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<h1 id="-">元编程实战</h1>
<h2 id="-1">问题1</h2>
<p> 此问题改编自Dave Thomas的屏播<a href="http://pragprog.com/screencasts/v-dtrubyom/the-ruby-object-model-and-metaprogramming" target="_blank">Episode 5: Nine Examples of Metaprogramming</a>。</p>
<p> 众所周知，RubyLearnin.org的Core Ruby课程已经开办8周了。每周我们都有一个满分10分的测验。8周结束后，学生可以知道他的分数百分比。例如，有一个学生，在过去的8周里，他的得分情况为：5、10、10、10、10、10、10、10。那么，他的得分百分比为93.75%。</p>
<p> 问题描述：每一批Core Ruby学习班有成百上千的学生。让我们假设我们有一个可以计算这个百分比并返回对应的值的Ruby方法。</p>
<h3 id="-">现存的类和方法</h3>
<p> 让我们先来看看已存在的类和方法，并修改它，来解决上述问题。</p>
<pre><code class="lang-ruby"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Result</span></span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>total(*scores)
percentage_calculation(*scores)
<span class="hljs-keyword">end</span>
private
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>percentage_calculation(*scores)
puts <span class="hljs-string">"Calculation for <span class="hljs-subst">#{scores.inspect}</span>"</span>
scores.inject {|sum, n| sum + n } * (<span class="hljs-number">100.0</span>/<span class="hljs-number">80.0</span>)
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
r = <span class="hljs-constant">Result</span>.new
puts r.total(<span class="hljs-number">5</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
puts r.total(<span class="hljs-number">5</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
puts r.total(<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
puts r.total(<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
</code></pre>
<p> 上述代码中，我们定义了<code>Result</code>类以及一个<code>total</code>方法。<code>total</code>方法用于列举每个学生的成绩。<code>score</code>代表了学生在课程的8次竞赛中获得的成绩。私有方法<code>percentage_calculation</code>用于计算等分率。为了测试如此，我们调用<code>total</code>方法四次。前两次和后两次调用时分别采用相同的数组，运行程序，我们得到下面的输出：</p>
<pre><code class="lang-ruby"><span class="hljs-constant">Calculation</span> <span class="hljs-keyword">for</span> [<span class="hljs-number">5</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>]
<span class="hljs-number">93.75</span>
<span class="hljs-constant">Calculation</span> <span class="hljs-keyword">for</span> [<span class="hljs-number">5</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>]
<span class="hljs-number">93.75</span>
<span class="hljs-constant">Calculation</span> <span class="hljs-keyword">for</span> [<span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>]
<span class="hljs-number">100.0</span>
<span class="hljs-constant">Calculation</span> <span class="hljs-keyword">for</span> [<span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>]
<span class="hljs-number">100.0</span>
</code></pre>
<p> 观察输出，我们意识到我们调用了<code>total</code>方法四次，这也意味着我们调用<code>percentage_calculation</code>方法四次。我们将要尝试减少调用<code>percentage_calculation</code>方法的次数。</p>
<h3 id="-">常规做法</h3>
<p> 减少对<code>percentage_calculation</code>方法的调用的一种途径是用某种方法存放之前计算出的数据。对此，我们需要定义一个叫做<code>MemoResult</code>的子类，该子类拥有一个叫做<code>@mem</code>的<code>Hash</code>类实例变量。代码如下：</p>
<pre><code class="lang-ruby"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Result</span></span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>total(*scores)
percentage_calculation(*scores)
<span class="hljs-keyword">end</span>
private
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>percentage_calculation(*scores)
puts <span class="hljs-string">"Calculation for <span class="hljs-subst">#{scores.inspect}</span>"</span>
scores.inject {|sum, n| sum + n } * (<span class="hljs-number">100.0</span>/<span class="hljs-number">80.0</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">MemoResult</span> <span class="hljs-inheritance">< <span class="hljs-parent">Result</span></span></span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>initialize
<span class="hljs-variable">@mem</span> = {}
<span class="hljs-keyword">end</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>total(*scores)
