操作注意手動(dòng)釋放
                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.
                // 當(dāng)繪制到Core Graphics時(shí)，我們會(huì)丟失方向信息，這意味著有時(shí)候由initWithCGIImage創(chuàng)建的
                // 圖片的方向不對(duì)。（不像在connectionDidFinishLoading中用initWithData創(chuàng)建的圖片）
                // 所以在這邊我們先保存這個(gè)信息并在后面使用
                orientation = [[self class] orientationFromPropertyValue:(orientationValue == -1 ? 1 : orientationValue)];
            }

        }

        // 圖片還未下載完成
        if (width + height > 0 && totalSize < self.expectedSize) {
            // 使用現(xiàn)有的數(shù)據(jù)創(chuàng)建圖片對(duì)象，如果數(shù)據(jù)中存有多張圖片，則取第一張：Create the image
            CGImageRef partialImageRef = CGImageSourceCreateImageAtIndex(imageSource, 0, NULL);

#ifdef TARGET_OS_IPHONE
            // 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) {
                // 對(duì)圖片進(jìn)行縮放、解碼操作：按道理說(shuō)這里比較耗資源，為啥不添加自動(dòng)釋放池？
                UIImage *image = [UIImage imageWithCGImage:partialImageRef scale:1 orientation:orientation];
                NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
                UIImage *scaledImage = [self scaledImageForKey:key image:image];
                if (self.shouldDecompressImages) {
                    image = [UIImage decodedImageWithImage:scaledImage];
                }
                else {
                    image = scaledImage;
                }
                CGImageRelease(partialImageRef);
                // 確保下載完的回調(diào)在主線(xiàn)程中執(zhí)行
                dispatch_main_sync_safe(^{
                    if (self.completedBlock) {
                        self.completedBlock(image, nil, nil, NO);
                    }
                });
            }
        }

        CFRelease(imageSource);
    }

    if (self.progressBlock) {
        self.progressBlock(self.imageData.length, self.expectedSize);
    }
}

// 告訴delegate是否把response存儲(chǔ)到緩存中
- (void)URLSession:(NSURLSession *)session
          dataTask:(NSURLSessionDataTask *)dataTask
 willCacheResponse:(NSCachedURLResponse *)proposedResponse
 completionHandler:(void (^)(NSCachedURLResponse *cachedResponse))completionHandler {

    responseFromCached = NO; // 如果這個(gè)代理方法被調(diào)用，說(shuō)明緩存中沒(méi)有對(duì)改response進(jìn)行緩存：If this method is called, it means the response wasn't read from cache
    NSCachedURLResponse *cachedResponse = proposedResponse;

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

#pragma mark -- NSURLSessionTaskDelegate
// 告訴delegate，task已經(jīng)完成：即一張圖片已完整下載完這個(gè)task完成時(shí)，對(duì)圖片進(jìn)行處理后回調(diào)
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
    @synchronized(self) {
        self.thread = nil;
        self.dataTask = nil;
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:self];
            if (!error) {
                [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadFinishNotification object:self];
            }
        });
    }
    
    if (error) {
        if (self.completedBlock) {
            self.completedBlock(nil, nil, error, YES);
        }
    } else {
        SDWebImageDownloaderCompletedBlock completionBlock = self.completedBlock;
        
        if (![[NSURLCache sharedURLCache] cachedResponseForRequest:_request]) {
            responseFromCached = NO;
        }
        
        if (completionBlock) {
            if (self.options & SDWebImageDownloaderIgnoreCachedResponse && responseFromCached) {
                completionBlock(nil, nil, nil, YES);
            } else if (self.imageData) {
                UIImage *image = [UIImage sd_imageWithData:self.imageData];
                NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
                image = [self scaledImageForKey:key image:image];
                
                // Gif圖片不要解碼：Do not force decoding animated GIFs
                if (!image.images) {
                    if (self.shouldDecompressImages) {
                        image = [UIImage decodedImageWithImage:image];
                    }
                }
                if (CGSizeEqualToSize(image.size, CGSizeZero)) {
                    // 空?qǐng)D片
                    completionBlock(nil, nil, [NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0 pixels"}], YES);
                }
                else {
                    completionBlock(image, self.imageData, nil, YES);
                }
            } else {
                completionBlock(nil, nil, [NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Image data is nil"}], YES);
            }
        }
    }
    
    self.completionBlock = nil;
    [self done];
}

// 告訴delegate, task已收到授權(quán)：處理服務(wù)器返回的證書(shū), 需要在該方法中告訴系統(tǒng)是否需要安裝服務(wù)器返回的證書(shū)
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential *credential))completionHandler {
    
    NSURLSessionAuthChallengeDisposition disposition = NSURLSessionAuthChallengePerformDefaultHandling;
    __block NSURLCredential *credential = nil;
    
    // 判斷服務(wù)器返回的證書(shū)是否是服務(wù)器信任的
    if ([challenge.protectionSpace.authenticationMethod isEqualToString:NSURLAuthenticationMethodServerTrust]) {
        if (!(self.options & SDWebImageDownloaderAllowInvalidSSLCertificates)) {
            // 默認(rèn)處理方式
            disposition = NSURLSessionAuthChallengePerformDefaultHandling;
        } else {
            // 使用指定證書(shū)
            credential = [NSURLCredential credentialForTrust:challenge.protectionSpace.serverTrust];
            disposition = NSURLSessionAuthChallengeUseCredential;
        }
    } else {
        if ([challenge previousFailureCount] == 0) {
            if (self.credential) {
                credential = self.credential;
                disposition = NSURLSessionAuthChallengeUseCredential;
            } else {
                disposition = NSURLSessionAuthChallengeCancelAuthenticationChallenge;
            }
        } else {
            disposition = NSURLSessionAuthChallengeCancelAuthenticationChallenge;
        }
    }
    
    // 安裝證書(shū)
    if (completionHandler) {
        completionHandler(disposition, credential);
    }
}

#pragma mark Helper methods

+ (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;
    }
}

/**
 *  放縮圖片：根據(jù)圖片的scale 或 圖片組 重新計(jì)算返回圖片
 *
 *  @param key   key
 *  @param image image
 *
 *  @return UIImage
 */
- (UIImage *)scaledImageForKey:(NSString *)key image:(UIImage *)image {
    return SDScaledImageForKey(key, image);
}

/**
 *  App在后臺(tái)運(yùn)行，相關(guān)任務(wù)是否繼續(xù)執(zhí)行
 *
 *  @return <#return value description#>
 */
- (BOOL)shouldContinueWhenAppEntersBackground {
    return self.options & SDWebImageDownloaderContinueInBackground;
}

@end