SDWebImage源码解读之SDWebImagePrefetcher

第十篇

前言

我们先看看SDWebImage主文件的组成模块：

可以看出来，每个模块即独立又相对关联，当最后拼接出SDWebImageManager的时候，我们就可以利用它来做一些有意思的事情。

本篇就主要讲解其中的一个使用场景：批量图片下载。记得之前有一位同学有这样的开发需求：他们公司要做一个漫画APP，漫画都是由图片组成的，每一个本漫画由很多章节组成，需要提供一个缓存功能，也就是把图片一组一组的下载下来。那么使用本篇的这个类就能完美的解决它的需求。

SDWebImagePrefetcherDelegate

这个代理提供了两个方法来监听事件：

• 每次下载完一个图片
• 所有的都下载完

代码：

@protocol SDWebImagePrefetcherDelegate <NSObject>

@optional

/**
 * Called when an image was prefetched.
 *
 * @param imagePrefetcher The current image prefetcher
 * @param imageURL        The image url that was prefetched
 * @param finishedCount   The total number of images that were prefetched (successful or not)
 * @param totalCount      The total number of images that were to be prefetched
 */
- (void)imagePrefetcher:(nonnull SDWebImagePrefetcher *)imagePrefetcher didPrefetchURL:(nullable NSURL *)imageURL finishedCount:(NSUInteger)finishedCount totalCount:(NSUInteger)totalCount;

/**
 * Called when all images are prefetched.
 * @param imagePrefetcher The current image prefetcher
 * @param totalCount      The total number of images that were prefetched (whether successful or not)
 * @param skippedCount    The total number of images that were skipped
 */
- (void)imagePrefetcher:(nonnull SDWebImagePrefetcher *)imagePrefetcher didFinishWithTotalCount:(NSUInteger)totalCount skippedCount:(NSUInteger)skippedCount;

@end

SDWebImagePrefetcher.h

属性：

/**
 *  The web image manager
 */
@property (strong, nonatomic, readonly, nonnull) SDWebImageManager *manager;

/**
 * Maximum number of URLs to prefetch at the same time. Defaults to 3.
 */
@property (nonatomic, assign) NSUInteger maxConcurrentDownloads;

/**
 * SDWebImageOptions for prefetcher. Defaults to SDWebImageLowPriority.
 */
@property (nonatomic, assign) SDWebImageOptions options;

/**
 * Queue options for Prefetcher. Defaults to Main Queue.
 */
@property (nonatomic, assign, nonnull) dispatch_queue_t prefetcherQueue;

@property (weak, nonatomic, nullable) id <SDWebImagePrefetcherDelegate> delegate;

初始化：

/**
 * Return the global image prefetcher instance.
 */
+ (nonnull instancetype)sharedImagePrefetcher;

/**
 * Allows you to instantiate a prefetcher with any arbitrary image manager.
 */
- (nonnull instancetype)initWithImageManager:(nonnull SDWebImageManager *)manager NS_DESIGNATED_INITIALIZER;

方法：

/**
 * Assign list of URLs to let SDWebImagePrefetcher to queue the prefetching,
 * currently one image is downloaded at a time,
 * and skips images for failed downloads and proceed to the next image in the list
 *
 * @param urls list of URLs to prefetch
 */
- (void)prefetchURLs:(nullable NSArray<NSURL *> *)urls;

/**
 * Assign list of URLs to let SDWebImagePrefetcher to queue the prefetching,
 * currently one image is downloaded at a time,
 * and skips images for failed downloads and proceed to the next image in the list
 *
 * @param urls            list of URLs to prefetch
 * @param progressBlock   block to be called when progress updates; 
 *                        first parameter is the number of completed (successful or not) requests, 
 *                        second parameter is the total number of images originally requested to be prefetched
 * @param completionBlock block to be called when prefetching is completed
 *                        first param is the number of completed (successful or not) requests,
 *                        second parameter is the number of skipped requests
 */
- (void)prefetchURLs:(nullable NSArray<NSURL *> *)urls
            progress:(nullable SDWebImagePrefetcherProgressBlock)progressBlock
           completed:(nullable SDWebImagePrefetcherCompletionBlock)completionBlock;

/**
 * Remove and cancel queued list
 */
- (void)cancelPrefetching;

SDWebImagePrefetcher.m

@interface SDWebImagePrefetcher ()

@property (strong, nonatomic, nonnull) SDWebImageManager *manager;
@property (strong, nonatomic, nullable) NSArray<NSURL *> *prefetchURLs;
@property (assign, nonatomic) NSUInteger requestedCount;
@property (assign, nonatomic) NSUInteger skippedCount;
@property (assign, nonatomic) NSUInteger finishedCount;
@property (assign, nonatomic) NSTimeInterval startedTime;
@property (copy, nonatomic, nullable) SDWebImagePrefetcherCompletionBlock completionBlock;
@property (copy, nonatomic, nullable) SDWebImagePrefetcherProgressBlock progressBlock;

@end

这里多了一个skippedCount属性，这个属性用来记录下载失败的次数，skip表示跳过的意思。

+ (nonnull instancetype)sharedImagePrefetcher {
    static dispatch_once_t once;
    static id instance;
    dispatch_once(&once, ^{
        instance = [self new];
    });
    return instance;
}

- (nonnull instancetype)init {
    return [self initWithImageManager:[SDWebImageManager new]];
}

