簡(jiǎn)書博客已經(jīng)暫停更新殉农，想看更多技術(shù)博客請(qǐng)到：

• 掘金 ：J_Knight_
• 個(gè)人博客: J_Knight_
• 個(gè)人公眾號(hào)：程序員維他命

相信對(duì)于廣大的iOS開發(fā)者蛛砰，對(duì)SDWebImage并不會(huì)陌生吨拗，這個(gè)框架通過給UIImageView和UIButton添加分類，實(shí)現(xiàn)一個(gè)異步下載圖片并且支持緩存的功能。整個(gè)框架的接口非常簡(jiǎn)潔，每個(gè)類的分工都很明確状知，是很值得大家學(xué)習(xí)的。

在使用這個(gè)框架的時(shí)候孽查，只需要提供一個(gè)下載的url和占位圖就可以在回調(diào)里拿到下載后的圖片：

[imageview sd_setImageWithURL:[NSURL URLWithString:@"pic.jpg"] placeholderImage:[UIImage imageNamed:@"placeholder"] completed:^(UIImage * _Nullable image, NSError * _Nullable error, SDImageCacheType cacheType, NSURL * _Nullable imageURL) {
        
        imageview.image = image;
        NSLog(@"圖片加載完成");
        
    }];

而且我們還可以不設(shè)置占位圖片饥悴，也可以不使用回調(diào)的block，非常靈活:

//圖片下載完成后直接顯示下載后的圖片
[imageview sd_setImageWithURL:[NSURL URLWithString:@"pic.jpg"]];

在最開始先簡(jiǎn)單介紹這個(gè)框架：

這個(gè)框架的核心類是SDWebImageManger盲再，在外部有UIImageView+WebCache 和 UIButton+WebCache 為下載圖片的操作提供接口西设。內(nèi)部有SDWebImageManger負(fù)責(zé)處理和協(xié)調(diào) SDWebImageDownloader 和 SDWebImageCache：SDWebImageDownloader負(fù)責(zé)具體的下載任務(wù)，SDWebImageCache負(fù)責(zé)關(guān)于緩存的工作：添加答朋，刪除贷揽，查詢緩存。

首先我們大致看一下這個(gè)框架的調(diào)用流程圖：

SDWebImage

從這個(gè)流程圖里可以大致看出梦碗，該框架分為兩個(gè)層：UIKit層（負(fù)責(zé)接收下載參數(shù)）和工具層（負(fù)責(zé)下載操作和緩存）禽绪。

OK～基本流程大概清楚了蓖救，我們看一下每個(gè)層具體實(shí)現(xiàn)吧～

UIKit層

該框架最外層的類是UIImageView +WebCache，我們將圖片的URL印屁，占位圖片直接給這個(gè)類循捺。下面是這個(gè)類的公共接口：

 // ==============  UIImageView + WebCache.h ============== //
- (void)sd_setImageWithURL:(nullable NSURL *)url;

- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder;

- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
                   options:(SDWebImageOptions)options;

- (void)sd_setImageWithURL:(nullable NSURL *)url
                 completed:(nullable SDExternalCompletionBlock)completedBlock;

- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
                 completed:(nullable SDExternalCompletionBlock)completedBlock;

- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
                   options:(SDWebImageOptions)options
                 completed:(nullable SDExternalCompletionBlock)completedBlock;

- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
                   options:(SDWebImageOptions)options
                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                 completed:(nullable SDExternalCompletionBlock)completedBlock;

可以看出，這個(gè)類提供的接口非常靈活雄人，可以根據(jù)我們自己的需求來調(diào)用其中某一個(gè)方法从橘，而這些方法到最后都會(huì)走到：

// ==============  UIView+ WebCache.m ============== //
- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
                   options:(SDWebImageOptions)options
                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                 completed:(nullable SDExternalCompletionBlock)completedBlock;

而這個(gè)方法里面，調(diào)用的是UIView+WebCache分類的：

// ==============  UIView+ WebCache.m ============== //
- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
                  placeholderImage:(nullable UIImage *)placeholder
                           options:(SDWebImageOptions)options
                      operationKey:(nullable NSString *)operationKey
                     setImageBlock:(nullable SDSetImageBlock)setImageBlock
                          progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                         completed:(nullable SDExternalCompletionBlock)completedBlock;

