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copied!<p>If you want it another way, take a look at "strategy" design patten: </p> <pre><code># test_mach.rb require 'rubygems' require 'mechanize' # this is the context class,which calls the different strategy implementation class Crawler def initialize(some_website_strategy) @strategy = some_website_strategy end def crawl begin @strategy.crawl #execute code in Amazon class here? rescue Exception => e puts "==== starts this exception comes from Parent Class" puts e.backtrace puts "==== ends this exception comes from Parent Class" end end end # strategy class for Amazon class Amazon def crawl puts "now crawling amazon" raise "oh ... some errors when crawling amazon" end end # strategy class for taobao.com class Taobao def crawl puts "now crawling taobao" raise "oh ... some errors when crawling taobao" end end </code></pre> <p>then run this code:</p> <pre><code>amazon = Crawler.new(Amazon.new) amazon.crawl taobao = Crawler.new(Taobao.new) taobao.crawl </code></pre> <p>result: </p> <pre><code>now crawling amazon ==== starts this exception comes from Parent Class test_mach.rb:27:in `crawl' test_mach.rb:13:in `crawl' test_mach.rb:38 ==== ends this exception comes from Parent Class now crawling taobao ==== starts this exception comes from Parent Class test_mach.rb:34:in `crawl' test_mach.rb:13:in `crawl' test_mach.rb:40 ==== ends this exception comes from Parent Class </code></pre> <p>BTW. for your implementation, basically I did the same way with you. except </p> <pre><code># my implementation class Crawler def stuff raise "abstract method called" end end </code></pre> <p>If you want it another way, take a look at "around alias" (<< metaprogramming ruby>>, page 155 ) . However I think "around alias" is the revert case from strategy. </p> <p>( I mean, </p> <ul> <li>in strategy pattern, you at first implement the "context" class, then you need to implement the "strategy" class. </li> <li>but in "around alias" spell, you need to first of all get an "strategy" implemented, then you write a new class that "around alias" the "strategy"... </li> </ul> <p>err... hope I didn't make you confused ^_^ )</p>

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