<span class="hljs-keyword">if</span> <span class="hljs-variable">@mem</span>.has_key?(scores)
<span class="hljs-variable">@mem</span>[scores]
<span class="hljs-keyword">else</span>
<span class="hljs-variable">@mem</span>[scores] = <span class="hljs-keyword">super</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
r = <span class="hljs-constant">MemoResult</span>.new
puts r.total(<span class="hljs-number">5</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
puts r.total(<span class="hljs-number">5</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
puts r.total(<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
puts r.total(<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
</code></pre>
<p> <code>Hash</code>类提供了<code>has_key?</code>方法用于检查某个散列是否拥有指定的键。在上述程序中，如果<code>has_key?</code>返回<code>true</code>，那么我们就直接使用<code>@mem</code>中存放的值，否则我们将调用<code>percentage_calculation(*scores)</code>重新计算改值并存放在<code>@mem</code>中。输出如下：</p>
<pre><code class="lang-ruby"><span class="hljs-constant">Calculation</span> <span class="hljs-keyword">for</span> [<span class="hljs-number">5</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>]
<span class="hljs-number">93.75</span>
<span class="hljs-number">93.75</span>
<span class="hljs-constant">Calculation</span> <span class="hljs-keyword">for</span> [<span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>]
<span class="hljs-number">100.0</span>
<span class="hljs-number">100.0</span>
</code></pre>
<p> 观察上述输出，我们不难得出，在第二次、第四次调用<code>r.total</code>的时候，我们直接使用的存储值。</p>
<h3 id="-class-new-define_method-">使用Class.new和define_method的做法</h3>
<p> 上面使用的<code>MemoResult</code>类与其父类精密相连。为了避免这样，我们使用目前学过的Ruby元编程知识动态创建这个子类。</p>
<p> 我们需要编写一个接受两个参数的方法<code>mem_result</code>：一个参数为父类的名字，另一个参数为方法的名字（而<code>mem_result</code>方法会返回定义好的类的名字）。下面是代码：</p>
<pre><code class="lang-ruby"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Result</span></span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>total(*scores)
percentage_calculation(*scores)
<span class="hljs-keyword">end</span>
private
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>percentage_calculation(*scores)
puts <span class="hljs-string">"Calculation for <span class="hljs-subst">#{scores.inspect}</span>"</span>
scores.inject {|sum, n| sum + n } * (<span class="hljs-number">100.0</span>/<span class="hljs-number">80.0</span>)
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>mem_result(klass, method)
mem = {}
<span class="hljs-constant">Class</span>.new(klass) <span class="hljs-keyword">do</span>
define_method(method) <span class="hljs-keyword">do</span> |*args|
<span class="hljs-keyword">if</span> mem.has_key?(args)
mem[args]
<span class="hljs-keyword">else</span>
mem[args] = <span class="hljs-keyword">super</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
r = mem_result(<span class="hljs-constant">Result</span>, <span class="hljs-symbol">:total</span>).new
puts r.total(<span class="hljs-number">5</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
puts r.total(<span class="hljs-number">5</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
puts r.total(<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
puts r.total(<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
</code></pre>
<p> 输出如下：</p>
<pre><code class="lang-ruby"><span class="hljs-constant">Calculation</span> <span class="hljs-keyword">for</span> [<span class="hljs-number">5</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>]
<span class="hljs-number">93.75</span>
<span class="hljs-number">93.75</span>
<span class="hljs-constant">Calculation</span> <span class="hljs-keyword">for</span> [<span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>, <span class="hljs-number">10</span>]
<span class="hljs-number">100.0</span>
<span class="hljs-number">100.0</span>
</code></pre>
<p> 代码<code>Class.new(klass)</code>以给定的<code>klass</code>为父类，创建了一个匿名类。块被用作类的体，包含了该类中的方法。而<code>define_method</code>定义了<code>method</code>所代表的方法（也就是<code>mem_result</code>的第二个参数）。</p>
