使用libxml的sax解析器時未定義的方法錯誤

Question

我試圖使用libxml的SAX解析器（如illustrated here），但我遇到未定義的方法錯誤。

我的代碼是

$domain_topics = Hash.new { |h,d| h[d] = [] } 
parser = LibXML::XML::SaxParser.io(
    File.open("content.rdf.u8", "r:UTF-8") 
) 
class Callbacks 
    include LibXML::XML::SaxParser::Callbacks 
    def initialize 
    @state = :top 
    @topics = nil 
    end 
    def on_start_element(element, attributes) 
    case @state 
    when :top 
     return unless element == 'ExternalPage' 
     @state = :ExternalPage 

     domain = attributes['about'].sub(%r!^\w+://([^"/]*)(?:/[^"]*)?$!, '\1') 
     @topics = $domain_topics[domain] 
    when :ExternalPage 
     return unless element == 'topic' 
     @state = :topic 
    end 
    end 
    def on_characters(characters) 
    if @state == :topic and @topics 
     @topics << characters 
    end 
    end 
    def on_end_element(element) 
    case @state 
    when :ExternalPage 
     @state = :top 
     @topics = nil 
    when :topic 
     @state = :ExternalPage 
    end 
    end 
end 
parser.callbacks = Callbacks 
parser.parse 

當我運行它：

% ./my_awesome_code.rb 
./my_awesome_code.rb:1337:in `parse': undefined method `on_start_document' for Callbacks:Class (NoMethodError) 

我在做什麼錯在這裏？ include LibXML::XML::SaxParser::Callbacks不應該給出默認定義 on_start_document？

IRB似乎也印證了我的直覺：

1.9.3p194 :009 > Callbacks.instance_methods.include? :on_start_document 
=> true 

Answer 1

我啞然，並錯過了#callbacks=需要傳遞一個實例與適當的回調，而不是一類。所以我只需要做

parser.callbacks = Callbacks.new