SDWebImage 360°无死角分析之下载

打算用几篇文章整理一下 SDWebImage 的源码
源码有点小多, 决定把每个模块分开来整理
这其中包括 : 调度模块、下载模块、缓存模块、解码模块和一些代码整理
调度模块看这里
 缓存模块看这里
 下载模块看这里
 解码模块看这里
 整理模块看这里

本文是下载模块

下载模块由 SDWebImageDownloader 管理
SDWebImageDownloader 是以单例存在
作用是创建 NSURLSession, 处理所有下载任务的回调并分发给对应的 SDWebImageDownloaderOperation
也对图片下载管理进行一些全局的配置, 比如：
1).设置最大并发数，下载时间默认15秒，是否压缩图片和下载顺序等。
2).设置operation的请求头信息，负责生成每个图片的下载单位 SDWebImageDownloaderOperation 以及取消operation 等。
3).设置下载的策略SDWebImageDownloaderOptions。

直接看核心方法 :
在调度一文中的 SDWebImageManager 的 Load 方法中
SDImageCache 在缓存中没有找到图片
那么就会来到 SDWebImageDownloader 的核心方法
去下载图片

- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
                                                   options:(SDWebImageDownloaderOptions)options
                                                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                                 completed:(nullableSDWebImageDownloaderCompletedBlock)completedBlock

四个参数很简单
分别是 : 下载链接下载策略进度回调完成回调
代码注解如下

- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
                                                   options:(SDWebImageDownloaderOptions)options
                                                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                                 completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock {
    // 该URL将用作回调字典的键，因此它不能为空。 如果为零则立即回调，没有图像数据。
    if (url == nil) {
        if (completedBlock != nil) {
            completedBlock(nil, nil, nil, NO);
        }
        return nil;
    }
    
    LOCK(self.operationsLock);
    // 先尝试在 URLOperations 中取下载任务, 有一种情况是操作可能被标记为已完成，但未从“self.URLOperations”中删除。
    /** 获取的是一个 SDWebImageDownloaderOperation  对象 */ 
    NSOperation<SDWebImageDownloaderOperationInterface> *operation = [self.URLOperations objectForKey:url];
    if (!operation || operation.isFinished) {
        //  crucial moment ! 关键时刻 ! 创建图片下载任务!
        operation = [self createDownloaderOperationWithUrl:url options:options];
        __weak typeof(self) wself = self;
        operation.completionBlock = ^{
            __strong typeof(wself) sself = wself;
            if (!sself) {
                return;
            }
            LOCK(sself.operationsLock);
            [sself.URLOperations removeObjectForKey:url];
            UNLOCK(sself.operationsLock);
        };
        [self.URLOperations setObject:operation forKey:url];
        // 将 SDWebImageDownloaderOperation 加入队列开始下载任务
        [self.downloadQueue addOperation:operation];
    }
    UNLOCK(self.operationsLock);

    id downloadOperationCancelToken = [operation addHandlersForProgress:progressBlock completed:completedBlock];
    
    // 生成下载 token 用于取消
    SDWebImageDownloadToken *token = [SDWebImageDownloadToken new];
    token.downloadOperation = operation;
    token.url = url;
    token.downloadOperationCancelToken = downloadOperationCancelToken;

    return token;
}

首先根据下载 URL 去取下载任务
如果下载任务不存在或者已完成, 就创建新的下载任务
把下载任务加入任务队列后执行下载任务
SDWebImageDownloadToken 用于取消任务

创建 SDWebImageDownloaderOperation 下载任务的关键时刻是在这个方法中实现的 :
其中包括一些网络请求的配置

/** 创建下载任务 */
- (NSOperation<SDWebImageDownloaderOperationInterface> *)createDownloaderOperationWithUrl:(nullable NSURL *)url
                                                                                  options:(SDWebImageDownloaderOptions)options {
    NSTimeInterval timeoutInterval = self.downloadTimeout;
    if (timeoutInterval == 0.0) {
        timeoutInterval = 15.0;
    }

