在开发iOS
的客户端应用时,经常需要从服务器下载图片,虽然系统提供了下载工具:NSData、NSURLSession等等方法,但是考虑到图片下载过程中,需要考虑的因素比较多,比如:异步下载、图片缓存、错误处理、编码解码等,以及实际需要中根据不同网络加载不同画质的图片等等需求,因此下载操作不是一个简单的下载动作就可以解决。
针对上述问题,目前常用的开源库就是SDWebImage
,它很好的解决了图片的异步下载、图片缓存、错误处理等问题,得到了广泛的应用,使得设置UIImageView
、UIButton
对象的图片十分方便。本文就从源码的角度,剖析一下这款优秀的开源库的具体实现。
类结构图
SDWebImage
的源码的类结构图和下载流程图在官方的说明文档里有介绍,通过UML
类结构图详细的介绍了该框架的内部结构,以及通过流程图介绍了具体的下载过程。
下图是我总结的SDWebImage
的结构图,简单的把SDWebImage
源码文件按照功能进行了划分,方便在阅读源码时,能快速的对源码有一个总体的认识,加快阅读效率。
关键类功能简介:
- 1.下载类
SDWebImageDownloader:提供下载的方法给SDWebImageManager
使用,提供了最大并发量的下载控制、超时时间、取消下载、下载挂起、是否解压图片等等功能。同时,还提供了开始下载和停止下载的通知,给使用者监测下载状态,如果使用者不用监测下载状态,就不用监测该通知,这种设计模式很灵活,给使用者提供了更方便的选择。
extern NSString * _Nonnull const SDWebImageDownloadStartNotification; extern NSString * _Nonnull const SDWebImageDownloadStopNotification;
SDWebImageDownloaderOperation:继承自NSOperation
,是图片下载的具体实现类,通过加入到NSOperationQueue
中,然后在start
方法中来开启下载操作。
- 2.图片缓存
SDImageCacheConfig:主要提供缓存的配置信息,如:是否解压图片、是否缓存到内存、最大缓存时间(默认是一周)和最大缓存的字节数等等。
SDImageCache:缓存实现类,提供最大缓存字节、最大缓存条目的控制,以及缓存到内存及磁盘、从内存或磁盘删除、查询检索和查询缓存信息等功能。
- 3.分类
UIImageView+WebCache:UIImageView
的分类,提供了设置UIImageView
对象图片的多种方法,下面的方法可以说是SDWebImage
框架中最常用的方法。
- (void)sd_setImageWithURL:(nullable NSURL *)url placeholderImage:(nullable UIImage *)placeholder options:(SDWebImageOptions)options; // 带完成block的赋值方法 - (void)sd_setImageWithURL:(nullable NSURL *)url placeholderImage:(nullable UIImage *)placeholder options:(SDWebImageOptions)options completed:(nullable SDExternalCompletionBlock)completedBlock;
UIButton+WebCache:UIButton
的分类,提供了设置按钮图片和按钮背景图片的功能
- (void)sd_setImageWithURL:(nullable NSURL *)url forState:(UIControlState)state placeholderImage:(nullable UIImage *)placeholder;
- 4.工具类
SDWebImageDecoder:图像解码的工具类,通过imageNames:
加载图片会立即进行解码,而通过imageWithContentsOfFile:
则不会
SDWebImagePrefetcher:批量图像下载工具,针对UI
界面中需要下载多个图片时,又要在滑动中保持流畅体验,则可以使用该工具类批量下载图片,然后在给具体的UI
控件设置图片时,就会直接从缓存中取
SDWebImageManager:下载管理类工具,是SDWebImage
的核心类,从官方文档的类图中也可以看出,提供了查看图片是否已经缓存、下载图片、缓存图片、取消所有的下载等等功能
- 5.图片格式类
NSData+ImageContentType:根据图片数据的第一个字节来获取图片的格式,可以区分PNG
、JPEG
、GIF
、TIFF
和WebP
。
以上只是对SDWebImage
类结构图的简单分析,如果需要进一步了解各个类的具体实现,请参考文末的资料,已有人详细的介绍了各个类的功能实现原理或方法。
应用
下面介绍一个在应用SDWebImage
设置UI
图片的源码实现过程
在UIImageView上的应用
设置图片
通过设置URL、占位图片、图片配置、图片下载进度回调和设置完成回调来给UIImageView
对象设置图片
// ViewController.m [self.imageView sd_setImageWithURL:[NSURL URLWithString:@"http://rescdn.qqmail.com/dyimg/20140302/73EB27F4A350.jpg"] placeholderImage:[UIImage imageNamed:@"gift-icon"] options:0 progress:nil completed:nil];
上述代码调用UIImageView+WebCache.m
里的方法
- (void)sd_setImageWithURL:(nullable NSURL *)url placeholderImage:(nullable UIImage *)placeholder options:(SDWebImageOptions)options progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock completed:(nullable SDExternalCompletionBlock)completedBlock { [self sd_internalSetImageWithURL:url placeholderImage:placeholder options:options operationKey:nil setImageBlock:nil progress:progressBlock completed:completedBlock]; }
然后调用UIView+WebCache.m
中的方法获取图片,然后根据option的类型进行不同的设置
