我们开发中经常会用到NSURL,有时候我们需要对一个url进行分析整理，当然是可以按照RFC 1808的定义去自己分析，但是苹果已经给我们提供了解析url的方法，就是iOS 7时添加的NSURLComponents。
初始化方法如下：
// - (instancetype)init; // - (nullable instancetype)initWithURL:(NSURL *)url resolvingAgainstBaseURL:(BOOL)resolve; // + (nullable instancetype)componentsWithURL:(NSURL *)url resolvingAgainstBaseURL:(BOOL)resolve; // - (nullable instancetype)initWithString:(NSString *)URLString; // + (nullable instancetype)componentsWithString:(NSString *)URLString;常用的属性如下：
@property (nullable, readonly, copy) NSURL *URL; @property (nullable, readonly, copy) NSString *string NS_AVAILABLE(10_10, 8_0); @property (nullable, copy) NSString *scheme; // Attempting to set the scheme with an invalid scheme string will cause an exception. @property (nullable, copy) NSString *user; @property (nullable, copy) NSString *password; @property (nullable, copy) NSString *host; @property (nullable, copy) NSNumber *port; // Attempting to set a negative port number will cause an exception. @property (nullable, copy) NSString *path; @property (nullable, copy) NSString *query; @property (nullable, copy) NSString *fragment; @property (nullable, copy) NSString *percentEncodedUser; @property (nullable, copy) NSString *percentEncodedPassword; @property (nullable, copy) NSString *percentEncodedHost; @property (nullable, copy) NSString *percentEncodedPath; @property (nullable, copy) NSString *percentEncodedQuery; @property (nullable, copy) NSString *percentEncodedFragment; @property (nullable, copy) NSArray<NSURLQueryItem *> *queryItems NS_AVAILABLE(10_10, 8_0);比如http://help.baidu.com:80/question?prod_id=1#!/feedback，scheme就是http, host是help.baidu.com, port就是80, path就是/question, query就是prod_id=1, fragment = !/feedback。
