SDWebImage-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 embedded 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 certificates.
     * Useful for testing purposes. Use with caution in production.
     */
    SDWebImageAllowInvalidSSLCertificates = 1 << 7,

    /**
     * By default, images are loaded in the order in which they were queued. This flag moves them to
     * the front of the queue.
     */
    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,
    
    /**
     * By default, image is added to the imageView after download. But in some cases, we want to
     * have the hand before setting the image (apply a filter or add it with cross-fade animation for instance)
     * Use this flag if you want to manually set the image in the completion when success
     */
    SDWebImageAvoidAutoSetImage = 1 << 11
};

SDWebImageRetryFailed = 1 << 0
默认情况下,如果一个url在下载的时候失败了,那么这个url会被加入黑名单并且library不会尝试再次下载,这个flag会阻止library把失败的url加入黑名单(简单来说如果选择了这个flag,那么即使某个url下载失败了,sdwebimage还是会尝试再次下载他

SDWebImageLowPriority = 1 << 1
默认情况下,图片会在交互发生的时候下载(例如你滑动tableview的时候),这个flag会禁止这个特性,导致的结果就是在scrollview减速的时候,才会开始下载(也就是你滑动的时候scrollview不下载,你手从屏幕上移走,scrollview开始减速的时候才会开始下载图片

SDWebImageCacheMemoryOnly = 1 << 2
这个flag禁止磁盘缓存,只有内存缓存

SDWebImageProgressiveDownload = 1 << 3
这个flag会在图片下载的时候就显示(就像你用浏览器浏览网页的时候那种图片下载,一截一截的显示(待确认))

SDWebImageRefreshCached = 1 << 4
一个图片缓存了,还是会重新请求.并且缓存侧略依据NSURLCache而不是SDWebImage.URL不变,图片会更新时使用

SDWebImageContinueInBackground = 1 << 5
启动后台下载,加入你进入一个页面,有一张图片正在下载这时候你让app进入后台,图片还是会继续下载(这个估计要开backgroundfetch才有用)

SDWebImageHandleCookies = 1 << 6
可以控制存在NSHTTPCookieStore的cookies.

SDWebImageAllowInvalidSSLCertificates = 1 << 7
允许不安全的SSL证书,在正式环境中慎用

SDWebImageHighPriority = 1 << 8
默认情况下,image在装载的时候是按照他们在队列中的顺序装载的(就是先进先出).这个flag会把他们移动到队列的前端,并且立刻装载,而不是等到当前队列装载的时候再装载.

SDWebImageDelayPlaceholder = 1 << 9
默认情况下,占位图会在图片下载的时候显示.这个flag开启会延迟占位图显示的时间,等到图片下载完成之后才会显示占位图.

SDWebImageTransformAnimatedImage = 1 << 10
是否transform图片