SDWebImage 源码解析-白红宇

相信对于广大的iOS开发者，对并不会陌生，这个框架通过给UIImageView和UIButton添加分类，实现一个异步下载图片并且支持缓存的功能。整个框架的接口非常简洁，每个类的分工都很明确，是很值得大家学习的。

在使用这个框架的时候，只需要提供一个下载的url和占位图就可以在回调里拿到下载后的图片：

[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(@"图片加载完成");        }];复制代码

而且我们还可以不设置占位图片，也可以不使用回调的block，非常灵活:

//图片下载完成后直接显示下载后的图片[imageview sd_setImageWithURL:[NSURL URLWithString:@"pic.jpg"]];复制代码

在最开始先简单介绍这个框架：

这个框架的核心类是SDWebImageManger，在外部有UIImageView+WebCache 和 UIButton+WebCache 为下载图片的操作提供接口。内部有SDWebImageManger负责处理和协调 SDWebImageDownloader 和 SDWebImageCache：SDWebImageDownloader负责具体的下载任务，SDWebImageCache负责关于缓存的工作：添加，删除，查询缓存。

首先我们大致看一下这个框架的调用流程图：

从这个流程图里可以大致看出，该框架分为两个层：UIKit层（负责接收下载参数）和工具层（负责下载操作和缓存）。

OK～基本流程大概清楚了，我们看一下每个层具体实现吧～

UIKit层

该框架最外层的类是UIImageView +WebCache，我们将图片的URL，占位图片直接给这个类。下面是这个类的公共接口：

// ==============  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;复制代码

可以看出，这个类提供的接口非常灵活，可以根据我们自己的需求来调用其中某一个方法，而这些方法到最后都会走到：

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

而这个方法里面，调用的是UIView+WebCache分类的：

// ==============  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;复制代码

为什么不是UIImageView+WebCache而要上一层到UIView的分类里呢？因为SDWebImage框架也支持UIButton的下载图片等方法，所以需要在它们的父类：UIView里面统一一个下载方法。

简单看一下这个方法的实现（省略的代码用...代替）：

// ==============  UIView+ WebCache.m ============== //    //valid key：UIImageView || UIButton    NSString *validOperationKey = operationKey ?: NSStringFromClass([self class]);    //UIView+WebCacheOperation 的 operationDictionary    //下面这行代码是保证没有当前正在进行的异步下载操作, 使它不会与即将进行的操作发生冲突    [self sd_cancelImageLoadOperationWithKey:validOperationKey];        //添加临时的占位图（在不延迟添加占位图的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下载图片        id 
    
      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 : 保证block能在主线程进行            dispatch_main_async_safe(^{                                if (!sself) {                    return;                }                               if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock) {                     //image，而且不自动替换 placeholder image                    completedBlock(image, error, cacheType, url);                    return;                                    } else if (image) {                    //存在image，需要马上替换 placeholder image                    [sself sd_setImage:image imageData:data basedOnClassOrViaCustomSetImageBlock:setImageBlock];                    [sself sd_setNeedsLayout];                                } else {                                        //没有image，在图片下载完之后显示 placeholder image                    if ((options & SDWebImageDelayPlaceholder)) {                        [sself sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];                        [sself sd_setNeedsLayout];                    }                }                                if (completedBlock && finished) {                    completedBlock(image, error, cacheType, url);                }            });        }];                //在操作缓存字典（operationDictionary）里添加operation，表示当前的操作正在进行        [self sd_setImageLoadOperation:operation forKey:validOperationKey];            } else {        //如果url不存在，就在completedBlock里传入error（url为空）        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，在这一层，使用一个字典``operationDictionary``专门用作存储操作的缓存，随时添加，删除操作任务。而这个字典是``UIView+WebCacheOperation``分类的关联对象，它的存取方法使用运行时来操作：```objc // ==============  UIView+WebCacheOperation.m ============== // //获取关联对象：operations（用来存放操作的字典）- (SDOperationsDictionary *)operationDictionary {    SDOperationsDictionary *operations = objc_getAssociatedObject(self, &loadOperationKey);    //存放操作的字典    if (operations) {        return operations;    }        //如果没有，就新建一个    operations = [NSMutableDictionary dictionary];        objc_setAssociatedObject(self, &loadOperationKey, operations, OBJC_ASSOCIATION_RETAIN_NONATOMIC);    return operations;}复制代码

为什么不直接在UIImageView+WebCache里直接关联这个对象呢？我觉得这里作者应该是遵从面向对象的单一职责原则（SRP：Single responsibility principle），就连类都要履行这个职责，何况分类呢？这里作者专门创造一个分类UIView+WebCacheOperation来管理操作缓存（字典）。