為什么不是UIImageView+WebCache而要上一層到UIView的分類里呢柠衍？
因?yàn)镾DWebImage框架也支持UIButton的下載圖片等方法洋满，所以需要在它們的父類：UIView里面統(tǒng)一一個(gè)下載方法。

簡(jiǎn)單看一下這個(gè)方法的實(shí)現(xiàn)（省略的代碼用...代替）：

 // ==============  UIView+ WebCache.m ============== //

    //valid key：UIImageView || UIButton
    NSString *validOperationKey = operationKey ?: NSStringFromClass([self class]);
    //UIView+WebCacheOperation 的 operationDictionary
    //下面這行代碼是保證沒有當(dāng)前正在進(jìn)行的異步下載操作, 使它不會(huì)與即將進(jìn)行的操作發(fā)生沖突
    [self sd_cancelImageLoadOperationWithKey:validOperationKey];
    

    //添加臨時(shí)的占位圖（在不延遲添加占位圖的option下）
    if (!(options & SDWebImageDelayPlaceholder)) {
        dispatch_main_async_safe(^{
            [self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
        });
    }
    
    //如果url存在
    if (url) {
       
       ...
        __weak __typeof(self)wself = self;

       //SDWebImageManager下載圖片
        id <SDWebImageOperation> operation = [SDWebImageManager.sharedManager loadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
          
            ...
            //dispatch_main_sync_safe : 保證block能在主線程進(jìn)行
            dispatch_main_async_safe(^{
                
                if (!sself) {
                    return;
                }
               
                if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock) {
                     //image珍坊，而且不自動(dòng)替換 placeholder image
                    completedBlock(image, error, cacheType, url);
                    return;
                    
                } else if (image) {
                    //存在image牺勾，需要馬上替換 placeholder image
                    [sself sd_setImage:image imageData:data basedOnClassOrViaCustomSetImageBlock:setImageBlock];
                    [sself sd_setNeedsLayout];
                
                } else {                    
                    //沒有image，在圖片下載完之后顯示 placeholder image
                    if ((options & SDWebImageDelayPlaceholder)) {
                        [sself sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
                        [sself sd_setNeedsLayout];
                    }
                }
                
                if (completedBlock && finished) {
                    completedBlock(image, error, cacheType, url);
                }
            });
        }];
        
        //在操作緩存字典（operationDictionary）里添加operation阵漏，表示當(dāng)前的操作正在進(jìn)行
        [self sd_setImageLoadOperation:operation forKey:validOperationKey];
        
    } else {
        //如果url不存在驻民，就在completedBlock里傳入error（url為空）
        dispatch_main_async_safe(^{
            [self sd_removeActivityIndicator];
            if (completedBlock) {
                NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
                completedBlock(nil, error, SDImageCacheTypeNone, url);
            }
        });
    }
    ```



