新知篇之一--枚举

目前开发中常用的枚举有两种：NS_ENUM 和NS_OPTIONS 。
不做过多铺垫，我们单刀直入，直接讲二者区别和用法。
1、定义：
NS_ENUM:普通枚举 、NS_OPTIONS:位移枚举
2、语法形式：
NS_ENUM ：

typedef NS_ENUM(NSInteger, NSWritingDirection) {
    NSWritingDirectionNatural = 0,     
    NSWritingDirectionLeftToRight,  
    NSWritingDirectionRightToLeft       
};

NS_OPTIONS :

typedef NS_OPTIONS(NSUInteger, UISwipeGestureRecognizerDirection) {
    UISwipeGestureRecognizerDirectionNone = 0,  //值为0
    UISwipeGestureRecognizerDirectionRight = 1 << 0,  //值为2的0次方
    UISwipeGestureRecognizerDirectionLeft = 1 << 1,  //值为2的1次方
    UISwipeGestureRecognizerDirectionUp = 1 << 2,  //值为2的2次方
    UISwipeGestureRecognizerDirectionDown = 1 << 3  //值为2的3次方
};

**注意区别：通常来讲普通枚举使用的是NSIterger,而NS_OPTIONS使用NSUInterger。原因，感兴趣同学可自行查阅。

3、如何使用：
通常来讲当有多个枚举值同时存在时，可以使用NS_OPTIONS。
例如：

UISwipeGestureRecognizer *swipeGR = [[UISwipeGestureRecognizer alloc] init];
swipeGR.direction = UISwipeGestureRecognizerDirectionDown | UISwipeGestureRecognizerDirectionLeft | UISwipeGestureRecognizerDirectionRight;

而NS_ENUM定义的枚举不能几个枚举项同时存在，只能选择其中一项。

4、位移枚举的运算：

//定义一个位移枚举
typedef NS_OPTIONS(NSUInteger, MyOption)  {
MyOptionNone = 0, //二进制0000,十进制0
MyOption1 = 1 << 0, //0001,1
MyOption2 = 1 << 1, //0010,2
MyOption3 = 1 << 2, //0100,4
MyOption4 = 1 << 3, //1000,8
};

//使用：判断、总结、移除枚举值
//声明定义枚举变量
MyOption option = MyOption1 | MyOption2; //0001 | 0010 = 0011,3

//检查是否包含某选型
if ( option & MyOption3 ) { //0011 & 0100 = 0000  通过位或运算来判断是否包含
     //包含MyOption3
}
else {
     //不包含MyOption3
}

//增加选项:
option = option | MyOption4;  //0011 | 1000 = 1011, 11
//减少选项
option = option & (~MyOption4); //1011 & (~1000) = 1011 & 0111 = 0011, 3

位移枚举是通过位或运算来实现：判断、增加、减少对应枚举值的

5、实际使用：SDWebImageDownloader中通过使用位移枚举提供多项可选操作。

if ((!cachedImage || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
            if (cachedImage && options & SDWebImageRefreshCached) {
                // If image was found in the cache but SDWebImageRefreshCached is provided, notify about the cached image
                // AND try to re-download it in order to let a chance to NSURLCache to refresh it from server.
                [self callCompletionBlockForOperation:weakOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
            }

            // download if no image or requested to refresh anyway, and download allowed by delegate
            SDWebImageDownloaderOptions downloaderOptions = 0;
//NOTE:Haoyu - 判断options中是否包含SDWebImageLowPriority枚举项，如包含，则将SDWebImageDownloaderLowPriority枚举值添加到downloaderOptions中。
            if (options & SDWebImageLowPriority) downloaderOptions |= SDWebImageDownloaderLowPriority;
            if (options & SDWebImageProgressiveDownload) downloaderOptions |= SDWebImageDownloaderProgressiveDownload;
            if (options & SDWebImageRefreshCached) downloaderOptions |= SDWebImageDownloaderUseNSURLCache;
            if (options & SDWebImageContinueInBackground) downloaderOptions |= SDWebImageDownloaderContinueInBackground;
            if (options & SDWebImageHandleCookies) downloaderOptions |= SDWebImageDownloaderHandleCookies;
            if (options & SDWebImageAllowInvalidSSLCertificates) downloaderOptions |= SDWebImageDownloaderAllowInvalidSSLCertificates;
            if (options & SDWebImageHighPriority) downloaderOptions |= SDWebImageDownloaderHighPriority;
            if (options & SDWebImageScaleDownLargeImages) downloaderOptions |= SDWebImageDownloaderScaleDownLargeImages;
            
            if (cachedImage && options & SDWebImageRefreshCached) {
                // force progressive off if image already cached but forced refreshing  移除某项枚举值
                downloaderOptions &= ~SDWebImageDownloaderProgressiveDownload;
                // ignore image read from NSURLCache if image if cached but force refreshing
                downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
            }

注：上述代码中用到了：|= 和 &= ~ ，二者分别等价于：options = options | SDWebImageDownloader和 options = options & （ ~SDWebImageDownloader）

6、总结：
两种方式同为枚举，根据不同需求，我么可以分别采用不同的枚举。二者都较为常见，但通常来讲大家只会用NS_ENUM方式，对NS_OPTIONS会陌生一些，在阅读源码时，我们经常会遇到NS_OPTIONS这种枚举方式。而且很有用，所以需要掌握该部分知识点。