NSString进行比较的方法和属性

#pragma mark *** String comparison and equality ***

//这是字符串用来比较的方法

/* In the compare: methods, the range argument specifies the subrange, rather than the whole, of the receiver to use in the comparison. The range is not applied to the search string.  For example, [@"AB" compare:@"ABC" options:0 range:NSMakeRange(0,1)]
compares "A" to "ABC", not "A" to "A", and will return NSOrderedAscending.

*/

- (NSComparisonResult)compare:(NSString *)string;

- (NSComparisonResult)compare:(NSString *)string options:(NSStringCompareOptions)mask;

- (NSComparisonResult)compare:(NSString *)string options:(NSStringCompareOptions)mask range:(NSRange)compareRange;

- (NSComparisonResult)compare:(NSString *)string options:(NSStringCompareOptions)mask range:(NSRange)compareRange
locale:(nullable id)locale; // locale arg used to be a dictionary pre-Leopard. We now accept NSLocale. Assumes the current locale if non-nil and non-NSLocale.
nil continues to mean canonical compare, which doesn't depend on user's locale choice.

- (NSComparisonResult)caseInsensitiveCompare:(NSString *)string;//忽略大小写进行字符串比较

- (NSComparisonResult)localizedCompare:(NSString *)string;//本地化比较

- (NSComparisonResult)localizedCaseInsensitiveCompare:(NSString *)string;//本地化比较并且不区分大小写

- (NSComparisonResult)localizedStandardCompare:(NSString *)string;//它对应的选项是 NSCaseInsensitiveSearch 、 NSNumericSearch 、NSWidthInsensitiveSearch 以及 NSForcedOrderingSearch 。如果我们要在UI上显示一个文件列表，用它就最合适不过了。

返回值NSComparisonResult是一个结构体类型

typedef NS_ENUM(NSInteger, NSComparisonResult) {NSOrderedAscending = -1L, NSOrderedSame, NSOrderedDescending};

NSOrderedAscending升序，代表左边的值比右边的小；

NSOrderedSame左右两个值大小相同；

NSOrderedDescending降序，代表左边的值比右边的大。

比较方法：根据输入的options值决定比较顺序，依次比较各个字符的值的大小（a-z依次递增），第一个不一样的值决定比较结果大小

参数类型NSStringCompareOptions是一个结构体类型

typedef NS_OPTIONS(NSUInteger, NSStringCompareOptions) {

    NSCaseInsensitiveSearch = 1,        /* 不区分大小写比较 */

    NSLiteralSearch = 2,
/* 区分大小写*/

    NSBackwardsSearch = 4,
/* 从字符串尾端搜索*/

    NSAnchoredSearch = 8,
/* 搜索限制范围的字符串*/

    NSNumericSearch = 64,
/* 以字符串中的数字为标准比较*/

    NSDiacriticInsensitiveSearch ,      /* 忽略“-”号*/

    NSWidthInsensitiveSearch ,
/* 忽略字符串长度 */

    NSForcedOrderingSearch , 
/* 忽略不区分大小写比较的选项，并强制返回 NSOrderedAscending 或者 NSOrderedDescending*/

    NSRegularExpressionSearch NS_ENUM_AVAILABLE(10_7, 3_2) = 1024 
  /* Applies to rangeOfString:..., stringByReplacingOccurrencesOfString:..., and replaceOccurrencesOfString:... methods only; the search string is treated as an ICU-compatible regular expression; if set, no other options can apply except
NSCaseInsensitiveSearch and NSAnchoredSearch */

};

location: 需要比较的字符串起始位置（以0为起始）