    // 配置
    NSURLRequestCachePolicy cachePolicy = options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData;
    //创建request 设置请求缓存策略 下载时间
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url
                                                                cachePolicy:cachePolicy
                                                            timeoutInterval:timeoutInterval];
    
    request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies);
    request.HTTPShouldUsePipelining = YES;
    //设置请求头
    if (self.headersFilter) {
        request.allHTTPHeaderFields = self.headersFilter(url, [self allHTTPHeaderFields]);
    } else {
        request.allHTTPHeaderFields = [self allHTTPHeaderFields];
    }
    // 重头戏 在这里创建 SDWebImageDownloaderOperation 对象
    NSOperation<SDWebImageDownloaderOperationInterface> *operation = [[self.operationClass alloc] initWithRequest:request inSession:self.session options:options];
    operation.shouldDecompressImages = self.shouldDecompressImages;
    
    if (self.urlCredential) {
        operation.credential = self.urlCredential;
    } else if (self.username && self.password) {
        operation.credential = [NSURLCredential credentialWithUser:self.username password:self.password persistence:NSURLCredentialPersistenceForSession];
    }
    //下载优先级
    if (options & SDWebImageDownloaderHighPriority) {
        operation.queuePriority = NSOperationQueuePriorityHigh;
    } else if (options & SDWebImageDownloaderLowPriority) {
        operation.queuePriority = NSOperationQueuePriorityLow;
    }
    //设置下载的顺序 是按照队列还是栈
    if (self.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {
        // Emulate LIFO execution order by systematically adding new operations as last operation's dependency
        [self.lastAddedOperation addDependency:operation];
        self.lastAddedOperation = operation;
    }

    return operation;
}

创建下载任务 SDWebImageDownloaderOperation 的时候, 传入了 self.session 也就是 NSURLSession
SDWebImageDownloaderOperation 拿到这个 NSURLSession 创建 NSURLSessionTask 进行图片请求
self.session 的创建是在 SDWebImageDownloader 单例初始化的时候创建的 :

- (nonnull instancetype)init {
    return [self initWithSessionConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]];
}

- (nonnull instancetype)initWithSessionConfiguration:(nullable NSURLSessionConfiguration *)sessionConfiguration {
    if ((self = [super init])) {
        // 下载类 
        _operationClass = [SDWebImageDownloaderOperation class];
        // 当前下载操作的图片是否应该压缩
        _shouldDecompressImages = YES;
        //  下载顺序 初始化为先进先出
        _executionOrder = SDWebImageDownloaderFIFOExecutionOrder;
        // 初始化 NSOperationQueue
        _downloadQueue = [NSOperationQueue new];
        // 最大并发出
        _downloadQueue.maxConcurrentOperationCount = 6;
        // 队列名称
        _downloadQueue.name = @"com.hackemist.SDWebImageDownloader";
        // 下载url作为key value是具体的下载 operation 用字典来存储，方便cancel等操作
        _URLOperations = [NSMutableDictionary new];
        // HTTP请求头
        SDHTTPHeadersMutableDictionary *headerDictionary = [SDHTTPHeadersMutableDictionary dictionary];
        NSString *userAgent = nil;
#if SD_UIKIT
        // User-Agent Header; see http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.43
        userAgent = [NSString stringWithFormat:@"%@/%@ (%@; iOS %@; Scale/%0.2f)", [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleExecutableKey] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleIdentifierKey], [[NSBundle mainBundle] infoDictionary][@"CFBundleShortVersionString"] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleVersionKey], [[UIDevice currentDevice] model], [[UIDevice currentDevice] systemVersion], [[UIScreen mainScreen] scale]];
#elif SD_WATCH
        // User-Agent Header; see http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.43
        userAgent = [NSString stringWithFormat:@"%@/%@ (%@; watchOS %@; Scale/%0.2f)", [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleExecutableKey] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleIdentifierKey], [[NSBundle mainBundle] infoDictionary][@"CFBundleShortVersionString"] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleVersionKey], [[WKInterfaceDevice currentDevice] model], [[WKInterfaceDevice currentDevice] systemVersion], [[WKInterfaceDevice currentDevice] screenScale]];
#elif SD_MAC
        userAgent = [NSString stringWithFormat:@"%@/%@ (Mac OS X %@)", [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleExecutableKey] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleIdentifierKey], [[NSBundle mainBundle] infoDictionary][@"CFBundleShortVersionString"] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleVersionKey], [[NSProcessInfo processInfo] operatingSystemVersionString]];
#endif
        if (userAgent) {
            if (![userAgent canBeConvertedToEncoding:NSASCIIStringEncoding]) {
                NSMutableString *mutableUserAgent = [userAgent mutableCopy];
                if (CFStringTransform((__bridge CFMutableStringRef)(mutableUserAgent), NULL, (__bridge CFStringRef)@"Any-Latin; Latin-ASCII; [:^ASCII:] Remove", false)) {
                    userAgent = mutableUserAgent;
                }
            }
            headerDictionary[@"User-Agent"] = userAgent;
        }
#ifdef SD_WEBP
        headerDictionary[@"Accept"] = @"image/webp,image/*;q=0.8";
#else
        headerDictionary[@"Accept"] = @"image/*;q=0.8";
#endif
        _HTTPHeaders = headerDictionary;
        // 一些信号量
        _operationsLock = dispatch_semaphore_create(1);
        _headersLock = dispatch_semaphore_create(1);
        // 超时时间
        _downloadTimeout = 15.0;
        // 创建 NSURLSession
        [self createNewSessionWithConfiguration:sessionConfiguration];
    }
    return self;
}

