汇总记录：
本文基于SDWebImage 4.2.3版本进行分析和整理（）。
整体目录结构：
SDWebImage
SDWebImageCompat 处理不同平台（iOS、TV、OS、Watch）宏，以及根据文件名@2x、@3x进行图片处理和缩放 |----SDWebImageOperation.h 添加cancel的delegate +----Cache |--------SDImageCache 主要处理缓存逻辑，重点集中在：NSCache（Memory）、Disk读写、清理Old File |--------SDImageCacheConfig 配置缓存参数：是否压缩、iCloud、InMemory、ReadingOption、时间和CacheSize +----Downloader |--------SDWebImageDownloaderOperation 主要提供下载的Operation操作 |--------SDWebImageDownloader 提供下载管理入口 +----Utils |--------SDWebImageManager 提供外层管理cache和download入口 |--------SDWebImagePrefetcher 预处理获取Image，主要应用预加载的地方 +----Categories |--------NSData+ImageContentType 提供类型判断和ImageIO类型转换 |--------UIImage+GIF Data转UIImage(GIF)扩展 |--------UIImage+MultiFormat 提供BitMap或者未知类型的Data转UIImage扩展 |--------UIImage+WebP Data转WebP扩展 |--------UIImage+ForceDecode 解压操作 |--------UIView+WebCacheOperation 提供顶层关于取消和下载记录的扩展 +----Decoder |--------SDWebImageCodersManager 整体Coders的入口，提供是否可Coder和Coder转发 |--------SDWebImageCoder 主要说明Coder Delegate 需要实现的接口 |--------SDWebImageImageIOCoder PNG/JPEG的Encode和解压操作 |--------SDWebImageGIFCoder GIF的Coder操作 |--------SDWebImageWebPCoder WebP的Coder操作 |--------SDWebImageFrame 辅助类，主要在GIF等动态图使用 |--------SDWebImageCoderHelper 辅助类，包括方向、Gif图合成等 整体组件结构
整体框架结构比较清晰，因为Decoder部分相对比较独立，业务逻辑处理主要在Cache、Downloader层级以及以上。
下文会以Cache、Downloader、Manager、Wrap的顺序进行分解，最后讨论Decoder部分。1、缓存部分解析
缓存部分逻辑主要是在SDImageCache，包括如下几个方面：
- 新增
- 删除
- 查询
- 缓存管理（过期）
SDWebImage的缓存中，主要走了一套NSCache管理内存和根据传入的Key转换MD5作为文件名存储。以及创建了一个IO操作的Queue进行管理IO操作。
这里重点注意，任何耗时：包括IO读写、转码等操作，都不应该放到主线程里面使用。缓存部分其他地方都比较简单易懂，直接看源码即可。
重点说下如下两个值得学习的地方：
1、通过NSOperation管理queue任务
- (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; } 复制代码 查询缓存的时候，这里采用了NSOperation进行是否取消的操作，因为当下载/缓存内容过多时，毕定存在先后处理顺序的问题，这时候可能由于用户操作等需要取消当前缓存处理，那么NSOperation这里起的唯一作用就是提供取消操作。可以参考具体的Manager里面缓存调起逻辑。
2、申请系统后台时间处理任务
- (void)backgroundDeleteOldFiles { Class UIApplicationClass = NSClassFromString(@"UIApplication"); if(!UIApplicationClass || ![UIApplicationClass respondsToSelector:@selector(sharedApplication)]) { return; } UIApplication *application = [UIApplication performSelector:@selector(sharedApplication)]; __block UIBackgroundTaskIdentifier bgTask = [application beginBackgroundTaskWithExpirationHandler:^{ // Clean up any unfinished task business by marking where you // stopped or ending the task outright. [application endBackgroundTask:bgTask]; bgTask = UIBackgroundTaskInvalid; }]; // Start the long-running task and return immediately. [self deleteOldFilesWithCompletionBlock:^{ [application endBackgroundTask:bgTask]; bgTask = UIBackgroundTaskInvalid; }];}复制代码 这里有个疑问点要注意，为啥会存在前后两部分都去释放Task任务。
iOS的后台任务有个背景，不管任何时候，都需要手动去调用endBackgroundTask结束后台任务，其实开启一个后台job的时候，因为时长有限，所以会存在两种结局：- 在允许的时间内执行完成
- 规定时间内未执行完成 如上两种情况，在结束后都必须手动调用endBackgroundTask：；
2、下载器（Downloader）
下载部分，主要是提供了一个Operation和一个Manager，其中SDWebImageDownloaderOperation里面提供了常用的Operation操作，也支持自定义的下载逻辑（实现SDWebImageDownloaderOperationInterface即可）。
