SDWebImage 第三方库（web 图像异步下载）

1、面试题 —— 如何防止一个URL对应的图片重复下载？
答：使用沙盒缓存。

2、面试题 —— SDWebImage 底层是怎么实现？
答：使用沙盒缓存。同上。

3、SDWebImage 默认缓存的时长是多少？
答：1周。

4、当app接受到内存警告时，SDWebImage 会做什么？
答：
1、SDWebImage会监听系统的 UIApplicationDidReceiveMemoryWarningNotification
通知，一旦收到通知，就会清理内存；
2、 应用程序将要终止的通知,UIApplicationWillTerminateNotification, 清理磁盘；
3、 应用程序进入后台的通知，UIApplicationDidEnterBackgroundNotification，也会清理磁盘；

（附，SDWebImage 的通知）

5、常用方法（4个）

- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder;- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options;- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder completed:(SDWebImageCompletionBlock)completedBlock;- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock;

其中，SDWebImageOptions 枚举（常用两个）
* SDWebImageRetryFailed : 下载失败后，会自动重新下载
* SDWebImageLowPriority : 当正在进行UI交互时，自动暂停内部的一些下载操作
* SDWebImageRetryFailed | SDWebImageLowPriority : 拥有上面2个功能

6、关于 SDWebImageOptions 枚举的类型

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.     * 默认情况下，当一个 URL 下载失败，会将该 URL 放在黑名单中，不再尝试下载     *     * This flag disable this blacklisting.     * 此标记取消黑名单     */    SDWebImageRetryFailed = 1 << 0,    /**     * By default, image downloads are started during UI interactions, this flags disable this feature,     * 默认情况下，在 UI 交互时也会启动图像下载，此标记取消这一特性     *     * leading to delayed download on UIScrollView deceleration for instance.     * 会推迟到滚动视图停止滚动之后再继续下载(这个注释是假的！)     */    SDWebImageLowPriority = 1 << 1,    /**     * This flag disables on-disk caching     * 此标记取消磁盘缓存，只有内存缓存     */    SDWebImageCacheMemoryOnly = 1 << 2,    /**     * This flag enables progressive download, the image is displayed progressively during download as a browser would do.     * 此标记允许渐进式下载，就像浏览器中那样，下载过程中，图像会逐步显示出来     *     * By default, the image is only displayed once completely downloaded.     * 默认情况下，图像会在下载完成后一次性显示     */    SDWebImageProgressiveDownload = 1 << 3,    /**     * Even if the image is cached, respect the HTTP response cache control, and refresh the image from remote location if needed.     * 即使图像被缓存，遵守 HTPP 响应的缓存控制，如果需要，从远程刷新图像     *     * The disk caching will be handled by NSURLCache instead of SDWebImage leading to slight performance degradation.     * 磁盘缓存将由 NSURLCache 处理，而不是 SDWebImage，这会对性能有轻微的影响     *     * This option helps deal with images changing behind the same request URL, e.g. Facebook graph api profile pics.     * 此选项有助于处理同一个请求 URL 的图像发生变化     *     * If a cached image is refreshed, the completion block is called once with the cached image and again with the final image.     * 如果缓存的图像被刷新，会调用一次 completion block，并传递最终的图像     *     * Use this flag only if you can't make your URLs static with embeded cache busting parameter.     * 仅在无法使用嵌入式缓存清理参数确定图像 URL 时，使用此标记     */    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     * 在 iOS 4+，当 App 进入后台后仍然会继续下载图像。这是向系统请求额外的后台时间以保证下载请求完成的     *     * extra time in background to let the request finish. If the background task expires the operation will be cancelled.     * 如果后台任务过期，请求将会被取消     */    SDWebImageContinueInBackground = 1 << 5,    /**     * Handles cookies stored in NSHTTPCookieStore by setting     * 通过设置     * NSMutableURLRequest.HTTPShouldHandleCookies = YES;     * 处理保存在 NSHTTPCookieStore 中的 cookies     */    SDWebImageHandleCookies = 1 << 6,    /**     * Enable to allow untrusted SSL ceriticates.     * 允许不信任的 SSL 证书     *     * Useful for testing purposes. Use with caution in production.     * 可以出于测试目的使用，在正式产品中慎用     */    SDWebImageAllowInvalidSSLCertificates = 1 << 7,    /**     * By default, image are loaded in the order they were queued. This flag move them to     * 默认情况下，图像会按照在队列中的顺序被加载，此标记会将它们移动到队列前部立即被加载     *     * the front of the queue and is loaded immediately instead of waiting for the current queue to be loaded (which     * 而不是等待当前队列被加载，等待队列加载会需要一段时间     * could take a while).     */    SDWebImageHighPriority = 1 << 8,    /**     * By default, placeholder images are loaded while the image is loading. This flag will delay the loading     * 默认情况下，在加载图像时，占位图像已经会被加载。而此标记会延迟加载占位图像，直到图像已经完成加载     *     * 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,     * 通常不会在可动画的图像上调用 transformDownloadedImage 代理方法，因为大多数转换代码会破坏动画文件     *     * as most transformation code would mangle it.     * Use this flag to transform them anyway.     * 使用此标记尝试转换     */    SDWebImageTransformAnimatedImage = 1 << 10,};