SDWebImage源码解析

但凡经过几年移动开发经验的人去大公司面试，都会有公司问到，使用过哪些第三方，看过他们的源码嘛？而SDWebImage就是经常被面试官和应聘者的提到的。下面将讲述SDWebImage的源码解析以及实现原理，希望可以帮助大家加深对SDWebImage实现原理的理解！！！

一、前言

SDWebImage专门用于iOS图片加载框架，提供了网络下载并缓存图片，通过使用SDWebImage来管理图片的相关操作，从而让研发人员专注业务逻辑实现，提高开发效率。

首先我们来看一下它的代码的整体结构。

看过上面的整体结构之后，紧接着看下代码的构成和分类，以及各个类的作用。

通过以上代码结构划分，可以看出SDWebImage是非常强大的，我们平常用到的不过是冰山一角。

那么SDWebImage的功能有哪些呢。

1. 提供UIImageView的category用来加载和缓存网络图片。
2. 提供异步方式下载网络图片。
3. 提供GIF动画。
4. 提供同一个URL的网络图片不会被重新下载两次。
5. 支持Arm64。
6. 耗时放在子线程，确保不会阻塞主线程。

二、概述

整个下载图片的过程如下：

1. 入口 setImageWithURL:placeholderImage:options:会先将placeholderImage显示，然后使用SDWebImageManager根据URL来处理图片；
2. 随之调用SDWebImageManagerdownloadWithURL:delegate:options:userInfo:，紧接着交给SDImageCache从缓存里面查找是否下载了，调用queryDiskCacheForKey:delegate:
3. 先从内存缓存中查找是否有该图片，如果内存有该图片，SDImageCacheDelegate中回调imageCache:didFindImage
4. 然后SDWebImageManagerDelegate回调webImageManager:didFinishWithImage:最后到UIImageView+WebCache展示图片
5. 如果内存缓存中没有生成，此时会生成NSInvocationOperation添加到队列，从硬盘查找图片是否已经缓存。
6. 根据URLKey查看硬盘，是否硬盘目录下图片文件信息，是在NSOperation进行的操作，所以需要回到主线程中回调notifyDelegate：
7. 如果上一操作从硬盘存在需要读取到的图，然后会将图片加入到内存缓存中（假如内存过小，会首先清空内存缓存，通过SDImageCacheDelegate回调imageCache:didFindImage:forKey:userInfo:从而展示图片）
8. 如果从硬盘里面没有读到图片，说明所有的缓存都没有存在该图片，需要重新下载图片，进而回调imageCache:didNotFindImageForKey:userInfo:
9. 利用重新共享或者重新生成一个下载器SDWebImageDownloader完成下载图片
10. 图片下载是由NSURLConnection来做，实现相关的delegate来判断图片下载时的状态：下载中、下载成功和下载失败。
11. 然后通过connection:didReceiveData:利用ImageIO做了下载进度的加载效果
12. connectionDidFinishLoading：数据下载完成之后会首先交给SDWebImageDecoder来进一步做图片解码处理
13. 对于图片解码会在NSOperationQueue中进行完成，不会阻塞主线程UI。所以说如果有对图片二次三次处理的，也可以在这里面处理。
14. 当解码完成之后，会在主线程中notifyDelegateOnMainThreadWithInfo通知解码完成，通过使用imageDecoder:didFinishDecodingImage:userInfo方法回调给SDWebImageDownloader。
15. 通过使用imageDownloader:didFinishWithImage:方法来回调给SDWebImageDownloader来检测图片是否已经下载完成了。
16. 紧着发通知给所有的downloadDelegates下载完成，进而回调给需要的地方展示。
17. 将图片保存到SDImageCache中，同时也会加入内存缓存和硬盘缓存中，写文件到硬盘中是在NSInvocationOperation中完成的，为了避免阻塞主线程。
18. 在初始化的时候会注册消息通知，遇到内存警告或者退到后台会清理内存管理缓存，应用结束的时候清理过期图片。

三、代码解析

3.1 .SDWebImageManager

SDWebImageManager是拥有一个SDWebImageCache和SDWebImageDownloader两个属性分别用于图片的缓存和下载需求。

我们经常使用的UIImageView以及其他视图通过UIView+WebCache分类的sd_internalSetImageWithURL来调用SDWebImageManager类实现图片加载。

3.1.1 下面这个方法是核心方法。

- (nullable id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
                                              options:(SDWebImageOptions)options
                                             progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                            completed:(nullable SDInternalCompletionBlock)completedBlock;

下面我们看一下实现方法

@param url 图片的url地址
 @param options 获取图片的属性
 @param progressBlock 加载进度回调
 @param completedBlock 加载完成回调
 @return 返回一个加载的载体对象。以便提供给后面取消删除等。
 */
- (id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
                                     options:(SDWebImageOptions)options
                                    progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                   completed:(nullable SDInternalCompletionBlock)completedBlock {
    /*
     如果传入的url是NSString格式的。则转换为NSURL类型再处理
     */
    if ([url isKindOfClass:NSString.class]) {
        url = [NSURL URLWithString:(NSString *)url];
    }