2.1 SDWebImageDownloaderOperation 逻辑
该文件里面重点是Delegate：SDWebImageDownloaderOperationInterface 的设计和一种实现方式 SDWebImageDownloaderOperation(PS:优秀的开源库基本都会设计一套接口，再做一套基础的实现)。
// 这里描述写的很清楚，如果需要自定义的Downloader op，那么需要继承NSOperation并且实现SDWebImageDownloaderOperation /** Describes a downloader operation. If one wants to use a custom downloader op, it needs to inherit from `NSOperation` and conform to this protocol */ @protocol SDWebImageDownloaderOperationInterface- (nonnull instancetype)initWithRequest:(nullable NSURLRequest *)request inSession:(nullable NSURLSession *)session options:(SDWebImageDownloaderOptions)options; - (nullable id)addHandlersForProgress:(nullable SDWebImageDownloaderProgressBlock)progressBlock completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock; - (BOOL)shouldDecompressImages; - (void)setShouldDecompressImages:(BOOL)value; - (nullable NSURLCredential *)credential; - (void)setCredential:(nullable NSURLCredential *)value; @end 复制代码
SDWebImageDownloaderOperation主要是提供内置的下载实现，重点是使用NSURLSessionTask进行下载，逻辑不复杂，详细的参考源码。里面重点有如下部分需要关注：
- 1、
_barrierQueue是共用的并发队列，而且不同的Op都是共用的同一个Queue，这里为啥要做成并发的呢？主要是优化多个线程查询callbacks的时间。
- (nullable NSArray*)callbacksForKey:(NSString *)key { __block NSMutableArray *callbacks = nil; dispatch_sync(self.barrierQueue, ^{ // We need to remove [NSNull null] because there might not always be a progress block for each callback callbacks = [[self.callbackBlocks valueForKey:key] mutableCopy]; [callbacks removeObjectIdenticalTo:[NSNull null]]; }); return [callbacks copy]; // strip mutability here } 复制代码
然后再配合dispatch_barrier_(a)sync操作来进行 增删 操作。
- 2、在URLSession的Delegate实现中，存在如下函数实现：
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential *credential))completionHandler { NSURLSessionAuthChallengeDisposition disposition = NSURLSessionAuthChallengePerformDefaultHandling; __block NSURLCredential *credential = nil; if ([challenge.protectionSpace.authenticationMethod isEqualToString:NSURLAuthenticationMethodServerTrust]) { if (!(self.options & SDWebImageDownloaderAllowInvalidSSLCertificates)) { disposition = NSURLSessionAuthChallengePerformDefaultHandling; } else { credential = [NSURLCredential credentialForTrust:challenge.protectionSpace.serverTrust]; disposition = NSURLSessionAuthChallengeUseCredential; } } else { if (challenge.previousFailureCount == 0) { if (self.credential) { credential = self.credential; disposition = NSURLSessionAuthChallengeUseCredential; } else { disposition = NSURLSessionAuthChallengeCancelAuthenticationChallenge; } } else { disposition = NSURLSessionAuthChallengeCancelAuthenticationChallenge; } } if (completionHandler) { completionHandler(disposition, credential); } } 复制代码 该部分重点是实现Https证书信任请求的，关于Https在iOS上的信任使用，可以参考
2.2 SDWebImageDownloader说明
SDWebImageDownloader主要是基于SDWebImageDownloaderOperation进行任务的添加和管理。使用NSOperationQueue进行任务执行操作。
- 设置了同一时间最多可执行task为6.