> 值得一提的是，在這一層履怯，使用一個(gè)字典``operationDictionary``專門用作存儲(chǔ)操作的緩存回还，隨時(shí)添加，刪除操作任務(wù)叹洲。
而這個(gè)字典是``UIView+WebCacheOperation``分類的關(guān)聯(lián)對(duì)象柠硕，它的存取方法使用運(yùn)行時(shí)來操作：


```objc
 // ==============  UIView+WebCacheOperation.m ============== //
 //獲取關(guān)聯(lián)對(duì)象：operations（用來存放操作的字典）
- (SDOperationsDictionary *)operationDictionary {
    SDOperationsDictionary *operations = objc_getAssociatedObject(self, &loadOperationKey);
    //存放操作的字典
    if (operations) {
        return operations;
    }
    
    //如果沒有，就新建一個(gè)
    operations = [NSMutableDictionary dictionary];
    
    objc_setAssociatedObject(self, &loadOperationKey, operations, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    return operations;
}

為什么不直接在UIImageView+WebCache里直接關(guān)聯(lián)這個(gè)對(duì)象呢运提？我覺得這里作者應(yīng)該是遵從面向?qū)ο蟮?strong>單一職責(zé)原則（SRP：Single responsibility principle）蝗柔，就連類都要履行這個(gè)職責(zé)，何況分類呢民泵？這里作者專門創(chuàng)造一個(gè)分類UIView+WebCacheOperation來管理操作緩存（字典）癣丧。

到這里，UIKit層上面的東西都講完了栈妆，現(xiàn)在開始正式講解工具層胁编。

工具層

上文提到過，SDWebImageManager同時(shí)管理SDImageCache和SDWebImageDownloader兩個(gè)類鳞尔，它是這一層的老大哥嬉橙。在下載任務(wù)開始的時(shí)候，SDWebImageManager首先訪問SDImageCache來查詢是否存在緩存寥假，如果有緩存憎夷，直接返回緩存的圖片。如果沒有緩存昧旨，就命令SDWebImageDownloader來下載圖片拾给，下載成功后祥得，存入緩存，顯示圖片蒋得。以上是SDWebImageManager大致的工作流程级及。

在詳細(xì)講解SDWebImageManager是如何下載圖片之前，我們先看一下這個(gè)類的幾個(gè)重要的屬性：

 // ==============  SDWebImageManager.h ============== //
@property (strong, nonatomic, readwrite, nonnull) SDImageCache *imageCache;//管理緩存
@property (strong, nonatomic, readwrite, nonnull) SDWebImageDownloader //下載器*imageDownloader;
@property (strong, nonatomic, nonnull) NSMutableSet<NSURL *> *failedURLs;//記錄失效url的名單
@property (strong, nonatomic, nonnull) NSMutableArray<SDWebImageCombinedOperation *> *runningOperations;//記錄當(dāng)前正在執(zhí)行的操作

SDWebImageManager下載圖片的方法只有一個(gè)：

[SDWebImageManager.sharedManager loadImageWithURL:options:progress:completed:]

看一下這個(gè)方法的具體實(shí)現(xiàn)：

 // ==============  SDWebImageManager.m ============== //
- (id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
                                     options:(SDWebImageOptions)options
                                    progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                   completed:(nullable SDInternalCompletionBlock)completedBlock {
     ...                             
    //在SDImageCache里查詢是否存在緩存的圖片
    operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {
        
        ...
        //（沒有緩存圖片） || （即使有緩存圖片额衙，也需要更新緩存圖片） || （代理沒有響應(yīng)imageManager:shouldDownloadImageForURL:消息饮焦，默認(rèn)返回yes，需要下載圖片）|| （imageManager:shouldDownloadImageForURL:返回yes窍侧，需要下載圖片）
        if ((!cachedImage || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
            
            //1. 存在緩存圖片 && 即使有緩存圖片也要下載更新圖片
            if (cachedImage && options & SDWebImageRefreshCached) {
                [self callCompletionBlockForOperation:weakOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
            }


            // 2. 如果不存在緩存圖片
            ...
            
            //開啟下載器下載
            //subOperationToken 用來標(biāo)記當(dāng)前的下載任務(wù)县踢，便于被取消
            SDWebImageDownloadToken *subOperationToken = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) {
                __strong __typeof(weakOperation) strongOperation = weakOperation;
                if (!strongOperation || strongOperation.isCancelled) {
                    // 1. 如果任務(wù)被取消，則什么都不做伟件，避免和其他的completedBlock重復(fù)
                
                } else if (error) {
                    
                    //2. 如果有錯(cuò)誤
                    //2.1 在completedBlock里傳入error
                    [self callCompletionBlockForOperation:strongOperation completion:completedBlock error:error url:url];

                            //2.2 在錯(cuò)誤url名單中添加當(dāng)前的url
                    if (   error.code != NSURLErrorNotConnectedToInternet
                        && error.code != NSURLErrorCancelled
                        && error.code != NSURLErrorTimedOut
                        && error.code != NSURLErrorInternationalRoamingOff
