1: 父类
继承于: NSObject
2: 属性
//所有空格字符的字符集
@property (readonly, class, copy) NSCharacterSet *whitespaceCharacterSet;
//所有空格和换行符的字符集
@property (readonly, class, copy) NSCharacterSet *whitespaceAndNewlineCharacterSet;
3: 方法
4:NSCharacterSet (NSURLUtilities)
//六个允许百分比编码的URL组件和子组件的预定义字符集, 这些字符集被传递给-stringByAddingPercentEncodingWithAllowedCharacters:
//返回url查询组件中允许的字符的字符集
@property (class, readonly, copy) NSCharacterSet *URLQueryAllowedCharacterSet;