    // Prevents app crashing on argument type error like sending NSNull instead of NSURL
    //如果url不会NSURL类型的对象。则置为nil
    if (![url isKindOfClass:NSURL.class]) {
        url = nil;
    }
    /*
     图片加载获取获取过程中绑定一个`SDWebImageCombinedOperation`对象。以方便后续再通过这个对象对url的加载控制。
     */
    __block SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
    __weak SDWebImageCombinedOperation *weakOperation = operation;

    BOOL isFailedUrl = NO;
    //当前url是否在失败url的集合里面
    if (url) {
        @synchronized (self.failedURLs) {
            isFailedUrl = [self.failedURLs containsObject:url];
        }
    }
    /*
     如果url是失败的url或者url有问题等各种问题。则直接根据opeation来做异常情况的处理
     */
    if (url.absoluteString.length ==  || (!(options & SDWebImageRetryFailed) && isFailedUrl)) {
        //构建回调Block
        [self callCompletionBlockForOperation:operation completion:completedBlock error:[NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorFileDoesNotExist userInfo:nil] url:url];
        return operation;
    }
    //把加载图片的一个载体存入runningOperations。里面是所有正在做图片加载过程的operation的集合。
    @synchronized (self.runningOperations) {
        [self.runningOperations addObject:operation];
    }
    //根据url获取url对应的key
    NSString *key = [self cacheKeyForURL:url];
    /*
    *如果图片是从内存加载，则返回的cacheOperation是nil，
    *如果是从磁盘加载，则返回的cacheOperation是`NSOperation`对象。
    *如果是从网络加载，则返回的cacheOperation对象是`SDWebImageDownloaderOperation`对象。
    */
    operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {
        //从缓存中获取图片数据返回
        //如果已经取消了操作。则直接返回并且移除对应的opetation对象
        if (operation.isCancelled) {
            [self safelyRemoveOperationFromRunning:operation];
            return;
        }

        if ((!cachedImage || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
            /**
             如果从缓存获取图片失败。或者设置了SDWebImageRefreshCached来忽略缓存。则先把缓存的图片返回。
             */
            if (cachedImage && options & SDWebImageRefreshCached) {
                //构建回调Block
                [self callCompletionBlockForOperation:weakOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
            }

            // download if no image or requested to refresh anyway, and download allowed by delegate
            /*
             把图片加载的`SDWebImageOptions`类型枚举转换为图片下载的`SDWebImageDownloaderOptions`类型的枚举
             */
            SDWebImageDownloaderOptions downloaderOptions = ;
            if (options & SDWebImageLowPriority) downloaderOptions |= SDWebImageDownloaderLowPriority;
            if (options & SDWebImageProgressiveDownload) downloaderOptions |= SDWebImageDownloaderProgressiveDownload;
            if (options & SDWebImageRefreshCached) downloaderOptions |= SDWebImageDownloaderUseNSURLCache;
            if (options & SDWebImageContinueInBackground) downloaderOptions |= SDWebImageDownloaderContinueInBackground;
            if (options & SDWebImageHandleCookies) downloaderOptions |= SDWebImageDownloaderHandleCookies;
            if (options & SDWebImageAllowInvalidSSLCertificates) downloaderOptions |= SDWebImageDownloaderAllowInvalidSSLCertificates;
            if (options & SDWebImageHighPriority) downloaderOptions |= SDWebImageDownloaderHighPriority;
            if (options & SDWebImageScaleDownLargeImages) downloaderOptions |= SDWebImageDownloaderScaleDownLargeImages;
            /*
             如果设置了强制刷新缓存的选项。则`SDWebImageDownloaderProgressiveDownload`选项失效并且添加`SDWebImageDownloaderIgnoreCachedResponse`选项。
             */
            if (cachedImage && options & SDWebImageRefreshCached) {
                // force progressive off if image already cached but forced refreshing
                downloaderOptions &= ~SDWebImageDownloaderProgressiveDownload;
                // ignore image read from NSURLCache if image if cached but force refreshing
                downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
            }
            /*
             新建一个网络下载的操作。
             */
            SDWebImageDownloadToken *subOperationToken = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) {
                __strong __typeof(weakOperation) strongOperation = weakOperation;
                //如果图片下载结束以后，对应的图片加载操作已经取消。则什么处理都不做
                if (!strongOperation || strongOperation.isCancelled) {
                   //如果operation已经被取消了，则什么也不做
                } else if (error) {
                    //如果加载出错。则直接返回回调。并且添加到failedURLs中
                    [self callCompletionBlockForOperation:strongOperation completion:completedBlock error:error url:url];

                    if (   error.code != NSURLErrorNotConnectedToInternet
                        && error.code != NSURLErrorCancelled
                        && error.code != NSURLErrorTimedOut
                        && error.code != NSURLErrorInternationalRoamingOff
                        && error.code != NSURLErrorDataNotAllowed
                        && error.code != NSURLErrorCannotFindHost
                        && error.code != NSURLErrorCannotConnectToHost
                        && error.code != NSURLErrorNetworkConnectionLost) {
                        @synchronized (self.failedURLs) {
                            [self.failedURLs addObject:url];
                        }
                    }
                }
                else {
                    //网络图片加载成功
                    if ((options & SDWebImageRetryFailed)) {
                        //如果有重试失败下载的选项。则把url从failedURLS中移除
                        @synchronized (self.failedURLs) {
                            [self.failedURLs removeObject:url];
                        }
                    }