到这里，UIKit层上面的东西都讲完了，现在开始正式讲解工具层。

工具层

上文提到过，SDWebImageManager同时管理SDImageCache和SDWebImageDownloader两个类，它是这一层的老大哥。在下载任务开始的时候，SDWebImageManager首先访问SDImageCache来查询是否存在缓存，如果有缓存，直接返回缓存的图片。如果没有缓存，就命令SDWebImageDownloader来下载图片，下载成功后，存入缓存，显示图片。以上是SDWebImageManager大致的工作流程。

在详细讲解SDWebImageManager是如何下载图片之前，我们先看一下这个类的几个重要的属性：

// ==============  SDWebImageManager.h ============== //@property (strong, nonatomic, readwrite, nonnull) SDImageCache *imageCache;//管理缓存@property (strong, nonatomic, readwrite, nonnull) SDWebImageDownloader //下载器*imageDownloader;@property (strong, nonatomic, nonnull) NSMutableSet
    
      *failedURLs;//记录失效url的名单@property (strong, nonatomic, nonnull) NSMutableArray
     
       *runningOperations;//记录当前正在执行的操作复制代码

SDWebImageManager下载图片的方法只有一个：

[SDWebImageManager.sharedManager loadImageWithURL:options:progress:completed:]复制代码

看一下这个方法的具体实现：

// ==============  SDWebImageManager.m ============== //- (id 
    
     )loadImageWithURL:(nullable NSURL *)url                                     options:(SDWebImageOptions)options                                    progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock                                   completed:(nullable SDInternalCompletionBlock)completedBlock {     ...                                 //在SDImageCache里查询是否存在缓存的图片    operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {                ...        //（没有缓存图片） || （即使有缓存图片，也需要更新缓存图片） || （代理没有响应imageManager:shouldDownloadImageForURL:消息，默认返回yes，需要下载图片）|| （imageManager:shouldDownloadImageForURL:返回yes，需要下载图片）        if ((!cachedImage || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {                        //1. 存在缓存图片 && 即使有缓存图片也要下载更新图片            if (cachedImage && options & SDWebImageRefreshCached) {                [self callCompletionBlockForOperation:weakOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];            }            // 2. 如果不存在缓存图片            ...                        //开启下载器下载            //subOperationToken 用来标记当前的下载任务，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. 如果任务被取消，则什么都不做，避免和其他的completedBlock重复                                } else if (error) {                                        //2. 如果有错误                    //2.1 在completedBlock里传入error                    [self callCompletionBlockForOperation:strongOperation completion:completedBlock error:error url:url];							//2.2 在错误url名单中添加当前的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. 下载成功                    //3.1 如果需要下载失败后重新下载，则将当前url从失败url名单里移除                    if ((options & SDWebImageRetryFailed)) {                        @synchronized (self.failedURLs) {                            [self.failedURLs removeObject:url];                        }                    }                                        //3.2 进行缓存                    BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);                                       if (options & SDWebImageRefreshCached && cachedImage && !downloadedImage) {                                            //（即使缓存存在，也要刷新图片） && 缓存图片 && 不存在下载后的图片：不做操作                                                               } else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)]) {                                  //（下载图片成功 && （没有动图||处理动图） && （下载之后，缓存之前处理图片）               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                                //缓存图片                                [self.imageCache storeImage:transformedImage imageData:(imageWasTransformed ? nil : downloadedData) forKey:key toDisk:cacheOnDisk completion:nil];                            }                            //将图片传入completedBlock                            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:transformedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];                        });                    } else {                                                //(图片下载成功并结束)                        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，从当前运行的操作列表里移除当前操作                if (finished) {                    [self safelyRemoveOperationFromRunning:strongOperation];                }            }];                        //取消的block            operation.cancelBlock = ^{                            //取消当前的token                [self.imageDownloader cancel:subOperationToken];                __strong __typeof(weakOperation) strongOperation = weakOperation;                //从当前运行的操作列表里移除当前操作                [self safelyRemoveOperationFromRunning:strongOperation];            };                } else if (cachedImage) {                        //存在缓存图片            __strong __typeof(weakOperation) strongOperation = weakOperation;                        //调用完成的block            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];                        //删去当前的的下载操作（线程安全）            [self safelyRemoveOperationFromRunning:operation];                } else {                        //没有缓存的图片，而且下载被代理终止了            __strong __typeof(weakOperation) strongOperation = weakOperation;                       // 调用完成的block            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:nil data:nil error:nil cacheType:SDImageCacheTypeNone finished:YES url:url];                        //删去当前的下载操作            [self safelyRemoveOperationFromRunning:operation];        }    }];    return operation;                                                             }复制代码