- (nonnull instancetype)initWithImageManager:(SDWebImageManager *)manager {
    if ((self = [super init])) {
        _manager = manager;
        _options = SDWebImageLowPriority;
        _prefetcherQueue = dispatch_get_main_queue();
        self.maxConcurrentDownloads = 3;
    }
    return self;
}

setter，getter：

- (void)setMaxConcurrentDownloads:(NSUInteger)maxConcurrentDownloads {
    self.manager.imageDownloader.maxConcurrentDownloads = maxConcurrentDownloads;
}

- (NSUInteger)maxConcurrentDownloads {
    return self.manager.imageDownloader.maxConcurrentDownloads;
}

这里是setter和getter方法，有意思的是setter并不以一定要给这个属性赋值，getter而不一定就一定返回该属性的值。

- (void)prefetchURLs:(nullable NSArray<NSURL *> *)urls
            progress:(nullable SDWebImagePrefetcherProgressBlock)progressBlock
           completed:(nullable SDWebImagePrefetcherCompletionBlock)completionBlock {
    [self cancelPrefetching]; // Prevent duplicate prefetch request
    self.startedTime = CFAbsoluteTimeGetCurrent();
    self.prefetchURLs = urls;
    self.completionBlock = completionBlock;
    self.progressBlock = progressBlock;

    if (urls.count == 0) {
        if (completionBlock) {
            completionBlock(0,0);
        }
    } else {
        // Starts prefetching from the very first image on the list with the max allowed concurrency
        NSUInteger listCount = self.prefetchURLs.count;
        for (NSUInteger i = 0; i < self.maxConcurrentDownloads && self.requestedCount < listCount; i++) {
            [self startPrefetchingAtIndex:i];
        }
    }
}

这个函数很有意思，首先调用了[self cancelPrefetching],我们看看该方法的实现：

- (void)cancelPrefetching {
    self.prefetchURLs = nil;
    self.skippedCount = 0;
    self.requestedCount = 0;
    self.finishedCount = 0;
    [self.manager cancelAll];
}

说明调用该方法后，所以的未完成的下载都会清空，也就是说SDWebImagePrefetcher只专注处理一组URLs，是无状态的下载。也就要求我们传入的URLs要完整。

那么我们如何实现支持多个图片并发下载呢？我们都知道SDWebImageManager的loadImage方法是异步执行的，因此只要多次调用loadImage方法就能做到了。也就是[self startPrefetchingAtIndex:i];

- (void)startPrefetchingAtIndex:(NSUInteger)index {
    /// 判断index是否越界
    if (index >= self.prefetchURLs.count) return;
    /// 已请求的个数加1
    self.requestedCount++;
    /// 使用self.manager下载图片
    [self.manager loadImageWithURL:self.prefetchURLs[index] options:self.options progress:nil completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
        /// 只有当finished完成之后，self.finishedCount加1
        if (!finished) return;
        self.finishedCount++;

        if (image) { // 下载成功后，调用progressBlock
            if (self.progressBlock) {
                self.progressBlock(self.finishedCount,(self.prefetchURLs).count);
            }
        }
        else { // 下载失败，也调用progressBlock，同时记录该次的下载失败
            if (self.progressBlock) {
                self.progressBlock(self.finishedCount,(self.prefetchURLs).count);
            }
            // Add last failed
            self.skippedCount++;
        }
        /// 调用delegate
        if ([self.delegate respondsToSelector:@selector(imagePrefetcher:didPrefetchURL:finishedCount:totalCount:)]) {
            [self.delegate imagePrefetcher:self
                            didPrefetchURL:self.prefetchURLs[index]
                             finishedCount:self.finishedCount
                                totalCount:self.prefetchURLs.count
             ];
        }
        /// 如果URLs的数量大于已经下载的数量，就说明还有没下载完的任务，继续下载下一个
        if (self.prefetchURLs.count > self.requestedCount) {
            dispatch_async(self.prefetcherQueue, ^{
                [self startPrefetchingAtIndex:self.requestedCount];
            });
        } else if (self.finishedCount == self.requestedCount) { // 当完成数等于已请求总数的时候，就宣告下载完毕
            /// 告诉代理，下载已经完毕
            [self reportStatus];
            /// 调用completionBlock,这里把completionBlock和progressBlock都设为nil是为了避免循环引用
            if (self.completionBlock) {
                self.completionBlock(self.finishedCount, self.skippedCount);
                self.completionBlock = nil;
            }
            self.progressBlock = nil;
        }
    }];
}

- (void)reportStatus {
    NSUInteger total = (self.prefetchURLs).count;
    if ([self.delegate respondsToSelector:@selector(imagePrefetcher:didFinishWithTotalCount:skippedCount:)]) {
        [self.delegate imagePrefetcher:self
               didFinishWithTotalCount:(total - self.skippedCount)
                          skippedCount:self.skippedCount
         ];
    }
}

总结

SDWebImagePrefetcher是SDWebImageManager很好的应用例子，下一篇我们总结一下UI控件使用SDWebImageManager获取图片的例子。

由于个人知识有限，如有错误之处，还望各路大侠给予指出啊

1. SDWebImage源码解读 之 NSData+ImageContentType 简书 博客园
2. SDWebImage源码解读 之 UIImage+GIF 简书 博客园
3. SDWebImage源码解读 之 SDWebImageCompat 简书 博客园
4. SDWebImage源码解读 之SDWebImageDecoder 简书 博客园
5. SDWebImage源码解读 之SDWebImageCache(上) 简书 博客园
6. SDWebImage源码解读之SDWebImageCache(下) 简书 博客园
7. SDWebImage源码解读之SDWebImageDownloaderOperation 简书 博客园
8. SDWebImage源码解读之SDWebImageDownloader 简书 博客园
9. SDWebImage源码解读之SDWebImageManager 简书 博客园