SDWebImage源码剖析（一）

  在开发项目的过程中会用到很多第三方库，比如AFNetWorking,SDWebImage,FMDB等，但一直都没去好好的研究一下，最近刚好项目不是太紧，闲下来可以给自己充充电，先研究一下SDWebImage的底层实现，源码地址：SDWebImage
  先介绍一下SDWebImage，我们使用较多的是它提供的UIImageView分类，支持从远程服务器下载并缓存图片。自从iOS5.0开始，NSURLCache也可以处理磁盘缓存，那么SDWebImage的优势在哪？首先NSURLCache是缓存原始数据(raw data)到磁盘或内存，因此每次使用的时候需要将原始数据转换成具体的对象，如UIImage等，这会导致额外的数据解析以及内存占用等，而SDWebImage则是缓存UIImage对象在内存，缓存在NSCache中，同时直接保存压缩过的图片到磁盘中；还有一个问题是当你第一次在UIImageView中使用image对象的时候，图片的解码是在主线程中运行的！而SDWebImage会强制将解码操作放到子线程中。下图是SDWebImage简单的类图关系：

SDWebImage.png

下面从UIImageView的图片加载开始看起，Let's go!

首先我们在给UIImageView设置图片的时候会调用方法：

- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder;

其中url为远程图片的地址，而placeholder为预显示的图片。
其实还可以添加一些额外的参数，比如图片选项SDWebImageOptions

typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
    SDWebImageRetryFailed = 1 

一般使用的是SDWebImageRetryFailed | SDWebImageLowPriority，下面看看具体的函数调用：

- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock 
{
    [self sd_cancelCurrentImageLoad];//取消正在下载的操作
    objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);//关联该view对应的图片URL  
   /*...*/ 
    if (url) {
        __weak UIImageView *wself = self;//防止retain cricle
        //由SDWebImageManager负责图片的获取
        id  operation = [SDWebImageManager.sharedManager downloadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
              /*获取图片到主线层显示*/ 
        }];
        [self sd_setImageLoadOperation:operation forKey:@"UIImageViewImageLoad"];
    } 
}

可以看出图片是从服务端、内存或者硬盘获取是由SDWebImageManager管理的，这个类有几个重要的属性：

@property (strong, nonatomic, readwrite) SDImageCache *imageCache;//负责管理cache，涉及内存缓存和硬盘保存
@property (strong, nonatomic, readwrite) SDWebImageDownloader *imageDownloader;//负责从网络下载图片
@property (strong, nonatomic) NSMutableArray *runningOperations;//包含所有当前正在下载的操作对象

manager会根据URL先去imageCache中查找对应的图片，如果没有在使用downloader去下载，并在下载完成缓存图片到imageCache，接着看实现：

- (id )downloadImageWithURL:(NSURL *)url options:(SDWebImageOptions)options
                                        progress:(SDWebImageDownloaderProgressBlock)progressBlock
                                       completed:(SDWebImageCompletionWithFinishedBlock)completedBlock
 {
     /*...*/
    //根据URL生成对应的key，没有特殊处理为[url absoluteString];
    NSString *key = [self cacheKeyForURL:url];
    //去imageCache中寻找图片
    operation.cacheOperation = [self.imageCache queryDiskCacheForKey:key done:^(UIImage *image, SDImageCacheType cacheType) 
    {
       /*...*/
       //如果图片没有找到，或者采用的SDWebImageRefreshCached选项，则从网络下载
        if ((!image || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
                dispatch_main_sync_safe(^{
                  //如果图片找到了，但是采用的SDWebImageRefreshCached选项，通知获取到了图片，并再次从网络下载，使NSURLCache重新刷新
                     completedBlock(image, nil, cacheType, YES, url);
                });
            }
            /*下载选项设置*/ 
            //使用imageDownloader开启网络下载
            id  subOperation = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *data, NSError *error, BOOL finished) {
                /*...*/
               if (downloadedImage && finished) {
                     //下载完成后，先将图片保存到imageCache中，然后主线程返回
                     [self.imageCache storeImage:downloadedImage recalculateFromImage:NO imageData:data forKey:key toDisk:cacheOnDisk];
                        }
                     dispatch_main_sync_safe(^{
                            if (!weakOperation.isCancelled) {
                                completedBlock(downloadedImage, nil, SDImageCacheTypeNone, finished, url);
                            }
                        });
                    }
                }
          /*...*/
       }
        else if (image) {
          //在cache中找到图片了，直接返回
            dispatch_main_sync_safe(^{
                if (!weakOperation.isCancelled) {
                    completedBlock(image, nil, cacheType, YES, url);
                }
            });
        }
    }];
    return operation;
}

下面先看downloader从网络下载的过程，下载是放在NSOperationQueue中进行的，默认maxConcurrentOperationCount为6，timeout时间为15s：