看完了SDWebImageManager的回调处理，我们分别看一下 SDImageCache和SDWebImageDownloader内部具体是如何工作的。首先看一下SDImageCache：

SDImageCache

属性

// ==============  SDImageCache.m ============== //@property (strong, nonatomic, nonnull) NSCache *memCache;//内存缓存@property (strong, nonatomic, nonnull) NSString *diskCachePath;//磁盘缓存路径@property (strong, nonatomic, nullable) NSMutableArray
    
      *customPaths;//@property (SDDispatchQueueSetterSementics, nonatomic, nullable) dispatch_queue_t //ioQueue唯一子线程;复制代码

核心方法：查询缓存

// ==============  SDImageCache.m ============== //- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key done:(nullable SDCacheQueryCompletedBlock)doneBlock {       if (!key) {        if (doneBlock) {            doneBlock(nil, nil, SDImageCacheTypeNone);        }        return nil;    }		    //================查看内存的缓存=================//    UIImage *image = [self imageFromMemoryCacheForKey:key];        // 如果存在，直接调用block，将image，data，CaheType传进去    if (image) {            NSData *diskData = nil;                //如果是gif，就拿到data，后面要传到doneBlock里。不是gif就传nil        if ([image isGIF]) {            diskData = [self diskImageDataBySearchingAllPathsForKey:key];        }                if (doneBlock) {            doneBlock(image, diskData, SDImageCacheTypeMemory);        }                //因为图片有缓存可供使用，所以不用实例化NSOperation，直接范围nil        return nil;    }    //================查看磁盘的缓存=================//    NSOperation *operation = [NSOperation new];        //唯一的子线程：self.ioQueue    dispatch_async(self.ioQueue, ^{                if (operation.isCancelled) {            // 在用之前就判断operation是否被取消了，作者考虑的非常严谨            return;        }        @autoreleasepool {                        NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];            UIImage *diskImage = [self diskImageForKey:key];                        if (diskImage && self.config.shouldCacheImagesInMemory) {                                // cost 被用来计算缓存中所有对象的代价。当内存受限或者所有缓存对象的总代价超过了最大允许的值时，缓存会移除其中的一些对象。                NSUInteger cost = SDCacheCostForImage(diskImage);                                //存入内存缓存中                [self.memCache setObject:diskImage forKey:key cost:cost];            }            if (doneBlock) {                dispatch_async(dispatch_get_main_queue(), ^{                    doneBlock(diskImage, diskData, SDImageCacheTypeDisk);                });            }        }    });    return operation;}复制代码

SDWebImageDownloader

属性

// ==============  SDWebImageDownloader.m ============== //@property (strong, nonatomic, nonnull) NSOperationQueue *downloadQueue;//下载队列@property (weak, nonatomic, nullable) NSOperation *lastAddedOperation;//最后添加的下载操作@property (assign, nonatomic, nullable) Class operationClass;//操作类@property (strong, nonatomic, nonnull) NSMutableDictionary
    
      *URLOperations;//操作数组@property (strong, nonatomic, nullable) SDHTTPHeadersMutableDictionary *HTTPHeaders;//HTTP请求头@property (SDDispatchQueueSetterSementics, nonatomic, nullable) dispatch_queue_t barrierQueue;//用来阻塞前面的下载线程（串行化）复制代码

核心方法：下载图片

// ==============  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                //创建下载请求        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;        }                //创建下载操作：SDWebImageDownloaderOperation用于请求网络资源的操作，它是一个 NSOperation 的子类        SDWebImageDownloaderOperation *operation = [[sself.operationClass alloc] initWithRequest:request inSession:sself.session options:options];        operation.shouldDecompressImages = sself.shouldDecompressImages;                //url证书        if (sself.urlCredential) {            operation.credential = sself.urlCredential;        } else if (sself.username && sself.password) {            operation.credential = [NSURLCredential credentialWithUser:sself.username password:sself.password persistence:NSURLCredentialPersistenceForSession];        }                //优先级        if (options & SDWebImageDownloaderHighPriority) {            operation.queuePriority = NSOperationQueuePriorityHigh;        } else if (options & SDWebImageDownloaderLowPriority) {            operation.queuePriority = NSOperationQueuePriorityLow;        }        //在下载队列里添加下载操作，执行下载操作        [sself.downloadQueue addOperation:operation];                //如果后进先出        if (sself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {            // Emulate LIFO execution order by systematically adding new operations as last operation's dependency            //addDependency:参数opertaion倍添加到NSOperationQueue后，只有等该opertion结束后才能执行其他的operation，实现了后进先出            [sself.lastAddedOperation addDependency:operation];            sself.lastAddedOperation = operation;        }        return operation;    }];}复制代码