                    BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);

                    if (options & SDWebImageRefreshCached && cachedImage && !downloadedImage) {
                        // Image refresh hit the NSURLCache cache, do not call the completion block
                        //如果成功下载图片。并且图片是动态图片。并且设置了SDWebImageTransformAnimatedImage属性。则处理图片
                    } else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)]) {
                        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, ), ^{
                            //获取transform以后的图片
                            UIImage *transformedImage = [self.delegate imageManager:self transformDownloadedImage:downloadedImage withURL:url];
                            //存储transform以后的的图片
                            if (transformedImage && finished) {
                                BOOL imageWasTransformed = ![transformedImage isEqual:downloadedImage];
                                // pass nil if the image was transformed, so we can recalculate the data from the image
                                [self.imageCache storeImage:transformedImage imageData:(imageWasTransformed ? nil : downloadedData) forKey:key toDisk:cacheOnDisk completion:nil];
                            }
                            //回调拼接
                            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:transformedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                        });
                    } else {
                        //如果成功下载图片。并且图片不是图片。则直接缓存和回调
                        if (downloadedImage && finished) {
                            [self.imageCache storeImage:downloadedImage imageData:downloadedData forKey:key toDisk:cacheOnDisk completion:nil];
                        }
                        //回调拼接
                        [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:downloadedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                    }
                }
                //从正在加载的图片操作集合中移除当前操作
                if (finished) {
                    [self safelyRemoveOperationFromRunning:strongOperation];
                }
            }];
            //重置cancelBlock，取消下载operation
            operation.cancelBlock = ^{
                [self.imageDownloader cancel:subOperationToken];
                __strong __typeof(weakOperation) strongOperation = weakOperation;
                [self safelyRemoveOperationFromRunning:strongOperation];
            };
        } else if (cachedImage) {
            //如果获取到了缓存图片。在直接通过缓存图片处理
            __strong __typeof(weakOperation) strongOperation = weakOperation;
            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
            [self safelyRemoveOperationFromRunning:operation];
        } else {
            // Image not in cache and download disallowed by delegate
            //图片么有缓存、并且图片也没有下载
            __strong __typeof(weakOperation) strongOperation = weakOperation;
            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:nil data:nil error:nil cacheType:SDImageCacheTypeNone finished:YES url:url];
            [self safelyRemoveOperationFromRunning:operation];
        }
    }];

    return operation;
}

3.1.2 SDWebImageCombinedOperation

loadImageWithURL方法会返回一个对象，该对象又个cancel方法，方法继承自SDWebImageOperation协议。

/**
 通过这个对象关联一个`SDWebImageDownloaderOperation`对象
 */
@interface SDWebImageCombinedOperation : NSObject <SDWebImageOperation>

/**
 用于判断Operation是否已经取消
 */
@property (assign, nonatomic, getter = isCancelled) BOOL cancelled;

/**
 取消回调
 */
@property (copy, nonatomic, nullable) SDWebImageNoParamsBlock cancelBlock;

/**
 SDWebImageDownloaderOperation对象。可以通过这个属性取消一个NSOperation
 */
@property (strong, nonatomic, nullable) NSOperation *cacheOperation;

@end

@implementation SDWebImageCombinedOperation

/**
 取消Operation的回调Block

 @param cancelBlock 回调Block
 */
- (void)setCancelBlock:(nullable SDWebImageNoParamsBlock)cancelBlock {
    // check if the operation is already cancelled, then we just call the cancelBlock
    if (self.isCancelled) {
        if (cancelBlock) {
            cancelBlock();
        }
        _cancelBlock = nil; // don't forget to nil the cancelBlock, otherwise we will get crashes
    } else {
        _cancelBlock = [cancelBlock copy];
    }
}

/**
 调用cancel方法。这个方法继承自`SDWebImageOperation`协议。方法里面会调用`SDWebImageDownlaoderOperation`或者`NSOperation`的cancel方法
 */
- (void)cancel {
    self.cancelled = YES;
    if (self.cacheOperation) {
        //调用`SDWebImageDownlaoderOperation`或者`NSOperation`的cancel方法
        [self.cacheOperation cancel];
        self.cacheOperation = nil;
    }
    if (self.cancelBlock) {
        self.cancelBlock();