// UIView+WebCache.m - (void)sd_internalSetImageWithURL:(nullable NSURL *)url placeholderImage:(nullable UIImage *)placeholder options:(SDWebImageOptions)options operationKey:(nullable NSString *)operationKey setImageBlock:(nullable SDSetImageBlock)setImageBlock progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock completed:(nullable SDExternalCompletionBlock)completedBlock { ... if (url) { ... __weak __typeof(self)wself = self; // 开始加载图片 id <SDWebImageOperation> operation = [SDWebImageManager.sharedManager loadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) { ... dispatch_main_async_safe(^{ if (!sself) { return; } if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock) { // 把图片放到completedBlock里处理,一般是手动设置图片,因为这样可以对图片做进一步处理 completedBlock(image, error, cacheType, url); return; } else if (image) { // 设置图片 [sself sd_setImage:image imageData:data basedOnClassOrViaCustomSetImageBlock:setImageBlock]; [sself sd_setNeedsLayout]; } else { // 延迟加载占位图(获取图片之后) if ((options & SDWebImageDelayPlaceholder)) { [sself sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock]; [sself sd_setNeedsLayout]; } } // 回调完成block,如果是nil,则不调用 if (completedBlock && finished) { completedBlock(image, error, cacheType, url); } }); }]; } else { // 处理url为nil的情况 dispatch_main_async_safe(^{ [self sd_removeActivityIndicator]; if (completedBlock) { NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}]; completedBlock(nil, error, SDImageCacheTypeNone, url); } }); } }
加载图片的具体实现代码在SDWebImageManager
里面,先从缓存中取图片,如果缓存中没有图片,就从网络下载,然后设置图片,最后再缓存该图片
// SDWebImageManager.m - (id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url options:(SDWebImageOptions)options progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock completed:(nullable SDInternalCompletionBlock)completedBlock { ... // 从缓存中取图片 operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) { if (operation.isCancelled) { // 如果操作被取消,就从runningOperations操作数组从把该操作删除 [self safelyRemoveOperationFromRunning:operation]; return; } if ((!cachedImage || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) { if (cachedImage && options & SDWebImageRefreshCached) { // 如果options设置为更新缓存,那么就需要从服务器从新下载图片,然后更新本地缓存 [self callCompletionBlockForOperation:weakOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url]; } ... // 创建下载器,从服务器下载图片 SDWebImageDownloadToken *subOperationToken = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) { __strong __typeof(weakOperation) strongOperation = weakOperation; ... else { // 设置了options为失败了重试,则会把失败的url加入failedURLs数组 if ((options & SDWebImageRetryFailed)) { @synchronized (self.failedURLs) { [self.failedURLs removeObject:url]; } } ... } else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)]) { // 对图片进行transform操作 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]; // pass nil if the image was transformed, so we can recalculate the data from the image [self.imageCache storeImage:transformedImage imageData:(imageWasTransformed ? nil : downloadedData) forKey:key toDisk:cacheOnDisk completion:nil]; } [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:transformedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url]; }); } else { // 缓存图片,有缓存到内存和磁盘两种方式 if (downloadedImage && finished) { [self.imageCache storeImage:downloadedImage imageData:downloadedData forKey:key toDisk:cacheOnDisk completion:nil]; } // 回调完成的block [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:downloadedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url]; } } if (finished) { // 下载完成,就从runningOperations数组中删除操作 [self safelyRemoveOperationFromRunning:strongOperation]; } }]; // 设置取消下载的回调 operation.cancelBlock = ^{ [self.imageDownloader cancel:subOperationToken]; __strong __typeof(weakOperation) strongOperation = weakOperation; [self safelyRemoveOperationFromRunning:strongOperation]; }; } else if (cachedImage) { // 从缓存在取到图片,回调完成block __strong __typeof(weakOperation) strongOperation = weakOperation; [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url]; [self safelyRemoveOperationFromRunning:operation]; } ... }]; return operation; }
