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    <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></nav>  <ul class="sidebar-links"><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>核心概念</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>高级指引</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading open"><span>Hook</span> <span class="arrow down"></span></p> <ul class="sidebar-links sidebar-group-items"><li><a href="/blog/pages/4c13b9/" aria-current="page" class="active sidebar-link">Hook概述</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level2"><a href="/blog/pages/4c13b9/#什么是-hook" class="sidebar-link">什么是 Hook?</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/4c13b9/#usestate-状态钩子" class="sidebar-link">useState（状态钩子）</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/4c13b9/#声明多个-state-变量" class="sidebar-link">声明多个 state 变量</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/4c13b9/#effect-hook-副作用钩子" class="sidebar-link">Effect Hook (副作用钩子)</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header level3"><a href="/blog/pages/4c13b9/#useeffect-生命周期钩子" class="sidebar-link">useEffect（生命周期钩子）</a></li><li class="sidebar-sub-header level3"><a href="/blog/pages/4c13b9/#清除-副作用" class="sidebar-link">“清除”副作用</a></li></ul></li><li class="sidebar-sub-header level2"><a href="/blog/pages/4c13b9/#hook-使用规则" class="sidebar-link">Hook 使用规则</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/4c13b9/#自定义-hook-hook的使用逻辑封装" class="sidebar-link">自定义 Hook （Hook的使用逻辑封装）</a></li><li class="sidebar-sub-header level2"><a href="/blog/pages/4c13b9/#其他-hook" class="sidebar-link">其他 Hook</a></li></ul></li><li><a href="/blog/pages/82baa3/" class="sidebar-link">使用State Hook</a></li><li><a href="/blog/pages/72710d/" class="sidebar-link">使用Effect Hook</a></li><li><a href="/blog/pages/723be7/" class="sidebar-link">Hook规则</a></li><li><a href="/blog/pages/cf1018/" class="sidebar-link">自定义 Hook</a></li></ul></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>案例演示</span> <span class="arrow right"></span></p> <!----></section></li></ul> </aside> <div><main class="page"><div class="theme-vdoing-wrapper "><div class="articleInfo-wrap" data-v-06970110><div class="articleInfo" data-v-06970110><ul class="breadcrumbs" data-v-06970110><li data-v-06970110><a href="/blog/" title="首页" class="iconfont icon-home router-link-active" data-v-06970110></a></li> <li data-v-06970110><a href="/blog/note/react/#《React》笔记" data-v-06970110>《React》笔记</a></li><li data-v-06970110><a href="/blog/note/react/#Hook" data-v-06970110>Hook</a></li></ul> <div class="info" data-v-06970110><div title="作者" class="author iconfont icon-touxiang" data-v-06970110><a href="https://github.com/heBody" target="_blank" title="作者" class="beLink" data-v-06970110>heBody</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06970110><a href="javascript:;" data-v-06970110>2021-03-31</a></div> <!----></div></div></div> <!----> <div class="content-wrapper"><div class="right-menu-wrapper"><div class="right-menu-margin"><div class="right-menu-title">目录</div> <div class="right-menu-content"></div></div></div> <h1><img src="data:image/png;base64,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">Hook概述<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="_01-hook概述"><a href="#_01-hook概述" class="header-anchor">#</a> 01. Hook概述</h1> <p>Hook可以让你在不编写 class 的情况下使用 state 以及其他的 React 特性。</p> <h2 id="什么是-hook"><a href="#什么是-hook" class="header-anchor">#</a> 什么是 Hook?</h2> <p>Hook 是一个特殊的函数，可以让你在函数组件里“钩入” React <strong>state</strong> 及 <strong>生命周期</strong>  等特性的函数。</p> <blockquote><ol><li><p>Hook 不能在 class 组件中使用 —— 这使得你不使用 class 也能使用 React。</p></li> <li><p>React 内置了一些像 <code>useState</code> 这样的 Hook。你也可以创建你自己的 Hook 来复用不同组件之间的状态逻辑。我们会先介绍这些内置的 Hook。</p></li></ol></blockquote> <h2 id="usestate-状态钩子"><a href="#usestate-状态钩子" class="header-anchor">#</a> useState（状态钩子）</h2> <p><strong>模式：</strong></p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">const</span> <span class="token punctuation">[</span><span class="token operator">&lt;</span>取值<span class="token operator">&gt;</span><span class="token punctuation">,</span> <span class="token operator">&lt;</span>设值<span class="token operator">&gt;</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token function">useState</span><span class="token punctuation">(</span><span class="token operator">&lt;</span>初始值<span class="token operator">&gt;</span><span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面的三个值均可自定义名称，分别是：</p> <ul><li>state取值变量名</li> <li>state设值方法名</li> <li>初始值
