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<h1 id="-">方法的调用</h1>
<p> 当你在调用某一个方法的时候，Ruby会完成下面的步骤：</p>
<ul>
<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>
<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>
</ul>
<h2 id="-">方法的查找</h2>
<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>
<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>
<p> 下面的代码演示了一次方法查找：</p>
<pre><code class="lang-ruby"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">A</span></span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>foo
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">B</span> <span class="hljs-inheritance">< <span class="hljs-parent">A</span></span></span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>bar
<span class="hljs-comment"># bar method in B</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">C</span> <span class="hljs-inheritance">< <span class="hljs-parent">B</span></span></span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>bar
<span class="hljs-comment"># bar method in C</span>
<span class="hljs-comment"># overwriting superclass' method</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
obj = <span class="hljs-constant">C</span>.new
obj.bar <span class="hljs-comment">#=> in C class</span>
obj.foo <span class="hljs-comment">#=> not in C class</span>
<span class="hljs-comment">#=> then go to C's superclass B</span>
<span class="hljs-comment">#=> not in B class</span>
<span class="hljs-comment">#=> then go to B's superclass A</span>
<span class="hljs-comment">#=> execute it</span>
</code></pre>
<h2 id="self">self</h2>
<ul>
<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>
<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>
<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>
</ul>
<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>
<pre><code class="lang-ruby">obj.do_method(param)
</code></pre>
<ul>
<li>将<code>self</code>指向<code>obj</code>；</li>
<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>
<li>调用方法<code>do_method(param)</code>；</li>
</ul>
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