从缓存中取图片,是先从内存中取,如果在内存中取到,就在当前线程中直接回调doneBlock;如果内存中没有,就开子线程从磁盘中取,如果取到图片,就回调doneBlock
// SDImageCache.m - (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key done:(nullable SDCacheQueryCompletedBlock)doneBlock { ... // First check the in-memory cache... UIImage *image = [self imageFromMemoryCacheForKey:key]; if (image) { NSData *diskData = nil; if ([image isGIF]) { diskData = [self diskImageDataBySearchingAllPathsForKey:key]; } if (doneBlock) { doneBlock(image, diskData, SDImageCacheTypeMemory); } return nil; } NSOperation *operation = [NSOperation new]; dispatch_async(self.ioQueue, ^{ if (operation.isCancelled) { // do not call the completion if cancelled return; } @autoreleasepool { // 从磁盘中取图片的data NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key]; // 从磁盘中直接取图片 UIImage *diskImage = [self diskImageForKey:key]; if (diskImage && self.config.shouldCacheImagesInMemory) { NSUInteger cost = SDCacheCostForImage(diskImage); // 缓存到内存中 [self.memCache setObject:diskImage forKey:key cost:cost]; } if (doneBlock) { dispatch_async(dispatch_get_main_queue(), ^{ doneBlock(diskImage, diskData, SDImageCacheTypeDisk); }); } } }); return operation; }
图片的下载过程是在SDWebImageDownloader.m
中进行的,实质是通过SDWebImageDownloaderOperation
(继承自NSOperation
)对象,把该对象加入到downloadQueue
里,然后在start
方法里通过NSURLSession
来下载图片。(其中,NSOperation
有两个方法:main
和start
,如果想使用同步,那么最简单方法的就是把逻辑写在main()
中,使用异步,需要把逻辑写到start()
中,然后加入到队列之中)
// SDWebImageDownloader.m - (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url options:(SDWebImageDownloaderOptions)options progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock { __weak SDWebImageDownloader *wself = self; return [self addProgressCallback:progressBlock completedBlock:completedBlock forURL:url createCallback:^SDWebImageDownloaderOperation *{ __strong __typeof (wself) sself = wself; NSTimeInterval timeoutInterval = sself.downloadTimeout; // 设置超时时间为15s if (timeoutInterval == 0.0) { timeoutInterval = 15.0; } // In order to prevent from potential duplicate caching (NSURLCache + SDImageCache) we disable the cache for image requests if told otherwise NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:(options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData) timeoutInterval:timeoutInterval]; request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies); request.HTTPShouldUsePipelining = YES; if (sself.headersFilter) { request.allHTTPHeaderFields = sself.headersFilter(url, [sself.HTTPHeaders copy]); } else { request.allHTTPHeaderFields = sself.HTTPHeaders; } SDWebImageDownloaderOperation *operation = [[sself.operationClass alloc] initWithRequest:request inSession:sself.session options:options]; operation.shouldDecompressImages = sself.shouldDecompressImages; ... // 加入操作队列,开始下载 [sself.downloadQueue addOperation:operation]; ... return operation; }]; }
把SDWebImageDownloaderOperation
对象加入到操作队列,就开始调用该对象的start
方法。