<ul><li>初始值可以是数字、字符串、对象、数组等</li></ul></li></ul> <p><strong>例子：</strong></p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">import</span> React<span class="token punctuation">,</span> <span class="token punctuation">{</span> useState <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'react'</span><span class="token punctuation">;</span>

<span class="token keyword">function</span> <span class="token function">Example</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// 声明一个新的叫做 “count” 的 state 变量</span>
  <span class="token keyword">const</span> <span class="token punctuation">[</span>count<span class="token punctuation">,</span> setCount<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token function">useState</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token keyword">return</span> <span class="token punctuation">(</span>
    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>div</span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
      </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>p</span><span class="token punctuation">&gt;</span></span><span class="token plain-text">You clicked </span><span class="token punctuation">{</span>count<span class="token punctuation">}</span><span class="token plain-text"> times</span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>p</span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
      </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>button</span> <span class="token attr-name">onClick</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token function">setCount</span><span class="token punctuation">(</span>count <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">}</span></span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
        Click me
      </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>button</span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
    </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>div</span><span class="token punctuation">&gt;</span></span>
  <span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><h3 id="声明多个-state-变量"><a href="#声明多个-state-变量" class="header-anchor">#</a> 声明多个 state 变量</h3> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">function</span> <span class="token function">ExampleWithManyStates</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// 声明多个 state 变量！</span>
  <span class="token keyword">const</span> <span class="token punctuation">[</span>age<span class="token punctuation">,</span> setAge<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token function">useState</span><span class="token punctuation">(</span><span class="token number">42</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">const</span> <span class="token punctuation">[</span>fruit<span class="token punctuation">,</span> setFruit<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token function">useState</span><span class="token punctuation">(</span><span class="token string">'banana'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">const</span> <span class="token punctuation">[</span>todos<span class="token punctuation">,</span> setTodos<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token function">useState</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token punctuation">{</span> <span class="token literal-property property">text</span><span class="token operator">:</span> <span class="token string">'Learn Hooks'</span> <span class="token punctuation">}</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token comment">// ...</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><h2 id="effect-hook-副作用钩子"><a href="#effect-hook-副作用钩子" class="header-anchor">#</a> Effect Hook (副作用钩子)</h2> <p>你之前可能已经在 React 组件中执行过<strong>数据获取、订阅 或 者手动修改 DOM</strong>。我们统一把这些操作称为“<strong>副作用</strong>”，或者简称为“作用”。</p> <h3 id="useeffect-生命周期钩子"><a href="#useeffect-生命周期钩子" class="header-anchor">#</a> useEffect（生命周期钩子）</h3> <p><code>useEffect</code> 就是一个 Effect Hook，<strong>给函数组件增加了操作副作用的能力</strong>。它跟 class 组件中的 <code>componentDidMount</code>、<code>componentDidUpdate</code> 和 <code>componentWillUnmount</code> 具有相同的用途，只不过被合并成了一个 API。（我们会在<a href="https://zh-hans.reactjs.org/docs/hooks-effect.html" target="_blank" rel="noopener noreferrer">使用 Effect Hook<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a> 里展示对比 <code>useEffect</code> 和这些方法的例子。）</p> <p>例如，下面这个组件在 React 更新 DOM 后会设置一个页面标题：</p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">import</span> React<span class="token punctuation">,</span> <span class="token punctuation">{</span> useState<span class="token punctuation">,</span> useEffect <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'react'</span><span class="token punctuation">;</span>