<blockquote>
<p>注意</p>
<p>我们并没有编写<code>initialize</code>方法和使用实例变量<code>@mem</code>。相反地，我们使用的是局部变量<code>mem</code>，这是因为块是一个闭包，而局部变量<code>mem</code>在块内部是有效的。</p>
</blockquote>
<h3 id="-">使用匿名类的做法</h3>
<pre><code class="lang-ruby"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Result</span></span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>total(*scores)
percentage_calculation(*scores)
<span class="hljs-keyword">end</span>
private
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>percentage_calculation(*scores)
puts <span class="hljs-string">"Calculation for <span class="hljs-subst">#{scores.inspect}</span>"</span>
scores.inject {|sum, n| sum + n } * (<span class="hljs-number">100.0</span>/<span class="hljs-number">80.0</span>)
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
r = <span class="hljs-constant">Result</span>.new
<span class="hljs-comment"># Anonymous class on object</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>r.total(*scores)
<span class="hljs-variable">@mem</span> ||= {}
<span class="hljs-keyword">if</span> <span class="hljs-variable">@mem</span>.has_key?(scores)
<span class="hljs-variable">@mem</span>[scores]
<span class="hljs-keyword">else</span>
<span class="hljs-variable">@mem</span>[scores] = <span class="hljs-keyword">super</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
puts r.total(<span class="hljs-number">5</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
puts r.total(<span class="hljs-number">5</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
puts r.total(<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
puts r.total(<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
</code></pre>
<h3 id="-">使用即时创建匿名类的做法</h3>
<pre><code class="lang-ruby"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Result</span></span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>total(*scores)
percentage_calculation(*scores)
<span class="hljs-keyword">end</span>
private
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>percentage_calculation(*scores)
puts <span class="hljs-string">"Calculation for <span class="hljs-subst">#{scores.inspect}</span>"</span>
scores.inject {|sum, n| sum + n } * (<span class="hljs-number">100.0</span>/<span class="hljs-number">80.0</span>)
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>mem_result(obj, method)
obj.<span class="hljs-keyword">class</span>.class_eval <span class="hljs-keyword">do</span>
mem ||= {}
define_method(method) <span class="hljs-keyword">do</span> |*args|
<span class="hljs-keyword">if</span> mem.has_key?(args)
mem[args]
<span class="hljs-keyword">else</span>
mem[args] = <span class="hljs-keyword">super</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
r = <span class="hljs-constant">Result</span>.new
mem_result(r, <span class="hljs-symbol">:total</span>)
puts r.total(<span class="hljs-number">5</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
puts r.total(<span class="hljs-number">5</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
puts r.total(<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
puts r.total(<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>,<span class="hljs-number">10</span>)
</code></pre>
<p> 上述的代码中，我们编写了新的<code>mem_result</code>方法。该方法的第一个参数<code>obj</code>接收一个对象，用于建立此对象的匿名类。该方法的第二个参数<code>method</code>接受一个方法，用于指明要在匿名类中创建的方法名字。</p>
<p> 在此之前，我们已经使用<code>define_method</code>即时地创建了一个方法。问题是，<code>define_method</code>只能对类或模块起作用，而我们这里处理的是对象。因此，我们使用<code>obj.class</code>来获得对象所属的类，然后对类使用<code>class_eval</code>和<code>define_method</code>方法在该类中添加一个实例方法<code>method</code>。现在，我们来运行一下代码并检查输出。</p>
<pre><code>result.rb:21:in `total': super: no superclass method `total' (NoMethodError)
from result.rb:30
</code></pre><p> 代码没有运行。</p>