                        && error.code != NSURLErrorDataNotAllowed
                        && error.code != NSURLErrorCannotFindHost
                        && error.code != NSURLErrorCannotConnectToHost) {
                        
                       @synchronized (self.failedURLs) {
                            [self.failedURLs addObject:url];
                        }
                    }
                }
                else {
                    
                    //3. 下載成功
                    //3.1 如果需要下載失敗后重新下載硼啤，則將當(dāng)前url從失敗url名單里移除
                    if ((options & SDWebImageRetryFailed)) {
                        @synchronized (self.failedURLs) {
                            [self.failedURLs removeObject:url];
                        }
                    }
                    
                    //3.2 進(jìn)行緩存
                    BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);

                   
                    if (options & SDWebImageRefreshCached && cachedImage && !downloadedImage) {
                    
                        //（即使緩存存在，也要刷新圖片） && 緩存圖片 && 不存在下載后的圖片：不做操作
                                           
                    } else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)]) {
                        
          //（下載圖片成功 && （沒有動(dòng)圖||處理動(dòng)圖） && （下載之后斧账，緩存之前處理圖片）               dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
                            UIImage *transformedImage = [self.delegate imageManager:self transformDownloadedImage:downloadedImage withURL:url];

                            if (transformedImage && finished) {
                                BOOL imageWasTransformed = ![transformedImage isEqual:downloadedImage];
                                // pass nil if the image was transformed, so we can recalculate the data from the image
                                //緩存圖片
                                [self.imageCache storeImage:transformedImage imageData:(imageWasTransformed ? nil : downloadedData) forKey:key toDisk:cacheOnDisk completion:nil];
                            }
                            //將圖片傳入completedBlock
                            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:transformedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                        });
                    } else {
                        
                        //(圖片下載成功并結(jié)束)
                        if (downloadedImage && finished) {
                            [self.imageCache storeImage:downloadedImage imageData:downloadedData forKey:key toDisk:cacheOnDisk completion:nil];
                        }
                        
                        [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:downloadedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                    }
                }

                     //如果完成谴返，從當(dāng)前運(yùn)行的操作列表里移除當(dāng)前操作
                if (finished) {
                    [self safelyRemoveOperationFromRunning:strongOperation];
                }
            }];
            
            //取消的block
            operation.cancelBlock = ^{
            
                //取消當(dāng)前的token
                [self.imageDownloader cancel:subOperationToken];
                __strong __typeof(weakOperation) strongOperation = weakOperation;
                //從當(dāng)前運(yùn)行的操作列表里移除當(dāng)前操作
                [self safelyRemoveOperationFromRunning:strongOperation];
            };
        
        } else if (cachedImage) {
            
            //存在緩存圖片
            __strong __typeof(weakOperation) strongOperation = weakOperation;
            
            //調(diào)用完成的block
            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
            
            //刪去當(dāng)前的的下載操作（線程安全）
            [self safelyRemoveOperationFromRunning:operation];
        
        } else {
            
            //沒有緩存的圖片，而且下載被代理終止了
            __strong __typeof(weakOperation) strongOperation = weakOperation;
           