- (void)createNewSessionWithConfiguration:(NSURLSessionConfiguration *)sessionConfiguration {
    [self cancelAllDownloads];
    if (self.session) {
        [self.session invalidateAndCancel];
    }
    sessionConfiguration.timeoutIntervalForRequest = self.downloadTimeout;
    self.session = [NSURLSession sessionWithConfiguration:sessionConfiguration
                                                 delegate:self
                                            delegateQueue:nil];
}

也就是说 NSURLSession 的代理的是 SDWebImageDownloader
所以所有图片下载任务的代理回调都由 SDWebImageDownloader 来处理
SDWebImageDownloader 再把每个下载任务的回调分发给对应的 SDWebImageDownloaderOperation
代码如下 :

#pragma mark NSURLSessionDataDelegate
/** 接收到服务器数据, 决定要不要进行 */
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveResponse:(NSURLResponse *)response
                                        completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler {
    // 确定运行此任务的操作，并将委托方法传递给它, 确认回调的是哪个任务
    NSOperation<SDWebImageDownloaderOperationInterface> *dataOperation = [self operationWithTask:dataTask];
    if ([dataOperation respondsToSelector:@selector(URLSession:dataTask:didReceiveResponse:completionHandler:)]) {
        [dataOperation URLSession:session dataTask:dataTask didReceiveResponse:response completionHandler:completionHandler];
    } else {
        if (completionHandler) {
            completionHandler(NSURLSessionResponseAllow);
        }
    }
}

#pragma mark NSURLSessionTaskDelegate
/** task已经完成. */
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
   // task 为唯一标识, 去查找对应的 operation
    NSOperation<SDWebImageDownloaderOperationInterface> *dataOperation = [self operationWithTask:task];
    if ([dataOperation respondsToSelector:@selector(URLSession:task:didCompleteWithError:)]) {
        [dataOperation URLSession:session task:task didCompleteWithError:error];
    }
}

SDWebImageDownloaderOperation

SDWebImageDownloaderOperation 继承自 NSOperation 是具体的图片下载单位
重写了 NSOperation 的 start 方法
使用 SDWebImageDownloader 的 NSURLSession 创建 NSURLSessionTask 发起图片下载
支持下载取消和后台下载
在下载中及时汇报下载进度
在下载成功后，对图片进行解码，缩放和压缩等操作

SDWebImageDownloaderOperation 的初始化:

