概述
SDWebImageManager是SDWebImage框架的核心调度类，负责图片的缓存和下载逻辑，本文分析一下该类的相关方法。
初始化方法
首先SDWebImageManager维护类几个属性，类的定义注释如下：
@interface SDWebImageManager ()
@property (strong, nonatomic, readwrite) SDImageCache *imageCache; //缓存类
@property (strong, nonatomic, readwrite) SDWebImageDownloader *imageDownloader; //下载类
@property (strong, nonatomic) NSMutableSet *failedURLs; //下载失败的url
@property (strong, nonatomic) NSMutableArray *runningOperations; //当前执行的operation
@end
imageCache用于缓存图片数据的类，具体可以参考第一篇文章，imageDownloader用于下载图片数据，具体可以参考上一篇文章，failedURLs是一个数组，里面存储了获取图片失败对应的url，runningOperations存储了当前正在执行的operation，operation是SDWebImageCombinedOperation类型的对象，定义注释如下：
@interface SDWebImageCombinedOperation : NSObject <SDWebImageOperation>
@property (assign, nonatomic, getter = isCancelled) BOOL cancelled;
@property (copy, nonatomic) SDWebImageNoParamsBlock cancelBlock; //cancel block
@property (strong, nonatomic) NSOperation *cacheOperation; //执行缓存操作的operaton
@end
SDWebImageOperation是protocol，SDWebImageCombinedOperation实现该protocol，实现了cancel方法。注释如下；
- (void)cancel {
self.cancelled = YES;
if (self.cacheOperation) {
[self.cacheOperation cancel]; //取消缓存operation的操作
self.cacheOperation = nil;
}
if (self.cancelBlock) {
self.cancelBlock(); //执行cancelBlock
_cancelBlock = nil;
}
}
然后SDWebImageManager通过-(void)initWithCache:downloader:方法初始化，注释如下：
- (instancetype)initWithCache:(SDImageCache *)cache downloader:(SDWebImageDownloader *)downloader {
if ((self = [super init])) {
_imageCache = cache; //赋值cache
_imageDownloader = downloader; //赋值downloader
_failedURLs = [NSMutableSet new];
_runningOperations = [NSMutableArray new];
}
return self;
}
下载图片方法
SDWebImageManager中的核心方法是-(void)downloadImageWithURL:options:progress:completed:方法，主要负责从缓存中取图片，然后根据策略下载图片。改方法的步骤主要分为以下几步：
-
判断前置条件：
if ([url isKindOfClass:NSString.class]) { url = [NSURL URLWithString:(NSString *)url]; } //url判空 if (![url isKindOfClass:NSURL.class]) { url = nil; } //创建operation对象 __block SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new]; __weak SDWebImageCombinedOperation *weakOperation = operation; BOOL isFailedUrl = NO; @synchronized (self.failedURLs) { isFailedUrl = [self.failedURLs containsObject:url]; } //当前url是否是FailedUrl，如果是直接返回上层。 if (url.absoluteString.length == 0 || (!(options & SDWebImageRetryFailed) && isFailedUrl)) { dispatch_main_sync_safe(^{ NSError *error = [NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorFileDoesNotExist userInfo:nil]; completedBlock(nil, error, SDImageCacheTypeNone, YES, url); }); return operation; }首先对url判空，如果不为空，判断当前url是否属于failedURLs中，因为failedURLs中的url属于之前下载失败的图片url，不能被下载。除非options中包含SDWebImageRetryFailed选项，表示之前下载失败过，但是本次需要重新下载。如果不包含SDWebImageRetryFailed选项，调用completedBlock回调。
-
将operation加入runningOperations中，从缓存中去图片数据：
operation.cacheOperation = [self.imageCache queryDiskCacheForKey:key done:^(UIImage *image, SDImageCacheType cacheType) { if (operation.isCancelled) { @synchronized (self.runningOperations) { [self.runningOperations removeObject:operation]; } return; } //不存在image，或者需要刷新image，重新下载 if ((!image || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) { ... } else if (image) { //存在image，直接返回 dispatch_main_sync_safe(^{ __strong __typeof(weakOperation) strongOperation = weakOperation; if (strongOperation && !strongOperation.isCancelled) { completedBlock(image, nil, cacheType, YES, url); } }); @synchronized (self.runningOperations) { [self.runningOperations removeObject:operation]; } } else { dispatch_main_sync_safe(^{ __strong __typeof(weakOperation) strongOperation = weakOperation; if (strongOperation && !weakOperation.isCancelled) { completedBlock(nil, nil, SDImageCacheTypeNone, YES, url); } }); @synchronized (self.runningOperations) { [self.runningOperations removeObject:operation]; } } }]该方法首先从缓存中取image，如果没有取到，说明是第一次下载图片，或者存在image，但是需要刷新图片，如果delegate实现shouldDownloadImageForURL方法，返回YES，说明需要下载，这些情况下开始下载图片。当不属于这些情况且image存在时，说明之前已经下载过url对应的图片了，则直接将缓存中的图片通过completedBlock返回，并且将operation从runningOperations中删除。如果image不存在，则返回nil给外部。
-
当需要下载图片时，设置标记位，将SDWebImageOptions转化成SDWebImageDownloaderOptions：
SDWebImageDownloaderOptions downloaderOptions = 0; if (options & SDWebImageLowPriority) downloaderOptions |= SDWebImageDownloaderLowPriority; if (options & SDWebImageProgressiveDownload) downloaderOptions |= SDWebImageDownloaderProgressiveDownload; if (options & SDWebImageRefreshCached) downloaderOptions |= SDWebImageDownloaderUseNSURLCache; //使用NSURLCache if (options & SDWebImageContinueInBackground) downloaderOptions |= SDWebImageDownloaderContinueInBackground; if (options & SDWebImageHandleCookies) downloaderOptions |= SDWebImageDownloaderHandleCookies; if (options & SDWebImageAllowInvalidSSLCertificates) downloaderOptions |= SDWebImageDownloaderAllowInvalidSSLCertificates; if (options & SDWebImageHighPriority) downloaderOptions |= SDWebImageDownloaderHighPriority; if (image && options & SDWebImageRefreshCached) { //禁用ProgressiveDownload downloaderOptions &= ~SDWebImageDownloaderProgressiveDownload; //开启忽略CachedResponse downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse; } -