            // 調(diào)用完成的block
            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:nil data:nil error:nil cacheType:SDImageCacheTypeNone finished:YES url:url];
            
            //刪去當(dāng)前的下載操作
            [self safelyRemoveOperationFromRunning:operation];
        }
    }];

    return operation;                                                             
}

看完了SDWebImageManager的回調(diào)處理咧织，我們分別看一下
SDImageCache和SDWebImageDownloader內(nèi)部具體是如何工作的嗓袱。首先看一下SDImageCache：

SDImageCache

屬性

 // ==============  SDImageCache.m ============== //
@property (strong, nonatomic, nonnull) NSCache *memCache;//內(nèi)存緩存
@property (strong, nonatomic, nonnull) NSString *diskCachePath;//磁盤緩存路徑
@property (strong, nonatomic, nullable) NSMutableArray<NSString *> *customPaths;//
@property (SDDispatchQueueSetterSementics, nonatomic, nullable) dispatch_queue_t //ioQueue唯一子線程;

核心方法：查詢緩存

 // ==============  SDImageCache.m ============== //
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key done:(nullable SDCacheQueryCompletedBlock)doneBlock {
   
    if (!key) {
        if (doneBlock) {
            doneBlock(nil, nil, SDImageCacheTypeNone);
        }
        return nil;
    }
        
    //================查看內(nèi)存的緩存=================//
    UIImage *image = [self imageFromMemoryCacheForKey:key];
    
    // 如果存在，直接調(diào)用block习绢，將image渠抹，data，CaheType傳進(jìn)去
    if (image) {
    
        NSData *diskData = nil;
        
        //如果是gif闪萄，就拿到data梧却，后面要傳到doneBlock里。不是gif就傳nil
        if ([image isGIF]) {
            diskData = [self diskImageDataBySearchingAllPathsForKey:key];
        }
        
        if (doneBlock) {
            doneBlock(image, diskData, SDImageCacheTypeMemory);
        }
        
        //因?yàn)閳D片有緩存可供使用桃煎，所以不用實(shí)例化NSOperation，直接范圍nil
        return nil;
    }

    //================查看磁盤的緩存=================//
    NSOperation *operation = [NSOperation new];
    
    //唯一的子線程：self.ioQueue
    dispatch_async(self.ioQueue, ^{
        
        if (operation.isCancelled) {
            // 在用之前就判斷operation是否被取消了大刊，作者考慮的非常嚴(yán)謹(jǐn)
            return;
        }

        @autoreleasepool {
            
            NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];
            UIImage *diskImage = [self diskImageForKey:key];
            
            if (diskImage && self.config.shouldCacheImagesInMemory) {
                
                // cost 被用來計(jì)算緩存中所有對(duì)象的代價(jià)为迈。當(dāng)內(nèi)存受限或者所有緩存對(duì)象的總代價(jià)超過了最大允許的值時(shí)，緩存會(huì)移除其中的一些對(duì)象缺菌。
                NSUInteger cost = SDCacheCostForImage(diskImage);
                
                //存入內(nèi)存緩存中
                [self.memCache setObject:diskImage forKey:key cost:cost];
            }

            if (doneBlock) {
                dispatch_async(dispatch_get_main_queue(), ^{
                    doneBlock(diskImage, diskData, SDImageCacheTypeDisk);
                });
            }
        }
    });

    return operation;
}

SDWebImageDownloader

屬性

 // ==============  SDWebImageDownloader.m ============== //
@property (strong, nonatomic, nonnull) NSOperationQueue *downloadQueue;//下載隊(duì)列
@property (weak, nonatomic, nullable) NSOperation *lastAddedOperation;//最后添加的下載操作
@property (assign, nonatomic, nullable) Class operationClass;//操作類
@property (strong, nonatomic, nonnull) NSMutableDictionary<NSURL *, SDWebImageDownloaderOperation *> *URLOperations;//操作數(shù)組
@property (strong, nonatomic, nullable) SDHTTPHeadersMutableDictionary *HTTPHeaders;//HTTP請(qǐng)求頭
@property (SDDispatchQueueSetterSementics, nonatomic, nullable) dispatch_queue_t barrierQueue;//用來阻塞前面的下載線程（串行化）

核心方法：下載圖片

 // ==============  SDWebImageDownloader.m ============== //
- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
                                                   options:(SDWebImageDownloaderOptions)options
                                                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                                 completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock {
