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 = << ,
/**
* 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 = << ,
/**
* This flag disables on-disk caching
* 此标记取消磁盘缓存，只有内存缓存
*/
SDWebImageCacheMemoryOnly = << ,
/**
* 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 = << ,
/**
* 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 = << ,
/**
* 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 = << ,
/**
* Handles cookies stored in NSHTTPCookieStore by setting
* 通过设置
* NSMutableURLRequest.HTTPShouldHandleCookies = YES;
* 处理保存在 NSHTTPCookieStore 中的 cookies
*/
SDWebImageHandleCookies = << ,
/**
* 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 = << ,
/**
* 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 = << ,
/**
* 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 = << ,
/**
* 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 = << ,
};