然后调用imageDownloader下载url对应图片数据，并处理不同情况：
id <SDWebImageOperation> subOperation = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *data, NSError *error, BOOL finished) { __strong __typeof(weakOperation) strongOperation = weakOperation; if (!strongOperation || strongOperation.isCancelled) { } else if (error) { //下载失败 dispatch_main_sync_safe(^{ if (strongOperation && !strongOperation.isCancelled) { completedBlock(nil, error, SDImageCacheTypeNone, finished, url); } }); if (error.code != NSURLErrorNotConnectedToInternet && error.code != NSURLErrorCancelled && error.code != NSURLErrorTimedOut && error.code != NSURLErrorInternationalRoamingOff && error.code != NSURLErrorDataNotAllowed && error.code != NSURLErrorCannotFindHost && error.code != NSURLErrorCannotConnectToHost) { @synchronized (self.failedURLs) { [self.failedURLs addObject:url]; //添加url进failedURLs } } } else { if ((options & SDWebImageRetryFailed)) { @synchronized (self.failedURLs) { [self.failedURLs removeObject:url]; //下载成功，将url从failedURLs中删除 } } BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly); if (options & SDWebImageRefreshCached && image && !downloadedImage) { } else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)]) { //将下载后的图片变换后写入缓存 dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{ UIImage *transformedImage = [self.delegate imageManager:self transformDownloadedImage:downloadedImage withURL:url]; if (transformedImage && finished) { BOOL imageWasTransformed = ![transformedImage isEqual:downloadedImage]; [self.imageCache storeImage:transformedImage recalculateFromImage:imageWasTransformed imageData:(imageWasTransformed ? nil : data) forKey:key toDisk:cacheOnDisk]; } dispatch_main_sync_safe(^{ //回调给外层 if (strongOperation && !strongOperation.isCancelled) { completedBlock(transformedImage, nil, SDImageCacheTypeNone, finished, url); } }); }); } else { //将下载后的图片数据写入缓存 if (downloadedImage && finished) { [self.imageCache storeImage:downloadedImage recalculateFromImage:NO imageData:data forKey:key toDisk:cacheOnDisk]; } dispatch_main_sync_safe(^{ //回调给外层 if (strongOperation && !strongOperation.isCancelled) { completedBlock(downloadedImage, nil, SDImageCacheTypeNone, finished, url); } }); } } if (finished) { @synchronized (self.runningOperations) { if (strongOperation) { //从runningOperations中删除 [self.runningOperations removeObject:strongOperation]; } } } }]; operation.cancelBlock = ^{ [subOperation cancel]; //取消下载 @synchronized (self.runningOperations) { __strong __typeof(weakOperation) strongOperation = weakOperation; if (strongOperation) { //从runningOperations中删除 [self.runningOperations removeObject:strongOperation]; } } };如果下载失败，则返回nil给外层，同时判断errorCode不是超时、取消、找不到主机地址等错误时，将url加入failedURLs。如果下载成功，且option标记为SDWebImageRetryFailed，则键url从failedURLs数组中删除。然后判断是否需要将下载的图片进行变换，如果需要transform，则进行transform后将图片写入缓存，否则直接将原图片写入缓存。当操作执行完后，将operation从runningOperations中删除。最后指定了operation的cancelBlock，该block主要讲负责下载的subOperation取消，然后将当前operation从runningOperations数组中删除。
其他方法
SDWebImageManager还提供了一些辅助方法，如下：
-
-(void)saveImageToCache: forURL:方法，该方法负责将图片存入缓存，url作为key。
- (void)saveImageToCache:(UIImage *)image forURL:(NSURL *)url { if (image && url) { NSString *key = [self cacheKeyForURL:url]; [self.imageCache storeImage:image forKey:key toDisk:YES]; } } -
-(void)cancelAll方法，该方法将runningOperations中的所有operation取消。
- (void)cancelAll { @synchronized (self.runningOperations) { NSArray *copiedOperations = [self.runningOperations copy]; [copiedOperations makeObjectsPerformSelector:@selector(cancel)]; //取消 [self.runningOperations removeObjectsInArray:copiedOperations]; } }调用makeObjectsPerformSelector方法遍历operation，调用cancel方法，SDWebImageCombinedOperation的cancel方法在上文有注释，取消cacheOperation，执行cancelBlock取消下载的operation。
-
-(BOOL)cachedImageExistsForURL:方法和-(BOOL)diskImageExistsForURL:方法
这两个同步方法返回图片是否在缓存中存在，前一个方法先判断图片是否在内存中存在，如果没有再去文件中查找，返回查找的结果。后一个方法直接去文件中查找，返回查找的结果。
-
-(void)cachedImageExistsForURL:completion:方法和-(void)diskImageExistsForURL:completion:方法
这两个方法的功能和上面两个是相同的，前一个方法同时从内存和文件缓存中查找图片是否存在，后一个方法只从文件缓存中查找，区别之处在于，这两个方法是通过completionBlock异步返回结果的。
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