    __weak SDWebImageDownloader *wself = self;

    return [self addProgressCallback:progressBlock completedBlock:completedBlock forURL:url createCallback:^SDWebImageDownloaderOperation *{
        
        __strong __typeof (wself) sself = wself;
        
        NSTimeInterval timeoutInterval = sself.downloadTimeout;
        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
        
        //創(chuàng)建下載請(qǐng)求
        NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:(options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData) timeoutInterval:timeoutInterval];
        request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies);
        request.HTTPShouldUsePipelining = YES;
        if (sself.headersFilter) {
            request.allHTTPHeaderFields = sself.headersFilter(url, [sself.HTTPHeaders copy]);
        }
        else {
            request.allHTTPHeaderFields = sself.HTTPHeaders;
        }
        
        //創(chuàng)建下載操作：SDWebImageDownloaderOperation用于請(qǐng)求網(wǎng)絡(luò)資源的操作葫辐，它是一個(gè) NSOperation 的子類
        SDWebImageDownloaderOperation *operation = [[sself.operationClass alloc] initWithRequest:request inSession:sself.session options:options];
        operation.shouldDecompressImages = sself.shouldDecompressImages;
        
        //url證書
        if (sself.urlCredential) {
            operation.credential = sself.urlCredential;
        } else if (sself.username && sself.password) {
            operation.credential = [NSURLCredential credentialWithUser:sself.username password:sself.password persistence:NSURLCredentialPersistenceForSession];
        }
        
        //優(yōu)先級(jí)
        if (options & SDWebImageDownloaderHighPriority) {
            operation.queuePriority = NSOperationQueuePriorityHigh;
        } else if (options & SDWebImageDownloaderLowPriority) {
            operation.queuePriority = NSOperationQueuePriorityLow;
        }

        //在下載隊(duì)列里添加下載操作，執(zhí)行下載操作
        [sself.downloadQueue addOperation:operation];
        
        //如果后進(jìn)先出
        if (sself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {
            // Emulate LIFO execution order by systematically adding new operations as last operation's dependency
            //addDependency:參數(shù)opertaion倍添加到NSOperationQueue后伴郁，只有等該opertion結(jié)束后才能執(zhí)行其他的operation耿战，實(shí)現(xiàn)了后進(jìn)先出
            [sself.lastAddedOperation addDependency:operation];
            sself.lastAddedOperation = operation;
        }

        return operation;
    }];
}

這里面還有一個(gè)addProgressCallback: progressBlock: completedBlock: forURL: createCallback:方法，用來保存progressBlock和completedBlock焊傅。我們看一下這個(gè)方法的實(shí)現(xiàn)：

 // ==============  SDWebImageDownloader.m ============== //
- (nullable SDWebImageDownloadToken *)addProgressCallback:(SDWebImageDownloaderProgressBlock)progressBlock
                                           completedBlock:(SDWebImageDownloaderCompletedBlock)completedBlock
                                                   forURL:(nullable NSURL *)url
                                           createCallback:(SDWebImageDownloaderOperation *(^)())createCallback {

    // url 用來作為回調(diào)字典的key剂陡，如果為空狈涮，立即返回失敗 
    if (url == nil) {
        if (completedBlock != nil) {
            completedBlock(nil, nil, nil, NO);
        }
        return nil;
    }

    __block SDWebImageDownloadToken *token = nil;

    //串行化前面所有的操作
    dispatch_barrier_sync(self.barrierQueue, ^{
    
            //當(dāng)前下載操作中取出SDWebImageDownloaderOperation實(shí)例
        SDWebImageDownloaderOperation *operation = self.URLOperations[url];
        
        if (!operation) {
        //如果沒有，就初始化它
            operation = createCallback();
            self.URLOperations[url] = operation;
            __weak SDWebImageDownloaderOperation *woperation = operation;
            
            operation.completionBlock = ^{
              SDWebImageDownloaderOperation *soperation = woperation;
              if (!soperation) return;
              if (self.URLOperations[url] == soperation) {
