require 'builder'  
 
x = Builder::XmlMarkup.new(:target =>
 $stdout, :indent => 1) 
#":target =＞$stdout"參數(shù)：指示輸出內(nèi)
容將被寫向標(biāo)準(zhǔn)輸出控制臺 
#":indent =＞1"參數(shù)：XML輸出形式將被縮
進一個空格字符x.instruct! :xml,
:version =>'1.1',:encoding => 'gb2312' 
x.comment! "書本信息" 
 
x.library("shelf" => "Recent Acquisitions") { 
x.section("name" => "ruby"){ 
x.book("isbn" => "0672310001"){ 
x.title "Programming Ruby"  
x.author "Yukihiro " 
x.description "Programming Ruby - 
The Pragmatic Programmer's Guide" 
} 
} 
}

p x #打印XML

Ruby創(chuàng)建XML輸出結(jié)果：

< ?xml version="1.1" encoding="gb2312"?> 
< !-- 書本信息 --> 
< library shelf="Recent Acquisitions"> 
< section name="ruby"> 
< book isbn="0672310001"> 
< title>Programming Ruby< /title> 
< author>Yukihiro < /author> 
< description>Programming Ruby - The 
Pragmatic Programmer's Guide
< /description> 
< /book> 
< /section> 
< /library> 
< inspect/> 
#< IO:0x2a06ae8>

使用ReXML解析XML

REXML 是一個完全用ruby寫的processor ,他有多種api，其中兩個經(jīng)典的api是通過DOM-like 和SAX-like 來進行區(qū)分的。第一種是將整個文件讀進內(nèi)存，然后存儲為一個分層的形式(也就是一棵樹了).而第二種是"parse as you go",當(dāng)你的文件很大，并且內(nèi)存受到限制的時候，比較適合用這種。

看下面的book.xml:

引用

<library shelf="Recent Acquisitions"> 
  <section name="Ruby"> 
    <book isbn="0672328844"> 
    <title>The Ruby Way</title> 
    <author>Hal Fulton</author> 
    <description> 
      Second edition. The book you are now reading. 
      Ain't recursion grand? 
    </description> 
    </book> 
  </section> 
  <section name="Space"> 
    <book isbn="0684835509"> 
      <title>The Case for Mars</title> 
      <author>Robert Zubrin</author> 
      <description>Pushing toward a second home for the human 
        race. 
      </description> 
    </book> 
    <book isbn="074325631X"> 
      <title>First Man: The Life of Neil A. Armstrong</title> 
      <author>James R. Hansen</author> 
      <description>Definitive biography of the first man on 
        the moon. 
      </description> 
    </book> 
  </section> 
</library>

1 Tree Parsing(也就是DOM-like)

我們需要require rexml/document 庫，并且include REXML :

require 'rexml/document' 
include REXML 
 
input = File.new("books.xml") 
doc = Document.new(input) 
 
root = doc.root 
puts root.attributes["shelf"]   # Recent Acquisitions 
 
doc.elements.each("library/section") { |e| puts e.attributes["name"] } 
# Output: 
#  Ruby 
#  Space 
 
doc.elements.each("*/section/book") { |e| puts e.attributes["isbn"] } 
# Output: 
#  0672328844 
#  0321445619 
#  0684835509 
#  074325631X 
 
sec2 = root.elements[2] 
author = sec2.elements[1].elements["author"].text    # Robert Zubrin

這里要注意的是xml中的屬性和值被表示為一個hash，因此我們能夠通過attributes[]來提取我們需要的值，元素的值還能通過類似于path的字符串或者整數(shù)來取得.其中用整數(shù)取的話，是1-based而不是0-based.

2 Stream Parsing（也就是SAX-like Parsing)

這邊使用了一個小技巧，那就是定義了一個listener 類，它將會在parse的時候被回調(diào):

require 'rexml/document' 
require 'rexml/streamlistener' 
include REXML 
 
class MyListener 
 include REXML::StreamListener 
 def tag_start(*args) 
  puts "tag_start: #{args.map {|x| x.inspect}.join(', ')}" 
 end 
 
 def text(data) 
  return if data =~ /^\w*$/   # whitespace only 
  abbrev = data[0..40] + (data.length > 40 ? "..." : "") 
  puts " text  :  #{abbrev.inspect}" 
 end 
end 
 
list = MyListener.new 
source = File.new "books.xml" 
Document.parse_stream(source, list)

這里介紹一下StreamListener 模塊，這個模塊它提供了幾個空的回調(diào)方法，因此你可以為了實現(xiàn)你自己的功能而覆蓋它.當(dāng)parser 進入一個tag時，就會調(diào)用tag_start方法.而text方法也是類似的，他只不過是當(dāng)讀取到數(shù)據(jù)時會被回調(diào),它的輸出是這樣的：

tag_start: "library", {"shelf"=>"Recent Acquisitions"} 
tag_start: "section", {"name"=>"Ruby"} 
tag_start: "book", {"isbn"=>"0672328844"} 
tag_start: "title", {} 
 text  :  "The Ruby Way" 
.........................................

3 XPath

REXML通過XPath 類來提供Xpath的支持. 它也同時支持DOM-like和SAX-like .還是前面的那個xml文件，我們使用Xpath可以這樣做：

book1 = XPath.first(doc, "http://book")  # Info for first book found 
p book1 
 
# Print out all titles 
XPath.each(doc, "http://title") { |e| puts e.text } 
 
# Get an array of all of the "author" elements in the document. 
names = XPath.match(doc, "http://author").map {|x| x.text } 
p names

輸出是類似于下面的：

<book isbn='0672328844'> ... </> 
The Ruby Way 
The Case for Mars 
First Man: The Life of Neil A. Armstrong 
["Hal Fulton", "Robert Zubrin", "James R. Hansen"]

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