加载流程

围绕SDWebImageManager
sd_setImageWithURL() --> sd_internalSetImageWithURL

//首先从self的任务operations:NSMapTable 中取消任务
    [self sd_cancelImageLoadOperationWithKey:validOperationKey];

manager --> loadImageWithURL
创建operation

    SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
    operation.manager = self;
 SD_LOCK(self.runningOperationsLock);
    [self.runningOperations addObject:operation];
    SD_UNLOCK(self.runningOperationsLock);

开始进入加载图片

 // Start the entry to load image from cache
    [self callCacheProcessForOperation:operation url:url options:options context:context progress:progressBlock completed:completedBlock];

开始查找缓存：内存查找 --> 硬盘查找
key: 可自定义cacheKeyFilter，默认url.absoluteString

// First check the in-memory cache...
    UIImage *image = [self imageFromMemoryCacheForKey:key];
  
 // Second check the disk cache...
    NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];

如果有缓存，加载缓存，如果需要刷新缓存，则继续请求网络

// If image was found in the cache but SDWebImageRefreshCached is provided, notify about the cached image
 // AND try to re-download it in order to let a chance to NSURLCache to refresh it from server.
    [self callCompletionBlockForOperation:operation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];

operation = [self createDownloaderOperationWithUrl:url options:options context:context];
[self.downloadQueue addOperation:operation];

开始缓存

   [self callStoreCacheProcessForOperation:operation url:url options:options context:context downloadedImage:downloadedImage downloadedData:downloadedData finished:finished progress:progressBlock completed:completedBlock];

缓存模块

缓存配置类：SDImageCacheConfig

• shouldCacheImagesInMemory 是否允许内存缓存
• shouldUseWeakMemoryCache 弱引用内存缓存，如果对象被释放，则缓存也释放
• maxMemoryCount 缓存图片个数限制，默认0不限制
• maxMemoryCost 最大内存占用空间，默认为0不限制，1 pixel is 4 bytes (32 bits)

内存缓存 SDMemoryCache
SDMemoryCache继承自NSCache,通过weakCache: NSMapTable来缓存图片,NSMapTabel对比NSDictionary有更多内存语义(strong、weak、copy、assign)

 self.weakCache = [[NSMapTable alloc] initWithKeyOptions:NSPointerFunctionsStrongMemory valueOptions:NSPointerFunctionsWeakMemory capacity:0];

value 弱引用，当图片被释放时，自动删除此key-value

- (void)setObject:(id)obj forKey:(id)key cost:(NSUInteger)g {
    [super setObject:obj forKey:key cost:g];
    if (!self.config.shouldUseWeakMemoryCache) {
        return;
    }
    if (key && obj) {
        // Store weak cache
        SD_LOCK(self.weakCacheLock);
        [self.weakCache setObject:obj forKey:key];
        SD_UNLOCK(self.weakCacheLock);
    }
}

内存缓存时会在NSCache缓存一份，我们自定义的weakCache中再缓存一份

//取出缓存
- (id)objectForKey:(id)key {
    id obj = [super objectForKey:key];
    if (!self.config.shouldUseWeakMemoryCache) {
        return obj;
    }
    if (key && !obj) {
        // Check weak cache
        SD_LOCK(self.weakCacheLock);
        obj = [self.weakCache objectForKey:key];
        SD_UNLOCK(self.weakCacheLock);
        if (obj) {
            // Sync cache
            NSUInteger cost = 0;
            if ([obj isKindOfClass:[UIImage class]]) {
                cost = [(UIImage *)obj sd_memoryCost];
            }
            [super setObject:obj forKey:key cost:cost];
        }
    }
    return obj;
}

从NSCache中取出缓存，在从weakCache中取出缓存，并同步到NSCache中
NSCache是系统缓存，不受我们控制，可能会被随时清空.

磁盘缓存

1. 创建目录
2. 为每一个文件生成一个MD5文件名
3. 清除磁盘缓存

• 删除超过截至日期的文件
• 按访问或修改日期排序删除更早的文件

下载模块

创建NSOperation
生成NSURLRequest，创建NSOperation

    NSOperation<SDWebImageDownloaderOperation> *operation = [[operationClass alloc] initWithRequest:request inSession:self.session options:options context:context];

控制任务优先级,先进先出或者后进先出

 if (self.config.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {
        // Emulate LIFO execution order by systematically adding new operations as last operation's dependency
        [self.lastAddedOperation addDependency:operation];
        self.lastAddedOperation = operation;
    }

SDWebImageDownloaderOperation重写了start方法,当[self.downloadQueue addOperation:operation];时，自动执行operation的start方法，[dataTask resume]则开始下载任务

//如果session不存在则创建session
            /**
             *  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];
//生成dataTask
        self.dataTask = [session dataTaskWithRequest:self.request];
//执行
  [self.dataTask resume];

NSURLSessionDataDelegate,NSURLSessionTaskDelegate代理方法，接收下载的数据

图片编解码
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