<span class="token keyword">function</span> <span class="token function">Example</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> <span class="token punctuation">[</span>count<span class="token punctuation">,</span> setCount<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token function">useState</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token comment">// 相当于 componentDidMount 和 componentDidUpdate:</span>
  <span class="token function">useEffect</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    <span class="token comment">// 第一次渲染DOM以及更新DOM后执行此函数</span>
    <span class="token comment">// 使用浏览器的 API 更新页面标题</span>
    document<span class="token punctuation">.</span>title <span class="token operator">=</span> <span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">You clicked </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>count<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string"> times</span><span class="token template-punctuation string">`</span></span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">return</span> <span class="token punctuation">(</span>
    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>div</span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
      </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>p</span><span class="token punctuation">&gt;</span></span><span class="token plain-text">You clicked </span><span class="token punctuation">{</span>count<span class="token punctuation">}</span><span class="token plain-text"> times</span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>p</span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
      </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>button</span> <span class="token attr-name">onClick</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token function">setCount</span><span class="token punctuation">(</span>count <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">}</span></span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
        Click me
      </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>button</span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
    </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>div</span><span class="token punctuation">&gt;</span></span>
  <span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br></div></div><p>当你调用 <code>useEffect</code> 时，就是在告诉 React <strong>在完成对 DOM 的更改后运行你的“副作用”函数</strong>。由于副作用函数是在组件内声明的，所以它们可以访问到组件的 props 和 state。默认情况下，React 会在每次渲染后调用副作用函数 —— <strong>包括</strong>第一次渲染的时候。</p> <h3 id="清除-副作用"><a href="#清除-副作用" class="header-anchor">#</a> “清除”副作用</h3> <p>副作用函数还可以通过返回一个函数来指定如何“清除”副作用。例如，在下面的组件中使用副作用函数来订阅好友的在线状态，并通过取消订阅来进行清除操作：</p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">import</span> React<span class="token punctuation">,</span> <span class="token punctuation">{</span> useState<span class="token punctuation">,</span> useEffect <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'react'</span><span class="token punctuation">;</span>

<span class="token keyword">function</span> <span class="token function">FriendStatus</span><span class="token punctuation">(</span><span class="token parameter">props</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> <span class="token punctuation">[</span>isOnline<span class="token punctuation">,</span> setIsOnline<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token function">useState</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token keyword">function</span> <span class="token function">handleStatusChange</span><span class="token punctuation">(</span><span class="token parameter">status</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token function">setIsOnline</span><span class="token punctuation">(</span>status<span class="token punctuation">.</span>isOnline<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>

  <span class="token function">useEffect</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span> <span class="token comment">// 组件挂载完成和更新完成后执行</span>
    <span class="token comment">// 创建订阅</span>
    ChatAPI<span class="token punctuation">.</span><span class="token function">subscribeToFriendStatus</span><span class="token punctuation">(</span>props<span class="token punctuation">.</span>friend<span class="token punctuation">.</span>id<span class="token punctuation">,</span> handleStatusChange<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span> <span class="token comment">// 组件销毁时执行</span>
      <span class="token comment">// 取消订阅</span>
