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。
- 提供UIImageView的category用来加载和缓存网络图片。
- 提供异步方式下载网络图片。
- 提供GIF动画。
- 提供同一个URL的网络图片不会被重新下载两次。
- 支持Arm64。
- 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：
- 入口 setImageWithURL:placeholderImage:options:会先将placeholderImage显示，然后使用SDWebImageManager根据URL来处理图片；
- 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:
- 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
- 然后SDWebImageManagerDelegate回调webImageManager:didFinishWithImage:最后到UIImageView+WebCache展示图片
- 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。
- 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：
- 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）
- 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:
- 利用重新共享或者重新生成一个下载器SDWebImageDownloader完成下载图片
- 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。
- 然后通过connection:didReceiveData:利用ImageIO做了下载进度的加载效果
- 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
- 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。
- 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。
- 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。
- 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。
- 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。
- 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 == 0 || (!(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 = 0; 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, 0), ^{ //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：
- 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。
- 如果接收到UIApplicationDidReceiveMemoryWarningNotification之后，会删除内存中所有的缓存图片。
- 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。
- 当接到UIApplicationDidEnterBackgroundNotification通知之后，此时会调用deleteOldFilesWithCompletionBlock方法用于清除缓存数据。
- 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。
- 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[0], r[1], r[2], r[3], r[4], r[5], r[6], r[7], r[8], r[9], r[10], r[11], r[12], r[13], r[14], r[15], ext.length == 0 ? @"" : [NSString stringWithFormat:@".%@", ext]]; return filename; } - (nullable NSString *)makeDiskCachePath:(nonnull NSString*)fullNamespace { NSArray<NSString *> *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES); return [paths[0] stringByAppendingPathComponent:fullNamespace]; }
3.存储图片
存储图片主要分为两模块
- 存储到内存：直接计算图片大小之后，然后用NSCache *memCache存储
- 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 = 0; // 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 > 0 && currentCacheSize > self.config.maxCacheSize) { // Target half of our maximum cache size for this cleanup pass. const NSUInteger desiredCacheSize = self.config.maxCacheSize / 2; // 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：
- 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。
- 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。
- 新建一个NSURLRequest对象，管理请求头的拼接。
- 对于每个请求，通过SDWebImageDownloaderOperation对象来操作网络下载。
- 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：
- SDWebImageDownloaderOperation是NSOperation子类，在start方法里面实现逻辑。
- NSURLSessionTaskDelegate和NSURLSessionDataDelegate来处理数据加载，以及进度的处理。
- 如果unownedSession为nil时，则初始化一个网络请求
- 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）。