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        <a href="." >Accesseurs</a>
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                                <h1 id="accesseurs">Accesseurs</h1>
<p>Parfois nous avons besoin de calculer des valeurs bas&#xE9;es sur l&apos;&#xE9;tat du store, par exemple pour filtrer une liste d&apos;&#xE9;l&#xE9;ments et les compter :</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>Si plus d&apos;un composant a besoin d&apos;utiliser cela, il nous faut ou bien dupliquer cette fonction, ou bien l&apos;extraire dans une fonction utilitaire s&#xE9;par&#xE9;e et l&apos;importer aux endroits n&#xE9;cessaires. Les deux id&#xE9;es sont loin d&apos;&#xEA;tre id&#xE9;ales.</p>
<p>Vuex nous permet de d&#xE9;finir des accesseurs (&#xAB; getters &#xBB;) dans le store. Voyez-les comme les propri&#xE9;t&#xE9;s calcul&#xE9;es des stores. Comme pour les propri&#xE9;t&#xE9;s calcul&#xE9;es, le r&#xE9;sultat de l&apos;accesseur est mis en cache en se basant sur ses d&#xE9;pendances et il ne sera r&#xE9;&#xE9;valu&#xE9; que lorsque l&apos;une de ses d&#xE9;pendances aura chang&#xE9;e.</p>
<p>Les accesseurs prennent l&apos;&#xE9;tat en premier argument :</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>Les accesseurs seront expos&#xE9;s sur l&apos;objet <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>Les accesseurs recevront &#xE9;galement les autres accesseurs en second argument :</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>Nous pouvons maintenant facilement les utiliser dans n&apos;importe quel composant :</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>Vous pouvez aussi passer des arguments aux accesseurs en retournant une fonction. Cela est particuli&#xE8;rement utile quand vous souhaitez interroger un tableau dans le store :</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="la-fonction-utilitaire-mapgetters">La fonction utilitaire <code>mapGetters</code></h3>
<p>La fonction utilitaire <code>mapGetters</code> attache simplement vos accesseurs du store aux propri&#xE9;t&#xE9;s calcul&#xE9;es locales :</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">// rajouter les accesseurs dans `computed` avec l&apos;op&#xE9;rateur de d&#xE9;composition</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>Si vous voulez attacher un accesseur avec un nom diff&#xE9;rent, utilisez un objet :</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">// attacher `this.doneCount` &#xE0; `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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