        // TODO: this is a temporary fix to #809.
        // Until we can figure the exact cause of the crash, going with the ivar instead of the setter
        //        self.cancelBlock = nil;
        _cancelBlock = nil;
    }
}

@end

如果我们在操作中，取消是放在UIView+WebCache中。

- (void)sd_cancelCurrentImageLoad {
    [self sd_cancelImageLoadOperationWithKey:NSStringFromClass([self class])];
}

/**
 取消当前key对应的所有`SDWebImageCombinedOperation`对象

 @param key Operation对应的key
 */
- (void)sd_cancelImageLoadOperationWithKey:(nullable NSString *)key {
    // Cancel in progress downloader from queue
    //获取当前View对应的所有key
    SDOperationsDictionary *operationDictionary = [self operationDictionary];
    //获取对应的图片加载Operation
    id operations = operationDictionary[key];
    //取消所有当前View对应的所有Operation
    if (operations) {
        if ([operations isKindOfClass:[NSArray class]]) {
            for (id <SDWebImageOperation> operation in operations) {
                if (operation) {
                    //SDWebImageCombinedOperation对象的cancel方法
                    [operation cancel];
                }
            }
        } else if ([operations conformsToProtocol:@protocol(SDWebImageOperation)]){
            [(id<SDWebImageOperation>) operations cancel];
        }
        [operationDictionary removeObjectForKey:key];
    }
}

3.2 SDImageCache

3.2.1 SDImageCache总结

SDImageCache主要是通过NSCache来管理内存以及根据传入的key转换为MD5作为文件存储。SDWebImageManager也是通过SDImageCache来获取和存储图片的，并且SDImageCache是一个单例对象。总结如下：

1. 通过AutoPurgeCache（也是NSCache的子类）来管理内存缓存。当收到内存警告的时候，移除内存中所有的对象。
2. 如果接收到UIApplicationDidReceiveMemoryWarningNotification之后，会删除内存中所有的缓存图片。
3. 如果接收到UIApplicationWillTerminateNotification通知后，会通过deleteOldFiles方法删除比较老的图片。下面是删除的具体细则：
   • 通过SDImageCacheConfig来设置缓存大小，过期日期以及是否允许缓存
   • 第一会根据迭代缓存目录下的所有文件，如果大于一周的图片会全部删除。
   • 然后会记录缓存目录的大小，如果当前缓存大于默认缓存，会按照创建日期的先后开始删除图片缓存，直到缓存小于默认缓存大小。
4. 当接到UIApplicationDidEnterBackgroundNotification通知之后，此时会调用deleteOldFilesWithCompletionBlock方法用于清除缓存数据。
5. 也定义了方法用于处理图片的获取，缓存以及其他工作。
   • 通过queryCacheOperationForKey方法查询指定的key，查找对应的缓存图片，再从内存，硬盘中找。
   • 通过removeImageForKey方法移除指定的缓存图片。
   • 通过diskImageDataBySearchingAllPathsForKey方法在磁盘查找key对应的图片。
   • 通过storeImageDataToDisk方法把指定的图片数据存入磁盘。
6. 通过cachedFileNameForKey方法获取图片对应的MD5加密的缓存名字。

3.2.2 具体方法

看完上面的些许总结：下面看一下SDWebImageCache的具体方法。

1.初始化方法

//单例
+ (nonnull instancetype)sharedImageCache;

//初始化一个namespace空间
- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns;

//初始化一个namespace空间和存储的路径
- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns
                       diskCacheDirectory:(nonnull NSString *)directory     NS_DESIGNATED_INITIALIZER;

下面是实现方式

//创建单例模式
+ (nonnull instancetype)sharedImageCache {
    static dispatch_once_t once;
    static id instance;
    dispatch_once(&once, ^{
        instance = [self new];
    });
    return instance;
}

//
- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns {
    NSString *path = [self makeDiskCachePath:ns];
    return [self initWithNamespace:ns diskCacheDirectory:path];
}

- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns
                       diskCacheDirectory:(nonnull NSString *)directory {
    if ((self = [super init])) {
        NSString *fullNamespace = [@"com.hackemist.SDWebImageCache." stringByAppendingString:ns];

        // Create IO serial queue
        _ioQueue = dispatch_queue_create("com.hackemist.SDWebImageCache", DISPATCH_QUEUE_SERIAL);

        _config = [[SDImageCacheConfig alloc] init];

        // Init the memory cache
        _memCache = [[AutoPurgeCache alloc] init];
        _memCache.name = fullNamespace;

        // Init the disk cache
        if (directory != nil) {
            _diskCachePath = [directory stringByAppendingPathComponent:fullNamespace];
        } else {
            NSString *path = [self makeDiskCachePath:ns];
            _diskCachePath = path;
        }

        dispatch_sync(_ioQueue, ^{
            _fileManager = [NSFileManager new];
        });