      ChatAPI<span class="token punctuation">.</span><span class="token function">unsubscribeFromFriendStatus</span><span class="token punctuation">(</span>props<span class="token punctuation">.</span>friend<span class="token punctuation">.</span>id<span class="token punctuation">,</span> handleStatusChange<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><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span>isOnline <span class="token operator">===</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token string">'Loading...'</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
  <span class="token keyword">return</span> isOnline <span class="token operator">?</span> <span class="token string">'Online'</span> <span class="token operator">:</span> <span class="token string">'Offline'</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br></div></div><p>在这个示例中，React 会在组件销毁时取消对 <code>ChatAPI</code> 的订阅，然后在后续渲染时重新执行副作用函数。（如果传给 <code>ChatAPI</code> 的 <code>props.friend.id</code> 没有变化，你也可以<a href="https://zh-hans.reactjs.org/docs/hooks-effect.html#tip-optimizing-performance-by-skipping-effects" target="_blank" rel="noopener noreferrer">告诉 React 跳过重新订阅<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>。）</p> <p>跟 <code>useState</code> 一样，你可以在组件中多次使用 <code>useEffect</code> ：</p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">function</span> <span class="token function">FriendStatusWithCounter</span><span class="token punctuation">(</span><span class="token parameter">props</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> <span class="token punctuation">[</span>count<span class="token punctuation">,</span> setCount<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token function">useState</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token function">useEffect</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    document<span class="token punctuation">.</span>title <span class="token operator">=</span> <span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">You clicked </span><span class="token interpolation"><span class="token interpolation-punctuation punctuation">${</span>count<span class="token interpolation-punctuation punctuation">}</span></span><span class="token string"> times</span><span class="token template-punctuation string">`</span></span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token keyword">const</span> <span class="token punctuation">[</span>isOnline<span class="token punctuation">,</span> setIsOnline<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token function">useState</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token function">useEffect</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    ChatAPI<span class="token punctuation">.</span><span class="token function">subscribeToFriendStatus</span><span class="token punctuation">(</span>props<span class="token punctuation">.</span>friend<span class="token punctuation">.</span>id<span class="token punctuation">,</span> handleStatusChange<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
      ChatAPI<span class="token punctuation">.</span><span class="token function">unsubscribeFromFriendStatus</span><span class="token punctuation">(</span>props<span class="token punctuation">.</span>friend<span class="token punctuation">.</span>id<span class="token punctuation">,</span> handleStatusChange<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><span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token keyword">function</span> <span class="token function">handleStatusChange</span><span class="token punctuation">(</span><span class="token parameter">status</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token function">setIsOnline</span><span class="token punctuation">(</span>status<span class="token punctuation">.</span>isOnline<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