// SDWebImageDownloaderOperation.m - (void)start { // 如果操作被取消,就reset设置 @synchronized (self) { if (self.isCancelled) { self.finished = YES; [self reset]; return; } ... NSURLSession *session = self.unownedSession; if (!self.unownedSession) { // 创建session的配置 NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration]; sessionConfig.timeoutIntervalForRequest = 15; // 创建session对象 self.ownedSession = [NSURLSession sessionWithConfiguration:sessionConfig delegate:self delegateQueue:nil]; session = self.ownedSession; } self.dataTask = [session dataTaskWithRequest:self.request]; self.executing = YES; } // 开始下载任务 [self.dataTask resume]; if (self.dataTask) { for (SDWebImageDownloaderProgressBlock progressBlock in [self callbacksForKey:kProgressCallbackKey]) { progressBlock(0, NSURLResponseUnknownLength, self.request.URL); } dispatch_async(dispatch_get_main_queue(), ^{ [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStartNotification object:self]; }); } else { // 创建任务失败 [self callCompletionBlocksWithError:[NSError errorWithDomain:NSURLErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Connection can't be initialized"}]]; } ... }
在下载过程中,会涉及鉴权、响应的statusCode
判断(404
、304
等等),以及收到数据后的进度回调等等,在最后的didCompleteWithError
里做最后的处理,然后回调完成的block
,下面仅分析一下didCompleteWithError
的方法
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error { ... if (error) { [self callCompletionBlocksWithError:error]; } else { if ([self callbacksForKey:kCompletedCallbackKey].count > 0) { /** * See #1608 and #1623 - apparently, there is a race condition on `NSURLCache` that causes a crash * Limited the calls to `cachedResponseForRequest:` only for cases where we should ignore the cached response * and images for which responseFromCached is YES (only the ones that cannot be cached). * Note: responseFromCached is set to NO inside `willCacheResponse:`. This method doesn't get called for large images or images behind authentication */ if (self.options & SDWebImageDownloaderIgnoreCachedResponse && responseFromCached && [[NSURLCache sharedURLCache] cachedResponseForRequest:self.request]) { // 如果options是忽略缓存,而图片又是从缓存中取的,就给回调传入nil [self callCompletionBlocksWithImage:nil imageData:nil error:nil finished:YES]; } else if (self.imageData) { UIImage *image = [UIImage sd_imageWithData:self.imageData]; // 缓存图片 NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL]; // 跳转图片的大小 image = [self scaledImageForKey:key image:image]; // Do not force decoding animated GIFs if (!image.images) { // 不是Gif图像 if (self.shouldDecompressImages) { if (self.options & SDWebImageDownloaderScaleDownLargeImages) { #if SD_UIKIT || SD_WATCH image = [UIImage decodedAndScaledDownImageWithImage:image]; [self.imageData setData:UIImagePNGRepresentation(image)]; #endif } else { image = [UIImage decodedImageWithImage:image]; } } } if (CGSizeEqualToSize(image.size, CGSizeZero)) { // 下载是图片大小的0 [self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0 pixels"}]]; } else { // 把下载的图片作为参数回调 [self callCompletionBlocksWithImage:image imageData:self.imageData error:nil finished:YES]; } } else { [self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Image data is nil"}]]; } } } ... }
以上就是给UIImageView对象设置图片的过程,可以看出还是比较复杂的,考虑的情况也比较多,不得不佩服作者的编码能力。至于UIButton
的图片设置过程,分析情况类似,在此不做分析。
SDWebImage
的源码中在设置图片的过程中,还应用了多种技术:GCD的线程组、锁机制、并发控制、队列、图像解码、缓存控制等等,是一个综合性十分强的项目了,通过阅读源码,对这些技术的使用也有了进一步的认知,对作者的编程功力的深厚深深折服。
SDWebImage
的解析到此结束,本文只是简单的从源码结构、UIImageView
的使用角度进行了简单的分析,希望对阅读源码的朋友有一些帮助,如果文中有不足之处,还望不吝指出,互相学习。
参考资料
来源:https://www.cnblogs.com/fishbay/p/7216164.html