#if SD_UIKIT
        // Subscribe to app events
        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(clearMemory)
                                                     name:UIApplicationDidReceiveMemoryWarningNotification
                                                   object:nil];

        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(deleteOldFiles)
                                                     name:UIApplicationWillTerminateNotification
                                                   object:nil];

        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(backgroundDeleteOldFiles)
                                                     name:UIApplicationDidEnterBackgroundNotification
                                                   object:nil];
#endif
    }

    return self;
}

可以看一下上面总结的，以及上面实现代码中- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns diskCacheDirectory:(nonnull NSString *)directory 标红的，就知道方法的调用过程。

2.存储路径

//存储路径
- (nullable NSString *)makeDiskCachePath:(nonnull NSString*)fullNamespace;

//添加只读cache的path
- (void)addReadOnlyCachePath:(nonnull NSString *)path;

下面看一下实现方法

#pragma mark - Cache paths

- (void)addReadOnlyCachePath:(nonnull NSString *)path {
    if (!self.customPaths) {
        self.customPaths = [NSMutableArray new];
    }

    if (![self.customPaths containsObject:path]) {
        [self.customPaths addObject:path];
    }
}

- (nullable NSString *)cachePathForKey:(nullable NSString *)key inPath:(nonnull NSString *)path {
    NSString *filename = [self cachedFileNameForKey:key];
    return [path stringByAppendingPathComponent:filename];
}

- (nullable NSString *)defaultCachePathForKey:(nullable NSString *)key {
    return [self cachePathForKey:key inPath:self.diskCachePath];
}

- (nullable NSString *)cachedFileNameForKey:(nullable NSString *)key {
    const char *str = key.UTF8String;
    if (str == NULL) {
        str = "";
    }
    unsigned char r[CC_MD5_DIGEST_LENGTH];
    CC_MD5(str, (CC_LONG)strlen(str), r);
    NSURL *keyURL = [NSURL URLWithString:key];
    NSString *ext = keyURL ? keyURL.pathExtension : key.pathExtension;
    NSString *filename = [NSString stringWithFormat:@"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%@",
                          r[], r[], r[], r[], r[], r[], r[], r[], r[], r[], r[],
                          r[], r[], r[], r[], r[], ext.length ==  ? @"" : [NSString stringWithFormat:@".%@", ext]];
    return filename;
}

- (nullable NSString *)makeDiskCachePath:(nonnull NSString*)fullNamespace {
    NSArray<NSString *> *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
    return [paths[] stringByAppendingPathComponent:fullNamespace];
}

3.存储图片

存储图片主要分为两模块

1. 存储到内存：直接计算图片大小之后，然后用NSCache *memCache存储
2. 存储到硬盘：首先将UIImage转化为NSData，然后用NSFileManager *_fileManager创建存储文件路径和图片存储文件

下面是存储图片的几个方法

//存储image到key
- (void)storeImage:(nullable UIImage *)image
            forKey:(nullable NSString *)key
        completion:(nullable SDWebImageNoParamsBlock)completionBlock;

//存储image到key，是否硬盘缓存
- (void)storeImage:(nullable UIImage *)image
            forKey:(nullable NSString *)key
            toDisk:(BOOL)toDisk
        completion:(nullable SDWebImageNoParamsBlock)completionBlock;

//存储imageData到key，是否硬盘缓存
- (void)storeImage:(nullable UIImage *)image
         imageData:(nullable NSData *)imageData
            forKey:(nullable NSString *)key
            toDisk:(BOOL)toDisk
        completion:(nullable SDWebImageNoParamsBlock)completionBlock;

//硬盘缓存，key
- (void)storeImageDataToDisk:(nullable NSData *)imageData forKey:(nullable NSString *)key;

下面看一下具体实现的过程

- (void)storeImage:(nullable UIImage *)image
            forKey:(nullable NSString *)key
        completion:(nullable SDWebImageNoParamsBlock)completionBlock {
    [self storeImage:image imageData:nil forKey:key toDisk:YES completion:completionBlock];
}

- (void)storeImage:(nullable UIImage *)image
            forKey:(nullable NSString *)key
            toDisk:(BOOL)toDisk
        completion:(nullable SDWebImageNoParamsBlock)completionBlock {
    [self storeImage:image imageData:nil forKey:key toDisk:toDisk completion:completionBlock];
}