- (nonnull instancetype)initWithRequest:(nullable NSURLRequest *)request
                              inSession:(nullable NSURLSession *)session
                                options:(SDWebImageDownloaderOptions)options {
    if ((self = [super init])) {
        //  NSURLRequest
        _request = [request copy];
        //  图片是否可以被压缩属性
        _shouldDecompressImages = YES;
        //  下载策略
        _options = options;
        //  将进度 progressBlock 和下载结束 completedBlock 封装成字典 SDCallbacksDictionary
        _callbackBlocks = [NSMutableArray new];
        //  是否正在执行
        _executing = NO;
        //  是否完成
        _finished = NO;
        //  期望data的大小size
        _expectedSize = 0;
        //  SDWebImageDownloader 传入的 NSURLSession
        _unownedSession = session;
        //  信号量
        _callbacksLock = dispatch_semaphore_create(1);
        //  解码队列
        _coderQueue = dispatch_queue_create("com.hackemist.SDWebImageDownloaderOperationCoderQueue", DISPATCH_QUEUE_SERIAL);
    }
    return self;
}

start 方法

SDWebImageDownloaderOperation 重写了 NSOperation 的 start 方法
当任务添加到 NSOperationQueue 后会执行该方法，启动下载任务

- (void)start {
    //添加同步锁，防止多线程数据竞争
    @synchronized (self) {
        if (self.isCancelled) {
            self.finished = YES;
            [self reset];
            return;
        }

#if SD_UIKIT
        //如调用者配置了在后台可以继续下载图片，那么在这里继续下载
        Class UIApplicationClass = NSClassFromString(@"UIApplication");
        BOOL hasApplication = UIApplicationClass && [UIApplicationClass respondsToSelector:@selector(sharedApplication)];
        if (hasApplication && [self shouldContinueWhenAppEntersBackground]) {
            __weak __typeof__ (self) wself = self;
            UIApplication * app = [UIApplicationClass performSelector:@selector(sharedApplication)];
            self.backgroundTaskId = [app beginBackgroundTaskWithExpirationHandler:^{
                __strong __typeof (wself) sself = wself;

                if (sself) {
                    [sself cancel];

                    [app endBackgroundTask:sself.backgroundTaskId];
                    sself.backgroundTaskId = UIBackgroundTaskInvalid;
                }
            }];
        }
#endif
        NSURLSession *session = self.unownedSession;
        if (!session) {
            // 判断unownedSession是否为了nil，如果是nil则重新创建一个ownedSession
            /** SDWebImageDownloaderOperation 在 初始化 的时候已经传入了 session, 使用 sd 自带单例的时候不会遇到为空的情况 */
            NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
            sessionConfig.timeoutIntervalForRequest = 150;
            
            /**
             *  Create the session for this task
             *  We send nil as delegate queue so that the session creates a serial operation queue for performing all delegate
             *  method calls and completion handler calls.
             */
            session = [NSURLSession sessionWithConfiguration:sessionConfig
                                                    delegate:self
                                               delegateQueue:nil];
            self.ownedSession = session;
        }
        
        // NSURLCache
        if (self.options & SDWebImageDownloaderIgnoreCachedResponse) {
            // Grab the cached data for later check
            NSURLCache *URLCache = session.configuration.URLCache;
            if (!URLCache) {
                URLCache = [NSURLCache sharedURLCache];
            }
            NSCachedURLResponse *cachedResponse;
            // NSURLCache's `cachedResponseForRequest:` is not thread-safe, see https://developer.apple.com/documentation/foundation/nsurlcache#2317483
            @synchronized (URLCache) {
                cachedResponse = [URLCache cachedResponseForRequest:self.request];
            }
            if (cachedResponse) {
                self.cachedData = cachedResponse.data;
            }
        }
        
        // 创建 task
        self.dataTask = [session dataTaskWithRequest:self.request];
        self.executing = YES;
    }

    if (self.dataTask) {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wunguarded-availability"
        if ([self.dataTask respondsToSelector:@selector(setPriority:)]) {
            if (self.options & SDWebImageDownloaderHighPriority) {
                self.dataTask.priority = NSURLSessionTaskPriorityHigh;
            } else if (self.options & SDWebImageDownloaderLowPriority) {
                self.dataTask.priority = NSURLSessionTaskPriorityLow;
            }
        }
#pragma clang diagnostic pop
        // 开启任务
        [self.dataTask resume];
        for (SDWebImageDownloaderProgressBlock progressBlock in [self callbacksForKey:kProgressCallbackKey]) {
            progressBlock(0, NSURLResponseUnknownLength, self.request.URL);
        }
        __weak typeof(self) weakSelf = self;
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStartNotification object:weakSelf];
        });
    } else {
        // dataTask 创建失败, 回调错误
        [self callCompletionBlocksWithError:[NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorUnknown userInfo:@{NSLocalizedDescriptionKey : @"Task can't be initialized"}]];
        [self done];
        return;
    }

