Tips：NS_OPTIONS

前言

在阅读sdwebimageView的时候,有一个NS_OPTIONS类的枚举类型

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.
     * 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,
    
    /**
     * By default, images are decoded respecting their original size. On iOS, this flag will scale down the
     * images to a size compatible with the constrained memory of devices.
     * If `SDWebImageProgressiveDownload` flag is set the scale down is deactivated.
     */
    SDWebImageScaleDownLargeImages = 1 << 12
};

这里的枚举值的可以多种状态同时存在，即可以只内存缓存，同时关闭失败重试，打开后台下载等各种功能。
不同于NS_ENUM类型的枚举变量，只能保存一个状态
NS_OPTIONS类型实现来一个枚举变量，保存多种状态的目的。

基本格式

typedef NS_OPTIONS(NSUInteger, MyEnum) {
    MyEnumValueA = 1 << 0, // 00000001
    MyEnumValueB = 1 << 1, // 00000010
    MyEnumValueC = 1 << 2,//  00000100
};

同时包含可以用 |

MyEnumValueA | MyEnumValueB 同时存在A和B两种状态,而且任意几个的状态的|不会等于其它的枚举值

添加一个状态用|=

MyEnum  value= MyEnumValueA |  MyEnumValueB；
value |= MyEnumValueC; // 添加MyEnumValueC状态

判断是否包含&

if(value & MyEnumValueC){
      NSLog(@"包含MyEnumValueC")；
}
// 位与如果value和MyEnumValueC在同一个位为1，那么整个结果肯定不为0，为True

DEMO

一个人可以有多种国籍，用Country枚举变量保存

- (void)viewDidLoad {
    [super viewDidLoad];
    Country country = China | Japan; // 位运算，表示同时存在，这个是有双重国籍的人
    country |= Korea; // 添加一个Korea, country = country | Korea;
    NSLog(@"%ld", country);
    if(country & Japan){  // country是否包含Japan
        NSLog(@"%ld", country);
    }

}