SDWebImage源码之SDWebImageManager

前面的两篇文章简单分析了SDImageCache和
SDWebImageDownloader的代码。它们分别实现了图片的缓存和下载。现在，还需要一个统一的类把它们的功能组合到一起，方便用户调用。这个类就是SDWebImageManager。

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以下代码和分析都基于041842bf085cbb711f0d9e099e6acbf6fd533b0c这个commit。

SDWebImageManager

- (nonnull instancetype)init {
    SDImageCache *cache = [SDImageCache sharedImageCache];
    SDWebImageDownloader *downloader = [SDWebImageDownloader sharedDownloader];
    return [self initWithCache:cache downloader:downloader];
}

- (nonnull instancetype)initWithCache:(nonnull SDImageCache *)cache downloader:(nonnull SDWebImageDownloader *)downloader {
    if ((self = [super init])) {
        _imageCache = cache;
        _imageDownloader = downloader;
        _failedURLs = [NSMutableSet new];
        _runningOperations = [NSMutableArray new];
    }
    return self;
}

SDWebImageManager提供了可以设置指定的SDImageCache和SDWebImageDownloader的init方法，方便调用方根据需要设置自己的SDImageCache和SDWebImageDownloader的实例。这种依赖注入的设计方式也可以方便单元测试时注入mock的对象。

/**
 * The cache filter is a block used each time SDWebImageManager need to convert an URL into a cache key. This can
 * be used to remove dynamic part of an image URL.
 *
 * The following example sets a filter in the application delegate that will remove any query-string from the
 * URL before to use it as a cache key:
 *
 * @code

[[SDWebImageManager sharedManager] setCacheKeyFilter:^(NSURL *url) {
    url = [[NSURL alloc] initWithScheme:url.scheme host:url.host path:url.path];
    return [url absoluteString];
}];

 * @endcode
 */
@property (nonatomic, copy, nullable) SDWebImageCacheKeyFilterBlock cacheKeyFilter;

- (nullable NSString *)cacheKeyForURL:(nullable NSURL *)url {
    if (!url) {
        return @"";
    }

    if (self.cacheKeyFilter) {
        return self.cacheKeyFilter(url);
    } else {
        return url.absoluteString;
    }
}

- (void)diskImageExistsForURL:(nullable NSURL *)url
                   completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock {
    NSString *key = [self cacheKeyForURL:url];
    
    [self.imageCache diskImageExistsWithKey:key completion:^(BOOL isInDiskCache) {
        // the completion block of checkDiskCacheForImageWithKey:completion: is always called on the main queue, no need to further dispatch
        if (completionBlock) {
            completionBlock(isInDiskCache);
        }
    }];
}

SDImageCache只管以key为唯一标识保存图片；SDWebImageDownloader只管依照图片URL下载图片，这里的cacheKeyFilter决定了URL应该被怎样处理成图片缓存用的key。

@interface SDWebImageCombinedOperation : NSObject <SDWebImageOperation>

@property (assign, nonatomic, getter = isCancelled) BOOL cancelled;
@property (copy, nonatomic, nullable) SDWebImageNoParamsBlock cancelBlock;
@property (strong, nonatomic, nullable) NSOperation *cacheOperation;

@end

@interface SDWebImageManager ()

@property (strong, nonatomic, nonnull) NSMutableArray<SDWebImageCombinedOperation *> *runningOperations;

@end

这里设计了一个SDWebImageCombinedOperation类，并维护了一个数组runningOperations对其进行管理。

loadImageWithURL加载图片有两个步骤，查询和下载。SDWebImageCombinedOperation同时代表了缓存查询和下载这两种行为。
这里的流程代码也比较多，用文字理一下：

1. 初始化：loadImageWithURL方法被调用时，把SDWebImageCombinedOperation加到runningOperations里。
2. 查询：到SDImageCache里查询图片是否存在，把查询缓存用的NSOperation保存在SDWebImageCombinedOperation里。
3. 查询结束：如果图片在缓存中，回调completedBlock，把SDWebImageCombinedOperation从runningOperations里去掉。否则开始下载。
4. 下载：调用SDWebImageDownloader下载图片，并且把SDWebImageCombinedOperation的cancelBlock设置成取消下载。
5. 下载结束：图片下载完成后，回调completedBlock，并把SDWebImageCombinedOperation从runningOperations里去掉。

可以看到，只要加载图片的过程在进行，无论是查询还是下载，这个任务都会在runningOperations里，外部可以统一的查询和管理图片加载的任务。

operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {
        if (operation.isCancelled) {
            [self safelyRemoveOperationFromRunning:operation];
            return;
        }
        ...
}];
            operation.cancelBlock = ^{
                [self.imageDownloader cancel:subOperationToken];
                __strong __typeof(weakOperation) strongOperation = weakOperation;
                [self safelyRemoveOperationFromRunning:strongOperation];
            };

要注意的是，如果在查询过程中，调用取消任务并不会中止查询流程，只是在最终返回时直接丢掉查询结果；而如果在下载过程中，调用cancel最后会调用到SDWebImageDownloader的cancel，真正取消下载任务。

- (void)cancelAll {
    @synchronized (self.runningOperations) {
        NSArray<SDWebImageCombinedOperation *> *copiedOperations = [self.runningOperations copy];
        [copiedOperations makeObjectsPerformSelector:@selector(cancel)];
        [self.runningOperations removeObjectsInArray:copiedOperations];
    }
}

- (BOOL)isRunning {
    BOOL isRunning = NO;
    @synchronized (self.runningOperations) {
        isRunning = (self.runningOperations.count > 0);
    }
    return isRunning;
}

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

为了防止self.runningOperations被多线程操作，所有对self.runningOperations的访问都加上了@synchronized临界区，@synchronized里的代码段不会同时在多个线程中被执行。

SDWebImageManagerTests

NSString *workingImageURL = @"http://s3.amazonaws.com/fast-image-cache/demo-images/FICDDemoImage001.jpg";

(void)test02ThatDownloadInvokesCompletionBlockWithCorrectParamsAsync {
    __block XCTestExpectation *expectation = [self expectationWithDescription:@"Image download completes"];

    NSURL *originalImageURL = [NSURL URLWithString:workingImageURL];
    
    [[SDWebImageManager sharedManager] loadImageWithURL:originalImageURL
                                                options:SDWebImageRefreshCached
                                               progress:nil
                                              completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
        expect(image).toNot.beNil();
        expect(error).to.beNil();
        expect(originalImageURL).to.equal(imageURL);

        [expectation fulfill];
        expectation = nil;
    }];
    expect([[SDWebImageManager sharedManager] isRunning]).to.equal(YES);

    [self waitForExpectationsWithTimeout:kAsyncTestTimeout handler:nil];
}

SDWebImage的单元测试依赖了网络访问，这个不太好。我们开发中需要做类似功能的单元测试时，可以考虑用OHHTTPStubs来搞些假的网络请求。

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SDWebImage源码系列

SDWebImage源码之SDImageCache
SDWebImage源码之SDWebImageDownloader
SDWebImage源码之SDWebImageManager