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‎.gitignore‎

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‎README.md‎

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# Ruby中的元编程
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## 简介
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　　本系列翻译自[Ruby Metaprogramming](http://ruby-metaprogramming.rubylearning.com/html/ruby_metaprogramming_1.html)站点上的课程笔记，并加入了我（[DeathKing](https://github.com/DeathKing)）的一些个人演绎、资料补充等。希望对大家有所帮助。
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　　该课程由[Satoshi Asakawa](http://www.workingwithrails.com/people/153056-satoshi)讲授，使用ruby 1.9.1p243 [i386-mingw32]。而我的测试环境则是ruby 1.9.2p180 [i386-mingw32]。
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　　本系列教程在2011年9月间翻译完毕，最初发布在我的博客上。于2014年5月间，藉由GitBook整理，并发布在GitHub上。在此期间，Paolo Perrotta所著的《Ruby元编程》也被翻译并出版。同时，Ruby版本也从1.9飞跃到了2.1，尽管如此，本系列介绍的Ruby元编程技术仍然可用，读者可以放心阅读。
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## 翻译的动机
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　　Ruby是动态的、魔幻而有趣的。而元编程（Metaprogramming）技术在Ruby中的应用也让我大开眼界，虽然以前也有浅显地介绍反射机制（Reflection），但我仍然觉得才疏学浅，不能让大家完全感受到元编程的优美。借此机会，我想借Satoshi Asakawa的本系列讲座，为大家展示一个绚丽的Ruby元编程世界。

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<section class="normal" id="section-gitbook_7">
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<h1 id="-">方法的调用</h1>
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<p>　　当你在调用某一个方法的时候，Ruby会完成下面的步骤：</p>
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<ul>
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<li>找到这个方法，我们把这个过程称作<strong>方法查找（method lookup）</strong>；</li>
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<li>执行这个方法，为了执行这个方法，Ruby需要一个叫做<code>self</code>的伪变量；</li>
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</ul>
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<h2 id="-">方法的查找</h2>
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<p>　　要理解Ruby的方法查找，你需要了解下面两个概念：<strong>接受者（receiver）</strong>和<strong>祖先链（ancestors chain）</strong>。接受者就是方法的调用者。例如，对调用<code>an_object.display()</code>来说，<code>an_object</code>就是接受者。而关于祖先链，请考察任何一个Ruby类的内部。想象一下从一个类移动到其父类，然后再移动到父类的父类，直到到达<code>Object</code>类（默认的父类），然后继续移动，最终到达<code>BasicObject</code>类（Ruby层级模型中的根）。你遍历这些类所走过的路径就是“祖先链”（祖先链中也包括模块）。</p>
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<p>　　因此，为了查找方法，Ruby首先进入receiver所属的类，并以此为起始，沿着祖先链不断前进，直到找到目标方法。这种行为也被称作<strong>“向右一步，再向上（one step to the right, then up）”</strong>规则：向右一步，进入reciver所属的类，然后再向上查找祖先链，直到找到目标方法。（译者注：如果遍历完祖先链也没有找到方法的话，会调用<code>method_missing</code>方法，如果这个方法没有被定义，则抛出<code>NoMethodError</code>）当你在一个类中包含一个模块时，Ruby创建了一个匿名类来封装这个模块，并将这个匿名类插入到祖先链中，也在这个类的上方。</p>
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<p>　　下面的代码演示了一次方法查找：</p>
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<pre><code class="lang-ruby"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">A</span></span>
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<span class="hljs-function"><span class="hljs-keyword">def</span> </span>foo
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<span class="hljs-keyword">end</span>
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<span class="hljs-keyword">end</span>
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<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">B</span> <span class="hljs-inheritance">&lt; <span class="hljs-parent">A</span></span></span>
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<span class="hljs-function"><span class="hljs-keyword">def</span> </span>bar
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<span class="hljs-comment"># bar method in B</span>
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<span class="hljs-keyword">end</span>
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<span class="hljs-keyword">end</span>
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<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">C</span> <span class="hljs-inheritance">&lt; <span class="hljs-parent">B</span></span></span>
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<span class="hljs-function"><span class="hljs-keyword">def</span> </span>bar
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<span class="hljs-comment"># bar method in C</span>
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<span class="hljs-comment"># overwriting superclass' method</span>
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<span class="hljs-keyword">end</span>
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<span class="hljs-keyword">end</span>
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obj = <span class="hljs-constant">C</span>.new
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obj.bar <span class="hljs-comment">#=&gt; in C class</span>
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obj.foo <span class="hljs-comment">#=&gt; not in C class</span>
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<span class="hljs-comment">#=&gt; then go to C's superclass B</span>
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<span class="hljs-comment">#=&gt; not in B class</span>
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<span class="hljs-comment">#=&gt; then go to B's superclass A</span>
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<span class="hljs-comment">#=&gt; execute it</span>
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</code></pre>
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<h2 id="self">self</h2>
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<ul>
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<li>在Ruby中，<code>self</code>是一个很特殊的变量，它总是指向当前对象（current object）；</li>
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<li><code>self</code>被认为是默认的receiver。也就是说，你如果没有明确指出receiver，那么系统将把<code>self</code>当做receiver；</li>
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<li>实例变量是在<code>self</code>（当前对象）中查找的。也就是说，如果我使用<code>@var</code>，那么Ruby就会在<code>self</code>所指向的对象中去寻找此实例变量。需要注意的是，实例变量并不是由类定义的，它们也和子类以及继承机制无关。</li>
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</ul>
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<p>　　因此，当我们在调用一个明确指出receiver的方法时，Ruby会按照下面的步骤执行：</p>
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<pre><code class="lang-ruby">obj.do_method(param)
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</code></pre>
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<ul>
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<li>将<code>self</code>指向<code>obj</code>；</li>
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<li>在<code>self</code>所属的类中查找<code>do_method(param)</code>方法（方法是存放在类中，而不是实例中！）；</li>
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<li>调用方法<code>do_method(param)</code>；</li>
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