编码和解码URL数据 <- URL会话编程指南

想要对部分URL字符串进行百分号编码，请使用NSString的stringByAddingPercentEncodingWithAllowedCharacters:方法，给URL组件或子组件传递合适的字符集：

• User: URLUserAllowedCharacterSet
• Password: URLPasswordAllowedCharacterSet
• Host: URLHostAllowedCharacterSet
• Path: URLPathAllowedCharacterSet
• Fragment（片段）: URLFragmentAllowedCharacterSet
• Query（查询）: URLQueryAllowedCharacterSet

重要：不要使用stringByAddingPercentEncodingWithAllowedCharacters:来编码整个URL字符串，因为每个URL组件或子组件对于有效字符都有不同的规则。

例如，想要对包含在URL片段中的UTF-8字符串进行百分号编码，请执行如下操作：

NSString *originalString = @"color-#708090";

NSCharacterSet *allowedCharacters = [NSCharacterSet URLFragmentAllowedCharacterSet];

NSString *percentEncodedString = [originalString stringByAddingPercentEncodingWithAllowedCharacters:allowedCharacters];

NSLog(@"%@", percentEncodedString"); // prints "color-%23708090"

let originalString = "color-#708090"

let allowedCharacters = NSCharacterSet.urlFragmentAllowed

let encodedString = originalString.addingPercentEncoding(withAllowedCharacters: allowedCharacters)

print(encodedString!) // prints "color-%23708090"

如果你想对URL组件进行百分号编码，使用NSURLComponents把URL分拆成它的组成部分，并访问相关的属性。

例如，想要得到URL片段百分号编码的UTF-8字符串值，请执行以下操作：

NSURL *URL = [NSURL URLWithString:@"https://example.com/#color-%23708090"];

NSURLComponents *components = [NSURLComponents componentsWithURL:URL resolvingAgainstBaseURL:NO];

NSString *fragment = components.fragment;

NSLog(@"%@", fragment); // prints "color-#708090"

let url = URL(string: "https://example.com/#color-%23708090")!

let components = URLComponents(url: url, resolvingAgainstBaseURL: true)!

let fragment = components.fragment!

print(fragment) // prints "color-#708090"