版本记录

前言

我们做APP，文字和图片是绝对不可缺少的元素，特别是图片一般存储在图床里面，一般公司可以委托第三方保存，NB的公司也可以自己存储图片，ios有很多图片加载的第三方框架，其中最优秀的莫过于SDWebImage，它几乎可以满足你所有的需求，用了好几年这个框架，今天想总结一下。感兴趣的可以看其他几篇。
1. SDWebImage探究（一）
2. SDWebImage探究（二）
3. SDWebImage探究（三）
4. SDWebImage探究（四）
5. SDWebImage探究（五）
6. SDWebImage探究（六） —— 图片类型判断深入研究
7. SDWebImage探究（七） —— 深入研究图片下载流程（一）之有关option的位移枚举的说明
8. SDWebImage探究（八） —— 深入研究图片下载流程（二）之开始下载并返回下载结果的总的方法
9. SDWebImage探究（九） —— 深入研究图片下载流程（三）之下载之前的缓存查询操作

回顾

从上面一篇可见，通过查询缓存获取了数据，这一篇就主要讲述一下查询后的block回调，一共回调三个参数UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType。

查询回调函数的处理

1. 检查操作是否被取消

首先是判断SDWebImageCombinedOperation *operation的是否别取消。

@property (assign, nonatomic, getter = isCancelled) BOOL cancelled;

if (operation.isCancelled) {
    [self safelyRemoveOperationFromRunning:operation];
    return;
}

如果operation被取消了，就会使用方法safelyRemoveOperationFromRunning:，将该操作从集合中移除。

- (void)safelyRemoveOperationFromRunning:(nullable SDWebImageCombinedOperation*)operation {
    @synchronized (self.runningOperations) {
        if (operation) {
            [self.runningOperations removeObject:operation];
        }
    }
}

2. 几个if-else逻辑处理

第1个条件判断

1）先看第一个判断条件

if ((!cachedImage || options & SDWebImageRefreshCached) 
&& (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] 
|| [self.delegate imageManager:self shouldDownloadImageForURL:url]))

然后进行进一步计算

if (cachedImage && options & SDWebImageRefreshCached) {
    // 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:weakOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
}

如果缓存图像不为空，并且options为SDWebImageRefreshCached，并且调用下面方法返回completionBlock

- (void)callCompletionBlockForOperation:(nullable SDWebImageCombinedOperation*)operation
                             completion:(nullable SDInternalCompletionBlock)completionBlock
                                  image:(nullable UIImage *)image
                                   data:(nullable NSData *)data
                                  error:(nullable NSError *)error
                              cacheType:(SDImageCacheType)cacheType
                               finished:(BOOL)finished
                                    url:(nullable NSURL *)url {
    dispatch_main_async_safe(^{
        if (operation && !operation.isCancelled && completionBlock) {
            completionBlock(image, data, error, cacheType, finished, url);
        }
    });
}

2） SDWebImageDownloaderOptions配置条件

下面就是这个枚举变量的配置逻辑。

// download if no image or requested to refresh anyway, and download allowed by delegate
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;

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;
}

这里根据传入的那个SDWebImageOptions类型配置SDWebImageDownloaderOptions类型的变量。我们可以看一下这个枚举定义的API

typedef NS_OPTIONS(NSUInteger, SDWebImageDownloaderOptions) {
    SDWebImageDownloaderLowPriority = 1 << 0,
    SDWebImageDownloaderProgressiveDownload = 1 << 1,

    /**
     * By default, request prevent the use of NSURLCache. With this flag, NSURLCache
     * is used with default policies.
     */
    SDWebImageDownloaderUseNSURLCache = 1 << 2,

    /**
     * Call completion block with nil image/imageData if the image was read from NSURLCache
     * (to be combined with `SDWebImageDownloaderUseNSURLCache`).
     * I think this option should be renamed to 'SDWebImageDownloaderUsingCachedResponseDontLoad'
     */
    SDWebImageDownloaderIgnoreCachedResponse = 1 << 3,
    
    /**
     * In iOS 4+, continue the download of the image if the app goes to background. This is achieved by asking the system for
     * extra time in background to let the request finish. If the background task expires the operation will be cancelled.
     */
    SDWebImageDownloaderContinueInBackground = 1 << 4,

    /**
     * Handles cookies stored in NSHTTPCookieStore by setting 
     * NSMutableURLRequest.HTTPShouldHandleCookies = YES;
     */
    SDWebImageDownloaderHandleCookies = 1 << 5,