<p> 代码<code>mem[args] = super</code>尝试在匿名类中调用<code>Result</code>类中的<code>total</code>方法。但问题是，我们是在<code>Result</code>类中直接定义的<code>total</code>方法。我们说过，<code>obj.class</code>返回的是<code>Result</code>，所以这并不起效。我们需要做得是创建一个匿名类，并将这个<code>total</code>方法放到这个匿名类中。同时，我们的匿名类应该是<code>Result</code>类的子类。</p>
<p> 让我们像下面一样创建需要的匿名类。</p>
<pre><code class="lang-ruby">anon = <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-inheritance"><</span><span class="hljs-inheritance">< <span class="hljs-parent">obj</span></span></span>
<span class="hljs-keyword">self</span>
<span class="hljs-keyword">end</span>
</code></pre>
<p> 上述代码中的<code>self</code>返回了我们需要的匿名类，并被变量<code>anon</code>所引用。大多数的Ruby程序员会像下面这样把这些代码写作一行，以表示他们正创建鬼魂类。</p>
<pre><code class="lang-ruby">anon = <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-inheritance"><</span><span class="hljs-inheritance">< <span class="hljs-parent">obj</span></span>;</span> <span class="hljs-keyword">self</span>; <span class="hljs-keyword">end</span>
</code></pre>
<p> 有了匿名类后，我们应该对其使用<code>class_eval</code>方法，就像下面这样：</p>
<pre><code>class Result
def total(*scores)
percentage_calculation(*scores)
end
private
def percentage_calculation(*scores)
puts "Calculation for #{scores.inspect}"
scores.inject {|sum, n| sum + n } * (100.0/80.0)
end
end
def mem_result(obj, method)
anon = class << obj; self; end
anon.class_eval do
mem ||= {}
define_method(method) do |*args|
if mem.has_key?(args)
mem[args]
else
mem[args] = super(*args)
end
end
end
end
r = Result.new
mem_result(r, :total)
puts r.total(5,10,10,10,10,10,10,10)
puts r.total(5,10,10,10,10,10,10,10)
puts r.total(10,10,10,10,10,10,10,10)
puts r.total(10,10,10,10,10,10,10,10)
</code></pre><p> 代码正常运行，并返回希望的结果。</p>
<h2 id="-2">问题2</h2>
<p> 本例根据Hal Fulton的文章[An Exercise in Metaprogramming with Ruby(<a href="http://an%20exercise%20in%20metaprogramming%20with%20ruby/)改编。" target="_blank">http://an%20exercise%20in%20metaprogramming%20with%20ruby/)改编。</a></p>
<p> 假设我们有两个CSV（comma-separated values，数据使用逗号分隔的）文件，其头部有一些描述性的文字，如下所示：</p>
<p> 文件：location.txt</p>
<pre><code>name,country
"Matz", "USA"
"Fabio Akita", "Brazil"
"Peter Cooper", "UK"
</code></pre><p> 文件：twitter.txt</p>
<pre><code>twitterid,url
"AkitaOnRails","http://www.akitaonrails.com/"
"peterc","http://www.petercooper.co.uk/"
</code></pre><p> 首先，我们建立一个名为datawrapper.rb的文件，并建立一个类。我们会调用<code>DataWrapper</code>类并定义一个名为<code>wrap</code>的类方法，此方法请求一个用于标识文件名的参数，并据此建立一个类。上面的两个文件的第一行都是由逗号分隔的属性名（attribute names）。因此，我们想要把这些文件当做是数据数组，我们将要读取这些数据，并以数组的形式存放。</p>
<pre><code class="lang-ruby"><span class="hljs-comment"># file: datawrapper.rb</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">DataWrapper</span></span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span><span class="hljs-keyword">self</span>.wrap(file_name)
data = <span class="hljs-constant">File</span>.new(file_name)
header = data.gets.chomp
data.close
puts header <span class="hljs-comment"># => name,country</span>
<span class="hljs-comment"># in the end we return the class name</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
</code></pre>
<p> 接着，我们编写一个小程序名为testdatawrapper.rb。尝试着读取location.txt文件。</p>
<pre><code class="lang-ruby"><span class="hljs-comment">#testdatawrapper.rb</span>
<span class="hljs-keyword">require</span> <span class="hljs-string">'datawrapper'</span>
<span class="hljs-constant">DataWrapper</span>.wrap(<span class="hljs-string">"location.txt"</span>)
</code></pre>
<p> 回到datawrapper.rb程序，我们需要新建一个类，并给他适当的名字：</p>
<pre><code class="lang-ruby"><span class="hljs-comment"># file: datawrapper.rb</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">DataWrapper</span></span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span><span class="hljs-keyword">self</span>.wrap(file_name)
data = <span class="hljs-constant">File</span>.new(file_name)
header = data.gets.chomp
data.close
class_name = <span class="hljs-constant">File</span>.basename(file_name, <span class="hljs-string">".txt"</span>).capitalize
klass = <span class="hljs-constant">Object</span>.const_set(class_name, <span class="hljs-constant">Class</span>.new)