- (void)storeImage:(nullable UIImage *)image
         imageData:(nullable NSData *)imageData
            forKey:(nullable NSString *)key
            toDisk:(BOOL)toDisk
        completion:(nullable SDWebImageNoParamsBlock)completionBlock {
    if (!image || !key) {
        if (completionBlock) {
            completionBlock();
        }
        return;
    }
    // if memory cache is enabled
    if (self.config.shouldCacheImagesInMemory) {
        NSUInteger cost = SDCacheCostForImage(image);
        [self.memCache setObject:image forKey:key cost:cost];
    }

    if (toDisk) {
        dispatch_async(self.ioQueue, ^{
            @autoreleasepool {
                NSData *data = imageData;
                if (!data && image) {
                    // If we do not have any data to detect image format, check whether it contains alpha channel to use PNG or JPEG format
                    SDImageFormat format;
                    if (SDCGImageRefContainsAlpha(image.CGImage)) {
                        format = SDImageFormatPNG;
                    } else {
                        format = SDImageFormatJPEG;
                    }
                    data = [[SDWebImageCodersManager sharedInstance] encodedDataWithImage:image format:format];
                }
                [self storeImageDataToDisk:data forKey:key];
            }

            if (completionBlock) {
                dispatch_async(dispatch_get_main_queue(), ^{
                    completionBlock();
                });
            }
        });
    } else {
        if (completionBlock) {
            completionBlock();
        }
    }
}

- (void)storeImageDataToDisk:(nullable NSData *)imageData forKey:(nullable NSString *)key {
    if (!imageData || !key) {
        return;
    }

    [self checkIfQueueIsIOQueue];

    if (![_fileManager fileExistsAtPath:_diskCachePath]) {
        [_fileManager createDirectoryAtPath:_diskCachePath withIntermediateDirectories:YES attributes:nil error:NULL];
    }

    // get cache Path for image key
    NSString *cachePathForKey = [self defaultCachePathForKey:key];
    // transform to NSUrl
    NSURL *fileURL = [NSURL fileURLWithPath:cachePathForKey];

    [_fileManager createFileAtPath:cachePathForKey contents:imageData attributes:nil];

    // disable iCloud backup
    if (self.config.shouldDisableiCloud) {
        [fileURL setResourceValue:@YES forKey:NSURLIsExcludedFromBackupKey error:nil];
    }
}

4.读取图片

读取图片分为从内存里面和从硬盘图片读取。

- (void)diskImageExistsWithKey:(nullable NSString *)key completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock;

- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key done:(nullable SDCacheQueryCompletedBlock)doneBlock;

- (nullable UIImage *)imageFromMemoryCacheForKey:(nullable NSString *)key;

- (nullable UIImage *)imageFromDiskCacheForKey:(nullable NSString *)key;

- (nullable UIImage *)imageFromCacheForKey:(nullable NSString *)key;

下面是实现方法

- (void)diskImageExistsWithKey:(nullable NSString *)key completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock {
    dispatch_async(_ioQueue, ^{
        BOOL exists = [_fileManager fileExistsAtPath:[self defaultCachePathForKey:key]];

        // fallback because of https://github.com/rs/SDWebImage/pull/976 that added the extension to the disk file name
        // checking the key with and without the extension
        if (!exists) {
            exists = [_fileManager fileExistsAtPath:[self defaultCachePathForKey:key].stringByDeletingPathExtension];
        }

        if (completionBlock) {
            dispatch_async(dispatch_get_main_queue(), ^{
                completionBlock(exists);
            });
        }
    });
}

- (nullable UIImage *)imageFromMemoryCacheForKey:(nullable NSString *)key {
    return [self.memCache objectForKey:key];
}

- (nullable UIImage *)imageFromDiskCacheForKey:(nullable NSString *)key {
    UIImage *diskImage = [self diskImageForKey:key];
    if (diskImage && self.config.shouldCacheImagesInMemory) {
        NSUInteger cost = SDCacheCostForImage(diskImage);
        [self.memCache setObject:diskImage forKey:key cost:cost];
    }

    return diskImage;
}

- (nullable UIImage *)imageFromCacheForKey:(nullable NSString *)key {
    // First check the in-memory cache...
    UIImage *image = [self imageFromMemoryCacheForKey:key];
    if (image) {
        return image;
    }

    // Second check the disk cache...
    image = [self imageFromDiskCacheForKey:key];
    return image;
}

- (nullable NSData *)diskImageDataBySearchingAllPathsForKey:(nullable NSString *)key {
    NSString *defaultPath = [self defaultCachePathForKey:key];
    NSData *data = [NSData dataWithContentsOfFile:defaultPath options:self.config.diskCacheReadingOptions error:nil];
    if (data) {
        return data;
    }

    // fallback because of https://github.com/rs/SDWebImage/pull/976 that added the extension to the disk file name
    // checking the key with and without the extension
    data = [NSData dataWithContentsOfFile:defaultPath.stringByDeletingPathExtension options:self.config.diskCacheReadingOptions error:nil];
    if (data) {
        return data;
    }

    NSArray<NSString *> *customPaths = [self.customPaths copy];
    for (NSString *path in customPaths) {
        NSString *filePath = [self cachePathForKey:key inPath:path];
        NSData *imageData = [NSData dataWithContentsOfFile:filePath options:self.config.diskCacheReadingOptions error:nil];
        if (imageData) {
            return imageData;
        }

