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。
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
@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 *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：
- 初始化：
loadImageWithURL方法被调用时，把SDWebImageCombinedOperation加到runningOperations里。 - 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。 - 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。 - 下载：调用
SDWebImageDownloader下载图片，并且把SDWebImageCombinedOperation的cancelBlock设置成取消下载。 - 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 *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。
SDWebImage源码系列
SDWebImage源码之SDImageCache
SDWebImage源码之SDWebImageDownloader
SDWebImage源码之SDWebImageManager