- (id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url options:(SDWebImageDownloaderOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageDownloaderCompletedBlock)completedBlock {
    __block SDWebImageDownloaderOperation *operation;
    __weak SDWebImageDownloader *wself = self;
    /*...*/
    //防止NSURLCache和SDImageCache重复缓存
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:(options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy :NSURLRequestReloadIgnoringLocalCacheData) timeoutInterval:timeoutInterval];
    request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies);
    request.HTTPShouldUsePipelining = YES;
    request.allHTTPHeaderFields = wself.HTTPHeaders;//设置http头部
    //SDWebImageDownloaderOperation派生自NSOperation，负责图片下载工作
    operation = [[SDWebImageDownloaderOperation alloc] initWithRequest:request
                                                          options:options
                                                         progress:^(NSInteger receivedSize, NSInteger expectedSize) {}
                                                        completed:^(UIImage *image, NSData *data, NSError *error, BOOL finished) {}
                                                        cancelled:^{}];
    operation.shouldDecompressImages = wself.shouldDecompressImages;//是否需要解码
    if (wself.username && wself.password) {
            operation.credential = [NSURLCredential credentialWithUser:wself.username password:wself.password persistence:NSURLCredentialPersistenceForSession];
    }
    if (options & SDWebImageDownloaderHighPriority) {
            operation.queuePriority = NSOperationQueuePriorityHigh;
        } else if (options & SDWebImageDownloaderLowPriority) {
            operation.queuePriority = NSOperationQueuePriorityLow;
    }
        [wself.downloadQueue addOperation:operation];
        if (wself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {
            // 如果下载顺序是后面添加的先运行
            [wself.lastAddedOperation addDependency:operation];
            wself.lastAddedOperation = operation;
        }
    }];
    return operation;
}

SDWebImageDownloaderOperation派生自NSOperation，通过NSURLConnection进行图片的下载，为了确保能够处理下载的数据，需要在后台运行runloop：

- (void)start {
  /*...*/
#if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0
        //开启后台下载
        if ([self shouldContinueWhenAppEntersBackground]) {
            __weak __typeof__ (self) wself = self;
            self.backgroundTaskId = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
                __strong __typeof (wself) sself = wself;
                if (sself) {
                    [sself cancel];
                    [[UIApplication sharedApplication] endBackgroundTask:sself.backgroundTaskId];
                    sself.backgroundTaskId = UIBackgroundTaskInvalid;
                }
            }];
        }
#endif
        self.executing = YES;
        self.connection = [[NSURLConnection alloc] initWithRequest:self.request delegate:self startImmediately:NO];
    }
    [self.connection start];

    if (self.connection) {
        if (self.progressBlock) {
            self.progressBlock(0, NSURLResponseUnknownLength);
        }
       //注册和发送通知需要在一个线程
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStartNotification object:self];
        });
       CFRunLoopRun();//在默认模式下运行当前runlooprun，直到调用CFRunLoopStop停止运行
        if (!self.isFinished) {
            [self.connection cancel];
            [self connection:self.connection didFailWithError:[NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorTimedOut userInfo:@{NSURLErrorFailingURLErrorKey : self.request.URL}]];
        }
    }
#if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0
    if (self.backgroundTaskId != UIBackgroundTaskInvalid) {
        [[UIApplication sharedApplication] endBackgroundTask:self.backgroundTaskId];
        self.backgroundTaskId = UIBackgroundTaskInvalid;
    }
#endif
}

下载过程中，在代理 － (void)connection:(NSURLConnection )connection didReceiveData:(NSData )data中将接收到的数据保存到NSMutableData中，[self.imageData appendData:data]，下载完成后在该线程完成图片的解码，并在完成的completionBlock中进行imageCache的缓存：

- (void)connectionDidFinishLoading:(NSURLConnection *)aConnection {
    SDWebImageDownloaderCompletedBlock completionBlock = self.completedBlock;
    @synchronized(self) {
        CFRunLoopStop(CFRunLoopGetCurrent());//停止当前对runloop
        /*...*/
        if (completionBlock) {
            /*...*/
            UIImage *image = [UIImage sd_imageWithData:self.imageData];
            NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
            image = [self scaledImageForKey:key image:image];

              // Do not force decoding animated GIFs
             if (!image.images) {
                 if (self.shouldDecompressImages) {
                    image = [UIImage decodedImageWithImage:image];//图片解码
                }
            }
            if (CGSizeEqualToSize(image.size, CGSizeZero)) {
                completionBlock(nil, nil, [NSError errorWithDomain:@"SDWebImageErrorDomain" code:0 userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0 pixels"}], YES);
            }
            else {
                completionBlock(image, self.imageData, nil, YES);
            }
        }
    }
    self.completionBlock = nil;
    [self done];
}

后续的图片缓存可以参考：SDWebImage源码剖析（二）