日常应用中经常使用到 NSURL，但对 HTTP URL 构成和 API 中接口返回值并不很了解，今天记录下以备后查。
HTTP URL 可以分成如下几部分：
- the protocol or scheme (here,
http) - the
://delimiter - the username and the password (here there isn't any, but it could be
username:password@hostname) - the host name (here,
digg.com) - the port (that would be :80 after the domain name for instance)
- the path (here,
/news/business/24hr) - the parameter string (anything that follows a semicolon)
- the query string (that would be if you had GET parameters like
?foo=bar&baz=frob) - the fragment (that would be if you had an anchor in the link, like
#foobar).
下例的URL拥有上述所有特征：
http://foobar:nicate@example.com:8080/some/path/file.html;params-here?foo=bar#baz
NSURL API 提供读取这些特征的方法，以上面的 URL 为例，各方法调用返回值如下：
-
-[NSURL scheme]= http -
-[NSURL resourceSpecifier]= (everything from // to the end of the URL) -
-[NSURL user]= foobar -
-[NSURL password]= nicate -
-[NSURL host]= example.com -
-[NSURL port]= 8080 -
-[NSURL path]= /some/path/file.html -
-[NSURL pathComponents]= @["/", "some", "path", "file.html"] (note that the initial / is part of it) -
-[NSURL lastPathComponent]= file.html -
-[NSURL pathExtension]= html -
-[NSURL parameterString]= params-here -
-[NSURL query]= foo=bar -
-[NSURL fragment]= baz
需要注意的是 - pathComponents 返回的数组中，第一个部分是 /。
日常开发中，我们会从已有的 URL 中取需要的部分构造新 URL：
NSURL* url = [NSURL URLWithString:@"http://digg.com/news/business/24hr"];
NSString* reducedUrl = [NSString stringWithFormat:
@"%@://%@/%@",
url.scheme,
url.host,
url.pathComponents[1]];
参考文章：
