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                                <h1 id="getters">Getters</h1>
<p>&#xC0;s vezes, talvez precisemos calcular o estado derivado com base no estado da <em>store</em>  , por exemplo, filtrar atrav&#xE9;s de uma lista de itens e cont&#xE1;-los:</p>
<pre class="language-"><code class="lang-js">computed<span class="token punctuation">:</span> <span class="token punctuation">{</span>
  doneTodosCount <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>$store<span class="token punctuation">.</span>state<span class="token punctuation">.</span>todos<span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span>todo <span class="token operator">=</span><span class="token operator">&gt;</span> todo<span class="token punctuation">.</span>done<span class="token punctuation">)</span><span class="token punctuation">.</span>length
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre>
<p>Se mais do que um componente precisa fazer uso disso, temos que duplicar a fun&#xE7;&#xE3;o, ou extra&#xED;-lo em um auxiliar compartilhado e import&#xE1;-lo em v&#xE1;rios lugares - ambos s&#xE3;o menos do que o ideal.
O Vuex nos permite definir &quot;getters&quot; na <em>store</em> . Voc&#xEA; pode pensar neles como dados computados para <em>stores</em> . Como os dados computados, o resultado de um getter &#xE9; armazenado em cache com base em suas depend&#xEA;ncias e s&#xF3; ir&#xE1; avaliar novamente quando algumas de suas depend&#xEA;ncias mudarem.</p>
<p>Getters receberam o estado como seu primeiro argumento:</p>
<pre class="language-"><code class="lang-js"><span class="token keyword">const</span> store <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Vuex<span class="token punctuation">.</span>Store</span><span class="token punctuation">(</span><span class="token punctuation">{</span>
  state<span class="token punctuation">:</span> <span class="token punctuation">{</span>
    todos<span class="token punctuation">:</span> <span class="token punctuation">[</span>
      <span class="token punctuation">{</span> id<span class="token punctuation">:</span> <span class="token number">1</span><span class="token punctuation">,</span> text<span class="token punctuation">:</span> <span class="token string">&apos;...&apos;</span><span class="token punctuation">,</span> done<span class="token punctuation">:</span> <span class="token boolean">true</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
      <span class="token punctuation">{</span> id<span class="token punctuation">:</span> <span class="token number">2</span><span class="token punctuation">,</span> text<span class="token punctuation">:</span> <span class="token string">&apos;...&apos;</span><span class="token punctuation">,</span> done<span class="token punctuation">:</span> <span class="token boolean">false</span> <span class="token punctuation">}</span>
    <span class="token punctuation">]</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
  getters<span class="token punctuation">:</span> <span class="token punctuation">{</span>
    doneTodos<span class="token punctuation">:</span> state <span class="token operator">=</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
      <span class="token keyword">return</span> state<span class="token punctuation">.</span>todos<span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span>todo <span class="token operator">=</span><span class="token operator">&gt;</span> todo<span class="token punctuation">.</span>done<span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre>
<p>Os getters ser&#xE3;o descritos no objeto <code>store.getters</code>:</p>
<pre class="language-"><code class="lang-js">store<span class="token punctuation">.</span>getters<span class="token punctuation">.</span>doneTodos <span class="token comment" spellcheck="true">// -&gt; [{ id: 1, text: &apos;...&apos;, done: true }]</span>
</code></pre>
<p>Getters tamb&#xE9;m receber&#xE1; outros getters como o segundo argumento:</p>
<pre class="language-"><code class="lang-js">getters<span class="token punctuation">:</span> <span class="token punctuation">{</span>
  <span class="token comment" spellcheck="true">// ...</span>
  doneTodosCount<span class="token punctuation">:</span> <span class="token punctuation">(</span>state<span class="token punctuation">,</span> getters<span class="token punctuation">)</span> <span class="token operator">=</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> getters<span class="token punctuation">.</span>doneTodos<span class="token punctuation">.</span>length
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre>
<pre class="language-"><code class="lang-js">store<span class="token punctuation">.</span>getters<span class="token punctuation">.</span>doneTodosCount <span class="token comment" spellcheck="true">// -&gt; 1</span>
</code></pre>
<p>Agora podemos usar facilmente isso dentro de qualquer componente:</p>
<pre class="language-"><code class="lang-js">computed<span class="token punctuation">:</span> <span class="token punctuation">{</span>
  doneTodosCount <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>$store<span class="token punctuation">.</span>getters<span class="token punctuation">.</span>doneTodosCount
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre>
<p>Voc&#xEA; tamb&#xE9;m pode passar argumentos para getters retornando uma fun&#xE7;&#xE3;o. Isso &#xE9; particularmente &#xFA;til quando voc&#xEA; deseja consultar um <em>array</em>  na <em>store</em> :</p>
<pre class="language-"><code class="lang-js">getters<span class="token punctuation">:</span> <span class="token punctuation">{</span>
  <span class="token comment" spellcheck="true">// ...</span>
  getTodoById<span class="token punctuation">:</span> <span class="token punctuation">(</span>state<span class="token punctuation">)</span> <span class="token operator">=</span><span class="token operator">&gt;</span> <span class="token punctuation">(</span>id<span class="token punctuation">)</span> <span class="token operator">=</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> state<span class="token punctuation">.</span>todos<span class="token punctuation">.</span><span class="token function">find</span><span class="token punctuation">(</span>todo <span class="token operator">=</span><span class="token operator">&gt;</span> todo<span class="token punctuation">.</span>id <span class="token operator">===</span> id<span class="token punctuation">)</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre>
<pre class="language-"><code class="lang-js">store<span class="token punctuation">.</span>getters<span class="token punctuation">.</span><span class="token function">getTodoById</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment" spellcheck="true">// -&gt; { id: 2, text: &apos;...&apos;, done: false }</span>
</code></pre>
<h3 id="o-auxiliar-mapgetters">O auxiliar <code>mapGetters</code></h3>
<p>O auxiliar <code>mapGetters</code> simplesmente mapeia os getters da <em>store</em>  para os dados computados locais:</p>
<pre class="language-"><code class="lang-js"><span class="token keyword">import</span> <span class="token punctuation">{</span> mapGetters <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">&apos;vuex&apos;</span>

<span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token punctuation">{</span>
  <span class="token comment" spellcheck="true">// ...</span>
  computed<span class="token punctuation">:</span> <span class="token punctuation">{</span>
    <span class="token comment" spellcheck="true">// misturar os getters em dados computados com o spread operator</span>
    <span class="token operator">...</span><span class="token function">mapGetters</span><span class="token punctuation">(</span><span class="token punctuation">[</span>
      <span class="token string">&apos;doneTodosCount&apos;</span><span class="token punctuation">,</span>
      <span class="token string">&apos;anotherGetter&apos;</span><span class="token punctuation">,</span>
      <span class="token comment" spellcheck="true">// ...</span>
    <span class="token punctuation">]</span><span class="token punctuation">)</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre>
<p>Se voc&#xEA; deseja mapear um getter para um nome diferente, use um objeto:</p>
<pre class="language-"><code class="lang-js"><span class="token operator">...</span><span class="token function">mapGetters</span><span class="token punctuation">(</span><span class="token punctuation">{</span>
  <span class="token comment" spellcheck="true">// mapeia `this.doneCount` para `store.getters.doneTodosCount`</span>
  doneCount<span class="token punctuation">:</span> <span class="token string">&apos;doneTodosCount&apos;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre>

                                
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