- 提供设置Http头信息入口
- 允许使用自定义的
SDWebImageDownloaderOperationInterfaceOperation进行操作，如果没有指定，那么就用默认的SDWebImageDownloaderOperation - 详细的逻辑参考代码即可，比较简单
3、针对通用组件封装（Wrap）
该部分的代码主要是在WebCache Categories文件夹下面，除了UIView+WebCache 以外，其他的涉及UIImage、UIButton等都是下载完成后赋值给到Image的区别，下载过程处理都是在UIView+WebCache中实现。
- (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 context:(nullable NSDictionary *)context 复制代码
详细的步骤逻辑如下：
// 第一步：取消老的下载，这就是UITableViewCell重用后，快速滑动时，中间部分的图片不会被下载的原因。 NSString *validOperationKey = operationKey ?: NSStringFromClass([self class]); [self sd_cancelImageLoadOperationWithKey:validOperationKey]; objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC); // // 第二步：看是否设置默认图片，这里一个点。如果[context valueForKey:SDWebImageInternalSetImageGroupKey]，那么就enter group，这个是干啥的呢，其实主要是给到FLAnimatedImageView+WebCache使用， // 这里是等待setImageBlock会调用的，会结合dispatch_group_leave 和 dispatch_group_notify 使用 if (!(options & SDWebImageDelayPlaceholder)) { if ([context valueForKey:SDWebImageInternalSetImageGroupKey]) { dispatch_group_t group = [context valueForKey:SDWebImageInternalSetImageGroupKey]; dispatch_group_enter(group); } dispatch_main_async_safe(^{ [self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock]; }); } if (url) { // 这里是状态栏的扩展，主要是显示流量那个菊花，可以忽略 // check if activityView is enabled or not if ([self sd_showActivityIndicatorView]) { [self sd_addActivityIndicator]; } /// // 第三步：获取Manager进行下载 SDWebImageManager *manager; if ([context valueForKey:SDWebImageExternalCustomManagerKey]) { manager = (SDWebImageManager *)[context valueForKey:SDWebImageExternalCustomManagerKey]; } else { manager = [SDWebImageManager sharedManager]; } __weak __typeof(self)wself = self; id operation = [manager loadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) { __strong __typeof (wself) sself = wself; [sself sd_removeActivityIndicator]; if (!sself) { return; } // 第四步：下载完成后，进行状态设置和相关状态回调 BOOL shouldCallCompletedBlock = finished || (options & SDWebImageAvoidAutoSetImage); BOOL shouldNotSetImage = ((image && (options & SDWebImageAvoidAutoSetImage)) || (!image && !(options & SDWebImageDelayPlaceholder))); SDWebImageNoParamsBlock callCompletedBlockClojure = ^{ if (!sself) { return; } if (!shouldNotSetImage) { [sself sd_setNeedsLayout]; } if (completedBlock && shouldCallCompletedBlock) { completedBlock(image, error, cacheType, url); } }; // case 1a: we got an image, but the SDWebImageAvoidAutoSetImage flag is set // OR // case 1b: we got no image and the SDWebImageDelayPlaceholder is not set if (shouldNotSetImage) { dispatch_main_async_safe(callCompletedBlockClojure); return; } UIImage *targetImage = nil; NSData *targetData = nil; if (image) { // case 2a: we got an image and the SDWebImageAvoidAutoSetImage is not set targetImage = image; targetData = data; } else if (options & SDWebImageDelayPlaceholder) { // case 2b: we got no image and the SDWebImageDelayPlaceholder flag is set targetImage = placeholder; targetData = nil; } if ([context valueForKey:SDWebImageInternalSetImageGroupKey]) { dispatch_group_t group = [context valueForKey:SDWebImageInternalSetImageGroupKey]; dispatch_group_enter(group); dispatch_main_async_safe(^{ [sself sd_setImage:targetImage imageData:targetData basedOnClassOrViaCustomSetImageBlock:setImageBlock]; }); // ensure completion block is called after custom setImage process finish dispatch_group_notify(group, dispatch_get_main_queue(), ^{ callCompletedBlockClojure(); }); } else { dispatch_main_async_safe(^{ [sself sd_setImage:targetImage imageData:targetData basedOnClassOrViaCustomSetImageBlock:setImageBlock]; callCompletedBlockClojure(); }); } }]; [self sd_setImageLoadOperation:operation forKey:validOperationKey]; } else { 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); } }); } 复制代码 4、Decoder模块