  <span class="token comment">// ...</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br></div></div><p>通过使用 Hook，你可以把组件内相关的副作用组织在一起（例如创建订阅及取消订阅），而不要把它们拆分到不同的生命周期函数里。</p> <blockquote><p>详细说明</p> <p>你可以在这一章节了解更多关于 <code>useEffect</code> 的内容：<a href="https://zh-hans.reactjs.org/docs/hooks-effect.html" target="_blank" rel="noopener noreferrer">使用 Effect Hook<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p></blockquote> <h2 id="hook-使用规则"><a href="#hook-使用规则" class="header-anchor">#</a> Hook 使用规则</h2> <p>Hook 就是 JavaScript 函数，但是使用它们会有两个额外的规则：</p> <ul><li>只能在<strong>函数最外层</strong>调用 Hook。不要在循环、条件判断或者子函数中调用。</li> <li>只能在 <strong>React 的函数组件</strong>中调用 Hook。不要在其他 JavaScript 函数中调用。（还有一个地方可以调用 Hook —— 就是自定义的 Hook 中，我们稍后会学习到。）</li></ul> <p>同时，我们提供了 <a href="https://www.npmjs.com/package/eslint-plugin-react-hooks" target="_blank" rel="noopener noreferrer">linter 插件<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>来自动执行这些规则。这些规则乍看起来会有一些限制和令人困惑，但是要让 Hook 正常工作，它们至关重要。</p> <blockquote><p>详细说明</p> <p>你可以在这章节了解更多关于这些规则的内容：<a href="https://zh-hans.reactjs.org/docs/hooks-rules.html" target="_blank" rel="noopener noreferrer">Hook 使用规则<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>。</p></blockquote> <h2 id="自定义-hook-hook的使用逻辑封装"><a href="#自定义-hook-hook的使用逻辑封装" class="header-anchor">#</a> 自定义 Hook （Hook的使用逻辑封装）</h2> <p>有时候我们会想要在组件之间重用一些状态逻辑。目前为止，有两种主流方案来解决这个问题：<a href="https://zh-hans.reactjs.org/docs/higher-order-components.html" target="_blank" rel="noopener noreferrer">高阶组件<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>和 <a href="https://zh-hans.reactjs.org/docs/render-props.html" target="_blank" rel="noopener noreferrer">render props<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>。自定义 Hook 可以让你在不增加组件的情况下达到同样的目的。</p> <p>前面，我们介绍了一个叫 <code>FriendStatus</code> 的组件，它通过调用 <code>useState</code> 和 <code>useEffect</code> 的 Hook 来订阅一个好友的在线状态。假设我们想在另一个组件里重用这个订阅逻辑。</p> <p>首先，我们把这个逻辑抽取到一个叫做 <code>useFriendStatus</code> 的自定义 Hook 里：</p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">import</span> React<span class="token punctuation">,</span> <span class="token punctuation">{</span> useState<span class="token punctuation">,</span> useEffect <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'react'</span><span class="token punctuation">;</span>

<span class="token comment">// 逻辑抽取，方便复用</span>
<span class="token keyword">function</span> <span class="token function">useFriendStatus</span><span class="token punctuation">(</span><span class="token parameter">friendID</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token comment">// 每次调用时的state都是独立的</span>
  <span class="token keyword">const</span> <span class="token punctuation">[</span>isOnline<span class="token punctuation">,</span> setIsOnline<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token function">useState</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token keyword">function</span> <span class="token function">handleStatusChange</span><span class="token punctuation">(</span><span class="token parameter">status</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token function">setIsOnline</span><span class="token punctuation">(</span>status<span class="token punctuation">.</span>isOnline<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>

  <span class="token function">useEffect</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    ChatAPI<span class="token punctuation">.</span><span class="token function">subscribeToFriendStatus</span><span class="token punctuation">(</span>friendID<span class="token punctuation">,</span> handleStatusChange<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
      ChatAPI<span class="token punctuation">.</span><span class="token function">unsubscribeFromFriendStatus</span><span class="token punctuation">(</span>friendID<span class="token punctuation">,</span> handleStatusChange<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><span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token keyword">return</span> isOnline<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br></div></div><p>它将 <code>friendID</code> 作为参数，并返回该好友是否在线：</p> <p>现在我们可以在两个组件中使用它：</p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">function</span> <span class="token function">FriendStatus</span><span class="token punctuation">(</span><span class="token parameter">props</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> isOnline <span class="token operator">=</span> <span class="token function">useFriendStatus</span><span class="token punctuation">(</span>props<span class="token punctuation">.</span>friend<span class="token punctuation">.</span>id<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span>isOnline <span class="token operator">===</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token string">'Loading...'</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
  <span class="token keyword">return</span> isOnline <span class="token operator">?</span> <span class="token string">'Online'</span> <span class="token operator">:</span> <span class="token string">'Offline'</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">function</span> <span class="token function">FriendListItem</span><span class="token punctuation">(</span><span class="token parameter">props</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> isOnline <span class="token operator">=</span> <span class="token function">useFriendStatus</span><span class="token punctuation">(</span>props<span class="token punctuation">.</span>friend<span class="token punctuation">.</span>id<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">return</span> <span class="token punctuation">(</span>