        // fallback because of https://github.com/rs/SDWebImage/pull/976 that added the extension to the disk file name
        // checking the key with and without the extension
        imageData = [NSData dataWithContentsOfFile:filePath.stringByDeletingPathExtension options:self.config.diskCacheReadingOptions error:nil];
        if (imageData) {
            return imageData;
        }
    }

    return nil;
}

- (nullable UIImage *)diskImageForKey:(nullable NSString *)key {
    NSData *data = [self diskImageDataBySearchingAllPathsForKey:key];
    if (data) {
        UIImage *image = [[SDWebImageCodersManager sharedInstance] decodedImageWithData:data];
        image = [self scaledImageForKey:key image:image];
        if (self.config.shouldDecompressImages) {
            image = [[SDWebImageCodersManager sharedInstance] decompressedImageWithImage:image data:&data options:@{SDWebImageCoderScaleDownLargeImagesKey: @(NO)}];
        }
        return image;
    } else {
        return nil;
    }
}

- (nullable UIImage *)scaledImageForKey:(nullable NSString *)key image:(nullable UIImage *)image {
    return SDScaledImageForKey(key, image);
}

- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key done:(nullable SDCacheQueryCompletedBlock)doneBlock {
    if (!key) {
        if (doneBlock) {
            doneBlock(nil, nil, SDImageCacheTypeNone);
        }
        return nil;
    }

    // First check the in-memory cache...
    UIImage *image = [self imageFromMemoryCacheForKey:key];
    if (image) {
        NSData *diskData = nil;
        if (image.images) {
            diskData = [self diskImageDataBySearchingAllPathsForKey:key];
        }
        if (doneBlock) {
            doneBlock(image, diskData, SDImageCacheTypeMemory);
        }
        return nil;
    }

    NSOperation *operation = [NSOperation new];
    dispatch_async(self.ioQueue, ^{
        if (operation.isCancelled) {
            // do not call the completion if cancelled
            return;
        }

        @autoreleasepool {
            NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];
            UIImage *diskImage = [self diskImageForKey:key];
            if (diskImage && self.config.shouldCacheImagesInMemory) {
                NSUInteger cost = SDCacheCostForImage(diskImage);
                [self.memCache setObject:diskImage forKey:key cost:cost];
            }

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

    return operation;
}

5.删除图片

删除图片分为两步；首先从cache删除缓存图片，然后再从硬盘删除对应的文件。

- (void)removeImageForKey:(nullable NSString *)key withCompletion:(nullable SDWebImageNoParamsBlock)completion;

- (void)removeImageForKey:(nullable NSString *)key fromDisk:(BOOL)fromDisk withCompletion:(nullable SDWebImageNoParamsBlock)completion;

下面是删除图片的实现代码

- (void)removeImageForKey:(nullable NSString *)key withCompletion:(nullable SDWebImageNoParamsBlock)completion {
    [self removeImageForKey:key fromDisk:YES withCompletion:completion];
}

- (void)removeImageForKey:(nullable NSString *)key fromDisk:(BOOL)fromDisk withCompletion:(nullable SDWebImageNoParamsBlock)completion {
    if (key == nil) {
        return;
    }

    if (self.config.shouldCacheImagesInMemory) {
        [self.memCache removeObjectForKey:key];
    }

    if (fromDisk) {
        dispatch_async(self.ioQueue, ^{
            [_fileManager removeItemAtPath:[self defaultCachePathForKey:key] error:nil];

            if (completion) {
                dispatch_async(dispatch_get_main_queue(), ^{
                    completion();
                });
            }
        });
    } else if (completion){
        completion();
    }

}

6.清理缓存

下面是清理缓存分为两步：首先清除所有过期图片，然后

//Clear all memory cached images
- (void)clearMemory;

//clear all disk cached images
- (void)clearDiskOnCompletion:(nullable SDWebImageNoParamsBlock)completion;

//remove all expired cached image from disk.
- (void)deleteOldFilesWithCompletionBlock:(nullable SDWebImageNoParamsBlock)completionBlock;

下面是实现方式

- (void)clearMemory {
    [self.memCache removeAllObjects];
}

- (void)clearDiskOnCompletion:(nullable SDWebImageNoParamsBlock)completion {
    dispatch_async(self.ioQueue, ^{
        [_fileManager removeItemAtPath:self.diskCachePath error:nil];
        [_fileManager createDirectoryAtPath:self.diskCachePath
                withIntermediateDirectories:YES
                                 attributes:nil
                                      error:NULL];

        if (completion) {
            dispatch_async(dispatch_get_main_queue(), ^{
                completion();
            });
        }
    });
}

- (void)deleteOldFiles {
    [self deleteOldFilesWithCompletionBlock:nil];
}

- (void)deleteOldFilesWithCompletionBlock:(nullable SDWebImageNoParamsBlock)completionBlock {
    dispatch_async(self.ioQueue, ^{
        NSURL *diskCacheURL = [NSURL fileURLWithPath:self.diskCachePath isDirectory:YES];
        NSArray<NSString *> *resourceKeys = @[NSURLIsDirectoryKey, NSURLContentModificationDateKey, NSURLTotalFileAllocatedSizeKey];