                  [self.URLOperations removeObjectForKey:url];
              };
            };
        }
        id downloadOperationCancelToken = [operation addHandlersForProgress:progressBlock completed:completedBlock];
        //這里 downloadOperationCancelToken 默認(rèn)是一個(gè)字典鸭栖，存放 progressBlock 和 completedBlock
        token = [SDWebImageDownloadToken new];
        token.url = url;
        token.downloadOperationCancelToken = downloadOperationCancelToken;
    });

    return token;
}

這里真正保存兩個(gè)block的方法是addHandlersForProgress: completed:：

- (nullable id)addHandlersForProgress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                            completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock {
    //實(shí)例化一個(gè)SDCallbacksDictionary歌馍，存放一個(gè)progressBlock 和 completedBlock
    SDCallbacksDictionary *callbacks = [NSMutableDictionary new];
    if (progressBlock) callbacks[kProgressCallbackKey] = [progressBlock copy];
    if (completedBlock) callbacks[kCompletedCallbackKey] = [completedBlock copy];
    dispatch_barrier_async(self.barrierQueue, ^{
        //添加到緩存中 self.callbackBlocks
        [self.callbackBlocks addObject:callbacks];
    });
    return callbacks;
}

到這里SDWebImage的核心方法都講解完畢了，其他沒有講到的部分以后會(huì)慢慢添加上去晕鹊。

最后看一下一些比較零散的知識(shí)點(diǎn)：

1. 運(yùn)行時(shí)存取關(guān)聯(lián)對(duì)象：

存：

objc_setAssociatedObject(self, &loadOperationKey, operations, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
//將operations對(duì)象關(guān)聯(lián)給self松却，地址為&loadOperationKey，語(yǔ)義是OBJC_ASSOCIATION_RETAIN_NONATOMIC溅话。

认汀：

SDOperationsDictionary *operations = objc_getAssociatedObject(self, &loadOperationKey);
//將operations對(duì)象通過地址&loadOperationKey從self里取出來

2. 數(shù)組的寫操作需要加鎖（多線程訪問，避免覆寫）

//給self.runningOperations加鎖
//self.runningOperations數(shù)組的添加操作
    @synchronized (self.runningOperations) {
        [self.runningOperations addObject:operation];
    }

//self.runningOperations數(shù)組的刪除操作
- (void)safelyRemoveOperationFromRunning:(nullable SDWebImageCombinedOperation*)operation {
    @synchronized (self.runningOperations) {
        if (operation) {
            [self.runningOperations removeObject:operation];
        }
    }
}

3. 確保在主線程的宏：

dispatch_main_async_safe(^{
                 //將下面這段代碼放在主線程中
            [self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
        });

//宏定義：
#define dispatch_main_async_safe(block)\
    if (strcmp(dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL), dispatch_queue_get_label(dispatch_get_main_queue())) == 0) {\
        block();\
    } else {\
        dispatch_async(dispatch_get_main_queue(), block);\
    }
#endif

4. 設(shè)置不能為nil的參數(shù)

- (nonnull instancetype)initWithCache:(nonnull SDImageCache *)cache downloader:(nonnull SDWebImageDownloader *)downloader {
    if ((self = [super init])) {
        _imageCache = cache;
        _imageDownloader = downloader;
        _failedURLs = [NSMutableSet new];
        _runningOperations = [NSMutableArray new];
    }
    return self;
}

如果在參數(shù)里添加了nonnull關(guān)鍵字飞几，那么編譯器就可以檢查傳入的參數(shù)是否為nil砚哆，如果是，則編譯器會(huì)有警告

5. 容錯(cuò)循狰，強(qiáng)制轉(zhuǎn)換類型

if ([url isKindOfClass:NSString.class]) {
        url = [NSURL URLWithString:(NSString *)url];
}

在傳入的參數(shù)為NSString時(shí)（但是方法參數(shù)要求是NSURL）窟社，自動(dòng)轉(zhuǎn)換為NSURL

貌似還有圖片解碼等內(nèi)容沒有詳細(xì)看，以后會(huì)逐漸補(bǔ)充噠～

本文已經(jīng)同步到我的個(gè)人技術(shù)博客：傳送門绪钥，歡迎常來^^

本文已在版權(quán)印備案灿里，如需轉(zhuǎn)載請(qǐng)?jiān)L問版權(quán)印。48422928

獲取授權(quán)

-------------------------------- 2018年7月17日更新 --------------------------------

注意注意３谈埂Ｏ坏酢！

筆者在近期開通了個(gè)人公眾號(hào)寸潦，主要分享編程色鸳，讀書筆記，思考類的文章见转。

• 編程類文章：包括筆者以前發(fā)布的精選技術(shù)文章命雀，以及后續(xù)發(fā)布的技術(shù)文章（以原創(chuàng)為主），并且逐漸脫離 iOS 的內(nèi)容斩箫，將側(cè)重點(diǎn)會(huì)轉(zhuǎn)移到提高編程能力的方向上吏砂。
• 讀書筆記類文章：分享編程類，思考類乘客，心理類狐血，職場(chǎng)類書籍的讀書筆記。
• 思考類文章：分享筆者平時(shí)在技術(shù)上易核，生活上的思考匈织。

因?yàn)楣娞?hào)每天發(fā)布的消息數(shù)有限制，所以到目前為止還沒有將所有過去的精選文章都發(fā)布在公眾號(hào)上，后續(xù)會(huì)逐步發(fā)布的缀匕。

而且因?yàn)楦鞔蟛┛推脚_(tái)的各種限制纳决，后面還會(huì)在公眾號(hào)上發(fā)布一些短小精干，以小見大的干貨文章哦~

掃下方的公眾號(hào)二維碼并點(diǎn)擊關(guān)注弦追，期待與您的共同成長(zhǎng)~

公眾號(hào)：程序員維他命