klass <span class="hljs-comment"># we return the class name</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
</code></pre>
<p> 变量<code>klass</code>指代的是我们新建的类。如果<code>file_name</code>参数所指向的文件为“location.txt”，那么新建立得类名会被命名为<code>Location</code>。</p>
<p> 再次运行修改后的程序。</p>
<pre><code class="lang-ruby"><span class="hljs-comment">#testdatawrapper.rb</span>
<span class="hljs-keyword">require</span> <span class="hljs-string">'datawrapper'</span>
data = <span class="hljs-constant">DataWrapper</span>.wrap(<span class="hljs-string">"location.txt"</span>) <span class="hljs-comment"># Capture return value and</span>
puts data <span class="hljs-comment"># => Location</span>
</code></pre>
<p> 然后就是大展身手的时候了。文件第一行读入的是表名（List Name）。我们只需要使用<code>split</code>方法即可快速读入。 修改后的datawrapper.rb如下：</p>
<pre><code class="lang-ruby"><span class="hljs-comment"># file: datawrapper.rb</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">DataWrapper</span></span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span><span class="hljs-keyword">self</span>.wrap(file_name)
data = <span class="hljs-constant">File</span>.new(file_name)
header = data.gets.chomp
data.close
class_name = <span class="hljs-constant">File</span>.basename(file_name, <span class="hljs-string">".txt"</span>).capitalize
klass = <span class="hljs-constant">Object</span>.const_set(class_name, <span class="hljs-constant">Class</span>.new)
<span class="hljs-comment"># get attribute names</span>
names = header.split(<span class="hljs-string">","</span>)
p names <span class="hljs-comment"># => ["name", "country"]</span>
klass <span class="hljs-comment"># we return the class name</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
</code></pre>
<p> 现在，在我们新建立的类的上下文中使用<code>class_eval</code>。此时，我们会定义一个<code>initialize</code>方法。并且，我们应该建立一个<code>to_s</code>方法，使得我们能够将其输出，记得使用<code>alias</code>关键字将<code>to_s</code>方法作为<code>inspect</code>方法的同义词。修改后的datawrapper.rb程序如下：</p>
<pre><code class="lang-ruby"><span class="hljs-comment"># file: datawrapper.rb</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">DataWrapper</span></span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span><span class="hljs-keyword">self</span>.wrap(file_name)
data = <span class="hljs-constant">File</span>.new(file_name)
header = data.gets.chomp
data.close
class_name = <span class="hljs-constant">File</span>.basename(file_name, <span class="hljs-string">".txt"</span>).capitalize
klass = <span class="hljs-constant">Object</span>.const_set(class_name, <span class="hljs-constant">Class</span>.new)
<span class="hljs-comment"># get attribute names</span>
names = header.split(<span class="hljs-string">","</span>)
klass.class_eval <span class="hljs-keyword">do</span>
<span class="hljs-keyword">attr_accessor</span> *names
define_method(<span class="hljs-symbol">:initialize</span>) <span class="hljs-keyword">do</span> |*values|
names.each_with_index <span class="hljs-keyword">do</span> |name, i|
instance_variable_set(<span class="hljs-string">"@"</span>+name, values[i])
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
define_method(<span class="hljs-symbol">:to_s</span>) <span class="hljs-keyword">do</span>
str = <span class="hljs-string">"<<span class="hljs-subst">#{<span class="hljs-keyword">self</span>.<span class="hljs-keyword">class</span>}</span>:"</span>
names.each {|name| str << <span class="hljs-string">" <span class="hljs-subst">#{name}</span>=<span class="hljs-subst">#{<span class="hljs-keyword">self</span>.send(name)}</span>"</span> }
str + <span class="hljs-string">">"</span>
<span class="hljs-keyword">end</span>
alias_method <span class="hljs-symbol">:inspect</span>, <span class="hljs-symbol">:to_s</span>
<span class="hljs-keyword">end</span>
klass <span class="hljs-comment"># we return the class name</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
</code></pre>