        // This enumerator prefetches useful properties for our cache files.
        NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtURL:diskCacheURL
                                                   includingPropertiesForKeys:resourceKeys
                                                                      options:NSDirectoryEnumerationSkipsHiddenFiles
                                                                 errorHandler:NULL];

        NSDate *expirationDate = [NSDate dateWithTimeIntervalSinceNow:-self.config.maxCacheAge];
        NSMutableDictionary<NSURL *, NSDictionary<NSString *, id> *> *cacheFiles = [NSMutableDictionary dictionary];
        NSUInteger currentCacheSize = ;

        // Enumerate all of the files in the cache directory.  This loop has two purposes:
        //
        //  1. Removing files that are older than the expiration date.
        //  2. Storing file attributes for the size-based cleanup pass.
        NSMutableArray<NSURL *> *urlsToDelete = [[NSMutableArray alloc] init];
        for (NSURL *fileURL in fileEnumerator) {
            NSError *error;
            NSDictionary<NSString *, id> *resourceValues = [fileURL resourceValuesForKeys:resourceKeys error:&error];

            // Skip directories and errors.
            if (error || !resourceValues || [resourceValues[NSURLIsDirectoryKey] boolValue]) {
                continue;
            }

            // Remove files that are older than the expiration date;
            NSDate *modificationDate = resourceValues[NSURLContentModificationDateKey];
            if ([[modificationDate laterDate:expirationDate] isEqualToDate:expirationDate]) {
                [urlsToDelete addObject:fileURL];
                continue;
            }

            // Store a reference to this file and account for its total size.
            NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
            currentCacheSize += totalAllocatedSize.unsignedIntegerValue;
            cacheFiles[fileURL] = resourceValues;
        }

        for (NSURL *fileURL in urlsToDelete) {
            [_fileManager removeItemAtURL:fileURL error:nil];
        }

        // If our remaining disk cache exceeds a configured maximum size, perform a second
        // size-based cleanup pass.  We delete the oldest files first.
        if (self.config.maxCacheSize >  && currentCacheSize > self.config.maxCacheSize) {
            // Target half of our maximum cache size for this cleanup pass.
            const NSUInteger desiredCacheSize = self.config.maxCacheSize / ;

            // Sort the remaining cache files by their last modification time (oldest first).
            NSArray<NSURL *> *sortedFiles = [cacheFiles keysSortedByValueWithOptions:NSSortConcurrent
                                                                     usingComparator:^NSComparisonResult(id obj1, id obj2) {
                                                                         return [obj1[NSURLContentModificationDateKey] compare:obj2[NSURLContentModificationDateKey]];
                                                                     }];

            // Delete files until we fall below our desired cache size.
            for (NSURL *fileURL in sortedFiles) {
                if ([_fileManager removeItemAtURL:fileURL error:nil]) {
                    NSDictionary<NSString *, id> *resourceValues = cacheFiles[fileURL];
                    NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
                    currentCacheSize -= totalAllocatedSize.unsignedIntegerValue;

                    if (currentCacheSize < desiredCacheSize) {
                        break;
                    }
                }
            }
        }
        if (completionBlock) {
            dispatch_async(dispatch_get_main_queue(), ^{
                completionBlock();
            });
        }
    });
}

3.3 SDWebImageDownloader

SDWebImageManager通过imageDownloader属性持有SDWebImageDownloader对象，并且可以调用它的downloadImageWithURL方法来从加载图片。

对于SDWebImageDownloader具体做了哪些工作如下。

SDWebImageManager是一个单例对象，做如下工作：

1. 定义SDWebImageDownloaderOptions属性（枚举属性）通过这个属性来设置从网络加载的不同状况。
2. 定义管理NSURLSession对象，用这个对象做网络请求，实现对象的代理方法。
3. 新建一个NSURLRequest对象，管理请求头的拼接。
4. 对于每个请求，通过SDWebImageDownloaderOperation对象来操作网络下载。
5. 管理网络下载和完成的回调工作，通过addProgressCallback方法实现。

3.4 SDWebImageDownloaderOperation

SDWebImageDownloaderOperation主要功能：

1. SDWebImageDownloaderOperation是NSOperation子类，在start方法里面实现逻辑。
2. NSURLSessionTaskDelegate和NSURLSessionDataDelegate来处理数据加载，以及进度的处理。
3. 如果unownedSession为nil时，则初始化一个网络请求
4. 发送SDWebImageDownloadStopNotification，SDWebImageDownloadStartNotification，SDWebImageDownloadFinishNotification，SDWebImageDownloadReceiveResponseNotification等通知，来通知Operation的状态。

SDWebImage主要的内容就是上面，当然还有一些辅助方法，大家可以不需要了解，看一下上面内容就会明白SDWebImage底层实现的逻辑。欢迎大家指正（后期会不断的更新）。