SDWebImage源碼解讀之SDWebImagePrefetcher

第十篇

前言

我們先看看SDWebImage主文件的組成模塊：

可以看出來，每個模塊即獨立又相對關(guān)聯(lián)，當最后拼接出SDWebImageManager的時候，我們就可以利用它來做一些有意思的事情。

本篇就主要講解其中的一個使用場景：批量圖片下載。記得之前有一位同學有這樣的開發(fā)需求：他們公司要做一個漫畫APP，漫畫都是由圖片組成的，每一個本漫畫由很多章節(jié)組成，需要提供一個緩存功能，也就是把圖片一組一組的下載下來。那么使用本篇的這個類就能完美的解決它的需求。

SDWebImagePrefetcherDelegate

這個代理提供了兩個方法來監(jiān)聽事件：

  • 每次下載完一個圖片
  • 所有的都下載完

代碼：

@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

屬性：

/**
 *  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

這里多了一個skippedCount屬性，這個屬性用來記錄下載失敗的次數(shù)，skip表示跳過的意思。

+ (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;
}

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

    • (NSUInteger)maxConcurrentDownloads {
      return self.manager.imageDownloader.maxConcurrentDownloads;
      }
      這里是setter和getter方法，有意思的是setter并不以一定要給這個屬性賦值，getter而不一定就一定返回該屬性的值。

    • (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];
      }
      }
      }
      這個函數(shù)很有意思，首先調(diào)用了[self cancelPrefetching],我們看看該方法的實現(xiàn)：

    • (void)cancelPrefetching {
      self.prefetchURLs = nil;
      self.skippedCount = 0;
      self.requestedCount = 0;
      self.finishedCount = 0;
      [self.manager cancelAll];
      }
      說明調(diào)用該方法后，所以的未完成的下載都會清空，也就是說SDWebImagePrefetcher只專注處理一組URLs，是無狀態(tài)的下載。也就要求我們傳入的URLs要完整。

那么我們?nèi)绾螌崿F(xiàn)支持多個圖片并發(fā)下載呢？我們都知道SDWebImageManager的loadImage方法是異步執(zhí)行的，因此只要多次調(diào)用loadImage方法就能做到了。也就是[self startPrefetchingAtIndex:i];

- (void)startPrefetchingAtIndex:(NSUInteger)index {
    /// 判斷index是否越界
    if (index >= self.prefetchURLs.count) return;
    /// 已請求的個數(shù)加1
    self.requestedCount++;
    /// 使用self.manager下載圖片
    [self.manager loadImageWithURL:self.prefetchURLs[index] options:self.options progress:nil completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
        /// 只有當finished完成之后，self.finishedCount加1
        if (!finished) return;
        self.finishedCount++;

        if (image) { // 下載成功后，調(diào)用progressBlock
            if (self.progressBlock) {
                self.progressBlock(self.finishedCount,(self.prefetchURLs).count);
            }
        }
        else { // 下載失敗，也調(diào)用progressBlock，同時記錄該次的下載失敗
            if (self.progressBlock) {
                self.progressBlock(self.finishedCount,(self.prefetchURLs).count);
            }
            // Add last failed
            self.skippedCount++;
        }
        /// 調(diào)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的數(shù)量大于已經(jīng)下載的數(shù)量，就說明還有沒下載完的任務，繼續(xù)下載下一個
        if (self.prefetchURLs.count > self.requestedCount) {
            dispatch_async(self.prefetcherQueue, ^{
                [self startPrefetchingAtIndex:self.requestedCount];
            });
        } else if (self.finishedCount == self.requestedCount) { // 當完成數(shù)等于已請求總數(shù)的時候，就宣告下載完畢
            /// 告訴代理，下載已經(jīng)完畢
            [self reportStatus];
            /// 調(diào)用completionBlock,這里把completionBlock和progressBlock都設為nil是為了避免循環(huán)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
         ];
    }
}

總結(jié)

SDWebImagePrefetcher是SDWebImageManager很好的應用例子，下一篇我們總結(jié)一下UI控件使用SDWebImageManager獲取圖片的例子。

由于個人知識有限，如有錯誤之處，還望各路大俠給予指出啊

  1. SDWebImage源碼解讀 之 NSData+ImageContentType 簡書 博客園
  2. SDWebImage源碼解讀 之 UIImage+GIF 簡書 博客園
  3. SDWebImage源碼解讀 之 SDWebImageCompat 簡書 博客園
  4. SDWebImage源碼解讀 之SDWebImageDecoder 簡書 博客園
  5. SDWebImage源碼解讀 之SDWebImageCache(上) 簡書 博客園
  6. SDWebImage源碼解讀之SDWebImageCache(下) 簡書 博客園
  7. SDWebImage源碼解讀之SDWebImageDownloaderOperation 簡書 博客園
  8. SDWebImage源碼解讀之SDWebImageDownloader 簡書 博客園
  9. SDWebImage源碼解讀之SDWebImageManager 簡書 博客園
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