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会自动进行内存缓存和磁盘缓存
SDWebImage是如何清空缓存 ?
SDWebImage是如何清除磁盘
SDWebImage可以直接播放GIF图片
SDWebImage它可以判断图片的类型
[cell.imageView sd_setImageWithURL:[NSURL URLWithString:app.icon] placeholderImage:[UIImage imageNamed:@"placehoder"]];
//下载并设置图片
/* 第一个参数:要下载图片的url地址 第二个参数:设置该imageView的占位图片 第三个参数:传一个枚举值,告诉程序你下载图片的策略是什么 第一个block块:获取当前图片数据的下载进度 receivedSize:已经下载完成的数据大小 expectedSize:该文件的数据总大小 第二个block块:当图片下载完成之后执行该block中的代码 image:下载得到的图片数据 error:下载出现的错误信息 SDImageCacheType:图片的缓存策略(不缓存,内存缓存,沙盒缓存) imageURL:下载的图片的url地址 */
[cell.imageView sd_setImageWithURL:[NSURL URLWithString:app.icon] placeholderImage:[UIImage imageNamed:@"placehoder"] options:SDWebImageRetryFailed progress:^(NSInteger receivedSize, NSInteger expectedSize) {
//计算当前图片的下载进度
NSLog(@"%.2f",1.0 *receivedSize / expectedSize);
} completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {
}];
//取消当前正在进行的所有下载操作
[[SDWebImageManager sharedManager] cancelAll];
//清除缓存数据(面试)
//cleanDisk:删除过期的文件数据,计算当前未过期的已经下载的文件数据的大小,如果发现该数据大小大于我们设置的最大缓存数据大小,那么程序内部会按照按文件数据缓存的时间从远到近删除,知道小于最大缓存数据为止。
//clearMemory:直接删除文件,重新创建新的文件夹
//[[SDWebImageManager sharedManager].imageCache cleanDisk];
[[SDWebImageManager sharedManager].imageCache clearMemory];
5-1 把用户传入的gif图片->NSData
5-2 根据该Data创建一个图片数据源(NSData->CFImageSourceRef)
5-3 计算该数据源中一共有多少帧,把每一帧数据取出来放到图片数组中
5-4 根据得到的数组+计算的动画时间-》可动画的image
[UIImage animatedImageWithImages:images duration:duration];
+ (NSString *)sd_contentTypeForImageData:(NSData *)data;
07 内部如何进行缓存处理?使用了NSCache类,使用和NSDictionary类似
08 沙盒缓存图片的命名方式为对该图片的URL进行MD5加密 echo -n "url" |MD5
09 当接收到内存警告之后,内部会自动清理内存缓存
10 图片的下载顺序,默认是先进先出的