    /**
     * Enable to allow untrusted SSL certificates.
     * Useful for testing purposes. Use with caution in production.
     */
    SDWebImageDownloaderAllowInvalidSSLCertificates = 1 << 6,

    /**
     * Put the image in the high priority queue.
     */
    SDWebImageDownloaderHighPriority = 1 << 7,
    
    /**
     * Scale down the image
     */
    SDWebImageDownloaderScaleDownLargeImages = 1 << 8,
};

3） 调用下载器SDWebImageDownloader进行下载

直接使用下载器进行下载

/**
 * Creates a SDWebImageDownloader async downloader instance with a given URL
 *
 * The delegate will be informed when the image is finish downloaded or an error has happen.
 *
 * @see SDWebImageDownloaderDelegate
 *
 * @param url            The URL to the image to download
 * @param options        The options to be used for this download
 * @param progressBlock  A block called repeatedly while the image is downloading
 *                       @note the progress block is executed on a background queue
 * @param completedBlock A block called once the download is completed.
 *                       If the download succeeded, the image parameter is set, in case of error,
 *                       error parameter is set with the error. The last parameter is always YES
 *                       if SDWebImageDownloaderProgressiveDownload isn't use. With the
 *                       SDWebImageDownloaderProgressiveDownload option, this block is called
 *                       repeatedly with the partial image object and the finished argument set to NO
 *                       before to be called a last time with the full image and finished argument
 *                       set to YES. In case of error, the finished argument is always YES.
 *
 * @return A token (SDWebImageDownloadToken) that can be passed to -cancel: to cancel this operation
 */
- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
                                                   options:(SDWebImageDownloaderOptions)options
                                                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                                 completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock;

这个方法返回的是SDWebImageDownloadToken对象。下载回调的参数如下UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished，一共四个参数。如果if (!strongOperation || strongOperation.isCancelled)就什么都不做；如果error不为空，作如下处理：

[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];
    }
}

这里，首先使用下面方法给出completionBlock回调

- (void)callCompletionBlockForOperation:(nullable SDWebImageCombinedOperation*)operation
                             completion:(nullable SDInternalCompletionBlock)completionBlock
                                  error:(nullable NSError *)error
                                    url:(nullable NSURL *)url {
    [self callCompletionBlockForOperation:operation completion:completionBlock image:nil data:nil error:error cacheType:SDImageCacheTypeNone finished:YES url:url];
}

- (void)callCompletionBlockForOperation:(nullable SDWebImageCombinedOperation*)operation
                             completion:(nullable SDInternalCompletionBlock)completionBlock
                                  image:(nullable UIImage *)image
                                   data:(nullable NSData *)data
                                  error:(nullable NSError *)error
                              cacheType:(SDImageCacheType)cacheType
                               finished:(BOOL)finished
                                    url:(nullable NSURL *)url {
    dispatch_main_async_safe(^{
        if (operation && !operation.isCancelled && completionBlock) {
            completionBlock(image, data, error, cacheType, finished, url);
        }
    });
}

并使用锁，将把失败的url放入集合中。

如果上面if不成立并且error为空时，就会做如下处理：

这里下载的图像回来就会根据条件进行图像的变换以及存储，如下所示：

/**
 * Asynchronously store an image into memory and disk cache at the given key.
 *
 * @param image           The image to store
 * @param imageData       The image data as returned by the server, this representation will be used for disk storage
 *                        instead of converting the given image object into a storable/compressed image format in order
 *                        to save quality and CPU
 * @param key             The unique image cache key, usually it's image absolute URL
 * @param toDisk          Store the image to disk cache if YES
 * @param completionBlock A block executed after the operation is finished
 */
- (void)storeImage:(nullable UIImage *)image
         imageData:(nullable NSData *)imageData
            forKey:(nullable NSString *)key
            toDisk:(BOOL)toDisk
        completion:(nullable SDWebImageNoParamsBlock)completionBlock;

根据指定的路径存在内存和disk。并调用callCompletionBlockForOperation:回调completionBlock。

第2个条件判断

条件是else if (cachedImage)，如果缓存图像不为空。

__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];

这里操作还是回调block和从操作数组中移除操作。

第3个条件判断

剩下的情况，如下处理：

// Image not in cache and download disallowed by delegate
__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];

这里操作还是回调block和从操作数组中移除操作。这个和上面不同的是图像、图像NSData数据和缓存类型分别为nil、nil和SDImageCacheTypeNone。

最后返回就是SDWebImageCombinedOperation *operation对象。

后记

本篇已结束，后面更精彩~~~