#if SD_UIKIT
    // 后台下载
    Class UIApplicationClass = NSClassFromString(@"UIApplication");
    if(!UIApplicationClass || ![UIApplicationClass respondsToSelector:@selector(sharedApplication)]) {
        return;
    }
    if (self.backgroundTaskId != UIBackgroundTaskInvalid) {
        UIApplication * app = [UIApplication performSelector:@selector(sharedApplication)];
        [app endBackgroundTask:self.backgroundTaskId];
        self.backgroundTaskId = UIBackgroundTaskInvalid;
    }
#endif
}

处理图片下载的数据回调

start 方法执行后, 下载任务开启
但是下载回调由 SDWebImageDownloader 接收, 然后分发给对应的 SDWebImageDownloaderOperation
SDWebImageDownloaderOperation 接收到数据的处理 :
省略了其他代理回调方法, 只看下载完成时的回调方法

#pragma mark NSURLSessionTaskDelegate
/** 下载完成或下载失败时的回调方法 */
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
    @synchronized(self) {
        self.dataTask = nil;
        __weak typeof(self) weakSelf = self;
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:weakSelf];
            if (!error) {
                [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadFinishNotification object:weakSelf];
            }
        });
    }
    
    // make sure to call `[self done]` to mark operation as finished
    if (error) {
        [self callCompletionBlocksWithError:error];
        [self done];
    } else {
        /** 图片下载结束的处理 */
        if ([self callbacksForKey:kCompletedCallbackKey].count > 0) {
            /**
             *   如果指定使用`NSURLCache`，那么您在此处获得的响应就是所需要的。
             */
            __block NSData *imageData = [self.imageData copy];
            if (imageData) {
                 / **
                 如果您指定仅通过`SDWebImageDownloaderIgnoreCachedResponse`使用缓存数据
                 然后我们应该检查缓存的数据是否等于图像数据
                 */
                if (self.options & SDWebImageDownloaderIgnoreCachedResponse && [self.cachedData isEqualToData:imageData]) {
                    // call completion block with nil
                    [self callCompletionBlocksWithImage:nil imageData:nil error:nil finished:YES];
                    [self done];
                } else {
                    // decode the image in coder queue
                    dispatch_async(self.coderQueue, ^{
                        @autoreleasepool {
                            // 这一步是 data 转 UIImage => UIImage *image = [[UIImage alloc] initWithData:data];
                            UIImage *image = [[SDWebImageCodersManager sharedInstance] decodedImageWithData:imageData];
                            NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
                            // 处理一下三倍图和二倍图, 以图片名称中是否有 @2x 和 @3x 为标准
                            image = [self scaledImageForKey:key image:image];
                            
                            // 图片是否需要解码
                            BOOL shouldDecode = YES;
                            // 不强制解码动画GIF和WebP
                            if (image.images) {
                                shouldDecode = NO;
                            } else {
#ifdef SD_WEBP
                                SDImageFormat imageFormat = [NSData sd_imageFormatForImageData:imageData];
                                if (imageFormat == SDImageFormatWebP) {
                                    shouldDecode = NO;
                                }
#endif
                            }
                            
                            if (shouldDecode) {
                                if (self.shouldDecompressImages) {
                                    BOOL shouldScaleDown = self.options & SDWebImageDownloaderScaleDownLargeImages;
                                    /** 解码图片 */
                                    image = [[SDWebImageCodersManager sharedInstance] decompressedImageWithImage:image data:&imageData options:@{SDWebImageCoderScaleDownLargeImagesKey: @(shouldScaleDown)}];
                                }
                            }
                            CGSize imageSize = image.size;
                            if (imageSize.width == 0 || imageSize.height == 0) {
                                [self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0 pixels"}]];
                            } else {
                                [self callCompletionBlocksWithImage:image imageData:imageData error:nil finished:YES];
                            }
                            [self done];
                        }
                    });
                }
            } else {
                [self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Image data is nil"}]];
                [self done];
            }
        } else {
            [self done];
        }
    }
}

图片下载结束并解码后回调
下载任务就完成了

SDWebImage 360°无死角分析之下载