Decoder模块部分的源码重点是在目录Decoder和Categories两个地方，其中Categories提供NSData、UIImage等的直接调用extend。核心实现还是基于Decoder。
- 为啥要压缩和Decompressed？
SDWebImageFrame和SDWebImageCoderHelper中animatedImageWithFrames的逻辑- 大图片缩放逻辑辅助说明
4.1 为啥要压缩和Decompressed
我们都知道，在iOS中，我加载PNG和JPEG，直接调用[UIImage imageNamed:@"pic"];就好，而且本来从网络上下载的图片也都是PNG或者JPEG的，又没用Zip等去进行压缩，为啥还有这么一个Decoder模块呢。
const size_t partialHeight = CGImageGetHeight(partialImageRef); CGColorSpaceRef colorSpace = SDCGColorSpaceGetDeviceRGB(); CGContextRef bmContext = CGBitmapContextCreate(NULL, _width, _height, 8, 0, colorSpace, kCGBitmapByteOrderDefault | kCGImageAlphaPremultipliedFirst); if (bmContext) { CGContextDrawImage(bmContext, (CGRect){.origin.x = 0.0f, .origin.y = 0.0f, .size.width = _width, .size.height = partialHeight}, partialImageRef); CGImageRelease(partialImageRef); partialImageRef = CGBitmapContextCreateImage(bmContext); CGContextRelease(bmContext); } 复制代码 4.2 SDWebImageFrame 和 SDWebImageCoderHelper 中animatedImageWithFrames的逻辑
这两个类里面，有部分函数主要是提供GIF图片的辅助，当我们从NSData拿到图片数据的时候，其实是拿到了多张图片的Data，当我们分成多张图（参考SDWebImageGIFCoder）后，我们要合成到UIImage的images中。
每个SDWebImageFrame就代表了一张图片，每个图片有个播放时长，但是总的一个animationImage并未对每个Image进行时长设置。他的逻辑是怎样的呢？ 假如我们有两张图AB，动画要显示A 3s，显示B 1s,那么我们可以指定显示四张图片：AAAB，总时长是4s。 所以当拿到GIF对应的UIImage和显示时长后，怎么去合成AAAB 和 4s 的时长呢，流程如下： -获取每个图片总时长，totalTimes -计算没个图片时长的最大公约数，再用每张图片的时长除以最大公约数，就是每张图片要显示的次数 -根据显示次数构建UIImage的Array，根据UIImage的Array和总时长，就能通过GIF转成UIImage动画了 4.3 大图片缩放逻辑辅助说明
图片缩放逻辑主要函数：
- (nullable UIImage *)sd_decompressedAndScaledDownImageWithImage:(nullable UIImage *)image { if (![[self class] shouldDecodeImage:image]) { return image; } if (![[self class] shouldScaleDownImage:image]) { return [self sd_decompressedImageWithImage:image]; } CGContextRef destContext; // autorelease the bitmap context and all vars to help system to free memory when there are memory warning. // on iOS7, do not forget to call [[SDImageCache sharedImageCache] clearMemory]; @autoreleasepool { CGImageRef sourceImageRef = image.CGImage; CGSize sourceResolution = CGSizeZero; sourceResolution.width = CGImageGetWidth(sourceImageRef); sourceResolution.height = CGImageGetHeight(sourceImageRef); float sourceTotalPixels = sourceResolution.width * sourceResolution.height; // Determine the scale ratio to apply to the input image // that results in an output image of the defined size. // see kDestImageSizeMB, and how it relates to destTotalPixels. float imageScale = kDestTotalPixels / sourceTotalPixels; CGSize destResolution = CGSizeZero; destResolution.width = (int)(sourceResolution.width*imageScale); destResolution.height = (int)(sourceResolution.height*imageScale); // current color space CGColorSpaceRef colorspaceRef = [[self class] colorSpaceForImageRef:sourceImageRef]; // kCGImageAlphaNone is not supported in CGBitmapContextCreate. // Since the original image here has no alpha info, use kCGImageAlphaNoneSkipLast // to create bitmap graphics contexts without alpha info. destContext = CGBitmapContextCreate(NULL, destResolution.width, destResolution.height, kBitsPerComponent, 