SDWebImage内部实现过程
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SDWebImage内部实现过程(新版本在各方法前加上了sd_前缀,以区分UIImageView+AFNetworking中的方法)
1.入口 setImageWithURL:placeholderImage:options:会先把 placeholderImage 显示,然后 SDWebImageManager 根据 URL 开始处理图片。
2.进入 SDWebImageManager-downloadWithURL:delegate:options:userInfo:,交给 SDImageCache 从缓存查找图片是否已经下载 queryDiskCacheForKey:delegate:userInfo:.
3.先从内存图片缓存查找是否有图片,如果内存中已经有图片缓存,SDImageCacheDelegate 回调 imageCache:didFindImage:forKey:userInfo: 到 SDWebImageManager。
4.SDWebImageManagerDelegate 回调 webImageManager:didFinishWithImage:到 UIImageView+WebCache 等前端展示图片。
5.如果内存缓存中没有,生成 NSInvocationOperation 添加到队列开始从硬盘查找图片是否已经缓存。
6.根据 URLKey 在硬盘缓存目录下尝试读取图片文件。这一步是在 NSOperation 进行的操作,所以回主线程进行结果回调 notifyDelegate:。
7.如果上一操作从硬盘读取到了图片,将图片添加到内存缓存中(如果空闲内存过小,会先清空内存缓存)。SDImageCacheDelegate 回调 imageCache:didFindImage:forKey:userInfo:。进而回调展示图片。
8.如果从硬盘缓存目录读取不到图片,说明所有缓存都不存在该图片,需要下载图片,回调 imageCache:didNotFindImageForKey:userInfo:。
9.共享或重新生成一个下载器 SDWebImageDownloader 开始下载图片。
10.图片下载由 NSURLConnection 来做,实现相关 delegate 来判断图片下载中、下载完成和下载失败。
11.connection:didReceiveData:中利用 ImageIO 做了按图片下载进度加载效果。
12.connectionDidFinishLoading:数据下载完成后交给 SDWebImageDecoder 做图片解码处理。
13.图片解码处理在一个 NSOperationQueue 完成,不会拖慢主线程 UI。如果有需要对下载的图片进行二次处理,最好也在这里完成,效率会好很多。
14.在主线程 notifyDelegateOnMainThreadWithInfo: 宣告解码完成,imageDecoder:didFinishDecodingImage:userInfo: 回调给 SDWebImageDownloader。
15.imageDownloader:didFinishWithImage: 回调给 SDWebImageManager 告知图片下载完成。
16.通知所有的 downloadDelegates 下载完成,回调给需要的地方展示图片。
17.将图片保存到 SDImageCache 中,内存缓存和硬盘缓存同时保存。写文件到硬盘也在以单独 NSInvocationOperation 完成,避免拖慢主线程。
18.SDImageCache 在初始化的时候会注册一些消息通知,在内存警告或退到后台的时候清理内存图片缓存,应用结束的时候清理过期图片。
19.SDWI 也提供了 UIButton+WebCache 和 MKAnnotationView+WebCache,方便使用。
20.SDWebImagePrefetcher 可以预先下载图片,方便后续使用。
从上面流程可以看出,当你调用setImageWithURL:方法的时候,他会自动去给你干这么多事,当你需要在某一具体时刻做事情的时候,你可以覆盖这些方法。比如在下载某个图片的过程中要响应一个事件,就覆盖这个方法:
覆盖方法,指哪打哪,这个方法是下载imagePath2的时候响应
SDWebImageManager *manager = [SDWebImageManager sharedManager]; [manager downloadImageWithURL:imagePath2 options:SDWebImageRetryFailed progress:^(NSInteger receivedSize, NSInteger expectedSize) { NSLog(@"显示当前进度"); } completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) { NSLog(@"下载完成"); }];
当服务器更新了某一张图片资源时,客户端需要重新加载,那么就可以设置SDWebImageOption为SDWebImageRefreshCached;附上全部的SDWebImageOptions:
typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) { /** * By default, when a URL fail to be downloaded, the URL is blacklisted so the library won't keep trying. * This flag disable this blacklisting. */ SDWebImageRetryFailed = 1 << 0, /** * By default, image downloads are started during UI interactions, this flags disable this feature, * leading to delayed download on UIScrollView deceleration for instance. */ SDWebImageLowPriority = 1 << 1, /** * This flag disables on-disk caching */ SDWebImageCacheMemoryOnly = 1 << 2, /** * This flag enables progressive download, the image is displayed progressively during download as a browser would do. * By default, the image is only displayed once completely downloaded. */ SDWebImageProgressiveDownload = 1 << 3, /** * Even if the image is cached, respect the HTTP response cache control, and refresh the image from remote location if needed. * The disk caching will be handled by NSURLCache instead of SDWebImage leading to slight performance degradation. * This option helps deal with images changing behind the same request URL, e.g. Facebook graph api profile pics. * If a cached image is refreshed, the completion block is called once with the cached image and again with the final image. * * Use this flag only if you can't make your URLs static with embeded cache busting parameter. */ SDWebImageRefreshCached = 1 << 4, /** * In iOS 4+, continue the download of the image if the app goes to background. This is achieved by asking the system for * extra time in background to let the request finish. If the background task expires the operation will be cancelled. */ SDWebImageContinueInBackground = 1 << 5, /** * Handles cookies stored in NSHTTPCookieStore by setting * NSMutableURLRequest.HTTPShouldHandleCookies = YES; */ SDWebImageHandleCookies = 1 << 6, /** * Enable to allow untrusted SSL ceriticates. * Useful for testing purposes. Use with caution in production. */ SDWebImageAllowInvalidSSLCertificates = 1 << 7, /** * By default, image are loaded in the order they were queued. This flag move them to * the front of the queue and is loaded immediately instead of waiting for the current queue to be loaded (which * could take a while). */ SDWebImageHighPriority = 1 << 8, /** * By default, placeholder images are loaded while the image is loading. This flag will delay the loading * of the placeholder image until after the image has finished loading. */ SDWebImageDelayPlaceholder = 1 << 9, /** * We usually don't call transformDownloadedImage delegate method on animated images, * as most transformation code would mangle it. * Use this flag to transform them anyway. */ SDWebImageTransformAnimatedImage = 1 << 10,};
- SDWebImage内部实现过程
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