<p> 下一步，建立一个类方法，用于读取整个文本，并返回一个代表文本中内容的素组对象。类方法的建立涉及到了单体类等概念，但此处不需要仔细考察。修改后的代码如下：</p>
<pre><code class="lang-ruby"><span class="hljs-comment"># file: datawrapper.rb</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">DataWrapper</span></span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span><span class="hljs-keyword">self</span>.wrap(file_name)
data = <span class="hljs-constant">File</span>.new(file_name)
header = data.gets.chomp
data.close
class_name = <span class="hljs-constant">File</span>.basename(file_name, <span class="hljs-string">".txt"</span>).capitalize
klass = <span class="hljs-constant">Object</span>.const_set(class_name, <span class="hljs-constant">Class</span>.new)
<span class="hljs-comment"># get attribute names</span>
names = header.split(<span class="hljs-string">","</span>)
klass.class_eval <span class="hljs-keyword">do</span>
<span class="hljs-keyword">attr_accessor</span> *names
define_method(<span class="hljs-symbol">:initialize</span>) <span class="hljs-keyword">do</span> |*values|
names.each_with_index <span class="hljs-keyword">do</span> |name, i|
instance_variable_set(<span class="hljs-string">"@"</span>+name, values[i])
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
define_method(<span class="hljs-symbol">:to_s</span>) <span class="hljs-keyword">do</span>
str = <span class="hljs-string">"<<span class="hljs-subst">#{<span class="hljs-keyword">self</span>.<span class="hljs-keyword">class</span>}</span>:"</span>
names.each {|name| str << <span class="hljs-string">" <span class="hljs-subst">#{name}</span>=<span class="hljs-subst">#{<span class="hljs-keyword">self</span>.send(name)}</span>"</span> }
str + <span class="hljs-string">">"</span>
<span class="hljs-keyword">end</span>
alias_method <span class="hljs-symbol">:inspect</span>, <span class="hljs-symbol">:to_s</span>
<span class="hljs-keyword">end</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>klass.read
array = []
data = <span class="hljs-constant">File</span>.new(<span class="hljs-keyword">self</span>.to_s.downcase+<span class="hljs-string">".txt"</span>)
data.gets <span class="hljs-comment"># throw away header</span>
data.each <span class="hljs-keyword">do</span> |line|
line.chomp!
values = eval(<span class="hljs-string">"[<span class="hljs-subst">#{line}</span>]"</span>)
array << <span class="hljs-keyword">self</span>.new(*values)
<span class="hljs-keyword">end</span>
data.close
array
<span class="hljs-keyword">end</span>
klass <span class="hljs-comment"># we return the class name</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
</code></pre>
<p> 修改testdatawrapper.rb并测试。</p>
<pre><code class="lang-ruby"><span class="hljs-comment">#testdatawrapper.rb</span>
<span class="hljs-keyword">require</span> <span class="hljs-string">'datawrapper'</span>
klass = <span class="hljs-constant">DataWrapper</span>.wrap(<span class="hljs-string">"location.txt"</span>)
list = klass.read
list.each <span class="hljs-keyword">do</span> |location|
puts(<span class="hljs-string">"<span class="hljs-subst">#{location.name}</span> is from the <span class="hljs-subst">#{location.country}</span>"</span>)
<span class="hljs-keyword">end</span>
</code></pre>
<p> 让我们来看看和location.txt文件完全不同的twitter.txt文件。针对于twitter.txt的测试文件如下：</p>
<pre><code class="lang-ruby"><span class="hljs-comment">#testdatawrapper.rb</span>
<span class="hljs-keyword">require</span> <span class="hljs-string">'datawrapper'</span>
klass = <span class="hljs-constant">DataWrapper</span>.wrap(<span class="hljs-string">"twitter.txt"</span>)
list = klass.read
list.each <span class="hljs-keyword">do</span> |twitter|
puts(<span class="hljs-string">"<span class="hljs-subst">#{twitter.twitterid}</span>'s site is <span class="hljs-subst">#{twitter.url}</span>"</span>)
<span class="hljs-keyword">end</span>
</code></pre>
<p> 即使我们使用了不同的数据，datawrapper.rb的代码也无需改变！这便是一个Ruby之中的元编程的例子与实践。</p>
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