这里面还有一个addProgressCallback: progressBlock: completedBlock: forURL: createCallback:方法，用来保存progressBlock和completedBlock。我们看一下这个方法的实现：

// ==============  SDWebImageDownloader.m ============== //- (nullable SDWebImageDownloadToken *)addProgressCallback:(SDWebImageDownloaderProgressBlock)progressBlock                                           completedBlock:(SDWebImageDownloaderCompletedBlock)completedBlock                                                   forURL:(nullable NSURL *)url                                           createCallback:(SDWebImageDownloaderOperation *(^)())createCallback {    // url 用来作为回调字典的key，如果为空，立即返回失败     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, ^{            	//当前下载操作中取出SDWebImageDownloaderOperation实例        SDWebImageDownloaderOperation *operation = self.URLOperations[url];                if (!operation) {        //如果没有，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];        //这里 downloadOperationCancelToken 默认是一个字典，存放 progressBlock 和 completedBlock        token = [SDWebImageDownloadToken new];        token.url = url;        token.downloadOperationCancelToken = downloadOperationCancelToken;    });    return token;}复制代码

这里真正保存两个block的方法是addHandlersForProgress: completed:：

- (nullable id)addHandlersForProgress:(nullable SDWebImageDownloaderProgressBlock)progressBlock                            completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock {    //实例化一个SDCallbacksDictionary，存放一个progressBlock 和 completedBlock    SDCallbacksDictionary *callbacks = [NSMutableDictionary new];    if (progressBlock) callbacks[kProgressCallbackKey] = [progressBlock copy];    if (completedBlock) callbacks[kCompletedCallbackKey] = [completedBlock copy];    dispatch_barrier_async(self.barrierQueue, ^{        //添加到缓存中 self.callbackBlocks        [self.callbackBlocks addObject:callbacks];    });    return callbacks;}复制代码

到这里SDWebImage的核心方法都讲解完毕了，其他没有讲到的部分以后会慢慢添加上去。

最后看一下一些比较零散的知识点：

1. 运行时存取关联对象：

存：

objc_setAssociatedObject(self, &loadOperationKey, operations, OBJC_ASSOCIATION_RETAIN_NONATOMIC);//将operations对象关联给self，地址为&loadOperationKey，语义是OBJC_ASSOCIATION_RETAIN_NONATOMIC。复制代码

取：

SDOperationsDictionary *operations = objc_getAssociatedObject(self, &loadOperationKey);//将operations对象通过地址&loadOperationKey从self里取出来复制代码

2. 数组的写操作需要加锁（多线程访问，避免覆写）

//给self.runningOperations加锁//self.runningOperations数组的添加操作    @synchronized (self.runningOperations) {        [self.runningOperations addObject:operation];    }//self.runningOperations数组的删除操作- (void)safelyRemoveOperationFromRunning:(nullable SDWebImageCombinedOperation*)operation {    @synchronized (self.runningOperations) {        if (operation) {            [self.runningOperations removeObject:operation];        }    }}复制代码

3. 确保在主线程的宏：

dispatch_main_async_safe(^{ 				 //将下面这段代码放在主线程中            [self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];        });//宏定义：#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复制代码

4. 设置不能为nil的参数

- (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;}复制代码

如果在参数里添加了nonnull关键字，那么编译器就可以检查传入的参数是否为nil，如果是，则编译器会有警告

5. 容错，强制转换类型

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

在传入的参数为NSString时（但是方法参数要求是NSURL），自动转换为NSURL

貌似还有图片解码等内容没有详细看，以后会逐渐补充哒～

本文已经同步到我的个人技术博客：

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

注意注意！！！

笔者在近期开通了个人公众号，主要分享编程，读书笔记，思考类的文章。

编程类文章：包括笔者以前发布的精选技术文章，以及后续发布的技术文章（以原创为主），并且逐渐脱离 iOS 的内容，将侧重点会转移到提高编程能力的方向上。

读书笔记类文章：分享编程类，思考类，心理类，职场类书籍的读书笔记。

思考类文章：分享笔者平时在技术上，生活上的思考。

因为公众号每天发布的消息数有限制，所以到目前为止还没有将所有过去的精选文章都发布在公众号上，后续会逐步发布的。

而且因为各大博客平台的各种限制，后面还会在公众号上发布一些短小精干，以小见大的干货文章哦~

扫下方的公众号二维码并点击关注，期待与您的共同成长~