0, colorspaceRef, kCGBitmapByteOrderDefault|kCGImageAlphaNoneSkipLast); if (destContext == NULL) { return image; } CGContextSetInterpolationQuality(destContext, kCGInterpolationHigh); // Now define the size of the rectangle to be used for the // incremental blits from the input image to the output image. // we use a source tile width equal to the width of the source // image due to the way that iOS retrieves image data from disk. // iOS must decode an image from disk in full width 'bands', even // if current graphics context is clipped to a subrect within that // band. Therefore we fully utilize all of the pixel data that results // from a decoding opertion by achnoring our tile size to the full // width of the input image. CGRect sourceTile = CGRectZero; sourceTile.size.width = sourceResolution.width; // The source tile height is dynamic. Since we specified the size // of the source tile in MB, see how many rows of pixels high it // can be given the input image width. sourceTile.size.height = (int)(kTileTotalPixels / sourceTile.size.width ); sourceTile.origin.x = 0.0f; // The output tile is the same proportions as the input tile, but // scaled to image scale. CGRect destTile; destTile.size.width = destResolution.width; destTile.size.height = sourceTile.size.height * imageScale; destTile.origin.x = 0.0f; // The source seem overlap is proportionate to the destination seem overlap. // this is the amount of pixels to overlap each tile as we assemble the ouput image. float sourceSeemOverlap = (int)((kDestSeemOverlap/destResolution.height)*sourceResolution.height); CGImageRef sourceTileImageRef; // calculate the number of read/write operations required to assemble the // output image. int iterations = (int)( sourceResolution.height / sourceTile.size.height ); // If tile height doesn't divide the image height evenly, add another iteration // to account for the remaining pixels. int remainder = (int)sourceResolution.height % (int)sourceTile.size.height; if(remainder) { iterations++; } // Add seem overlaps to the tiles, but save the original tile height for y coordinate calculations. float sourceTileHeightMinusOverlap = sourceTile.size.height; sourceTile.size.height += sourceSeemOverlap; destTile.size.height += kDestSeemOverlap; for( int y = 0; y < iterations; ++y ) { @autoreleasepool { sourceTile.origin.y = y * sourceTileHeightMinusOverlap + sourceSeemOverlap; destTile.origin.y = destResolution.height - (( y + 1 ) * sourceTileHeightMinusOverlap * imageScale + kDestSeemOverlap); sourceTileImageRef = CGImageCreateWithImageInRect( sourceImageRef, sourceTile ); if( y == iterations - 1 && remainder ) { float dify = destTile.size.height; destTile.size.height = CGImageGetHeight( sourceTileImageRef ) * imageScale; dify -= destTile.size.height; destTile.origin.y += dify; } CGContextDrawImage( destContext, destTile, sourceTileImageRef ); CGImageRelease( sourceTileImageRef ); } } CGImageRef destImageRef = CGBitmapContextCreateImage(destContext); CGContextRelease(destContext); if (destImageRef == NULL) { return image; } UIImage *destImage = [UIImage imageWithCGImage:destImageRef scale:image.scale orientation:image.imageOrientation]; CGImageRelease(destImageRef); if (destImage == nil) { return image; } return destImage; } } 复制代码 这里光看代码，肯定会一头雾水，其实逻辑很简单，避免要缩放的图片太大，采用的方式是将图片分割成一系列大小的小方块，然后每个方块去获取Image并draw到目标BitmapContext上。重点还是在于内存优化方面。
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