    <span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>li</span> <span class="token attr-name">style</span><span class="token script language-javascript"><span class="token script-punctuation punctuation">=</span><span class="token punctuation">{</span><span class="token punctuation">{</span> <span class="token literal-property property">color</span><span class="token operator">:</span> isOnline <span class="token operator">?</span> <span class="token string">'green'</span> <span class="token operator">:</span> <span class="token string">'black'</span> <span class="token punctuation">}</span><span class="token punctuation">}</span></span><span class="token punctuation">&gt;</span></span><span class="token plain-text">
      </span><span class="token punctuation">{</span>props<span class="token punctuation">.</span>friend<span class="token punctuation">.</span>name<span class="token punctuation">}</span><span class="token plain-text">
    </span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>li</span><span class="token punctuation">&gt;</span></span>
  <span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><p><strong>每个组件间的 state 是完全独立的。Hook 是一种复用<em>状态逻辑</em>的方式，它不复用 state 本身</strong>。事实上 Hook 的每次<em>调用</em>都有一个完全独立的 state —— 因此你可以在单个组件中多次调用同一个自定义 Hook。</p> <p>自定义 Hook 更像是一种约定而不是功能。<strong>如果函数的名字以 “<code>use</code>” 开头并调用其他 Hook，我们就说这是一个自定义 Hook</strong>。 <code>useSomething</code> 的命名约定可以让我们的 linter 插件在使用 Hook 的代码中找到 bug。</p> <p>你可以创建涵盖各种场景的自定义 Hook，如表单处理、动画、订阅声明、计时器，甚至可能还有更多我们没想到的场景。我们很期待看到 React 社区会出现什么样的自定义 Hook。</p> <blockquote><p>详细说明</p> <p>我们会在这一章节介绍更多关于自定义 Hook 的内容： <a href="https://zh-hans.reactjs.org/docs/hooks-custom.html" target="_blank" rel="noopener noreferrer">创建你自己的 Hook<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>。</p></blockquote> <h2 id="其他-hook"><a href="#其他-hook" class="header-anchor">#</a> 其他 Hook</h2> <p>除此之外，还有一些使用频率较低的但是很有用的 Hook。比如，<a href="https://zh-hans.reactjs.org/docs/hooks-reference.html#usecontext" target="_blank" rel="noopener noreferrer"><code>useContext</code><span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a> 让你不使用组件嵌套就可以订阅 React 的 Context。</p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">function</span> <span class="token function">Example</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> locale <span class="token operator">=</span> <span class="token function">useContext</span><span class="token punctuation">(</span>LocaleContext<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">const</span> theme <span class="token operator">=</span> <span class="token function">useContext</span><span class="token punctuation">(</span>ThemeContext<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token comment">// ...</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>另外 <a href="https://zh-hans.reactjs.org/docs/hooks-reference.html#usereducer" target="_blank" rel="noopener noreferrer"><code>useReducer</code><span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a> 可以让你通过 reducer 来管理组件本地的复杂 state。</p> <div class="language-jsx line-numbers-mode"><pre class="language-jsx"><code><span class="token keyword">function</span> <span class="token function">Todos</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> <span class="token punctuation">[</span>todos<span class="token punctuation">,</span> dispatch<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token function">useReducer</span><span class="token punctuation">(</span>todosReducer<span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// ...</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><blockquote><p>详细说明</p> <p>你可以在这一章节了解更多关于所有内置 Hook 的内容：<a href="https://zh-hans.reactjs.org/docs/hooks-reference.html" target="_blank" rel="noopener noreferrer">Hook API 索引<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a>。</p></blockquote></div></div>  <div class="page-edit"><div class="edit-link"><a href="https://github.com/heBody/blog/edit/master/docs/《React》笔记/03.Hook/01.Hook概述.md" target="_blank" rel="noopener noreferrer">编辑</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <div class="tags"><a href="/blog/tags/?tag=React" title="标签">#React</a></div> <div class="last-updated"><span class="prefix">上次更新:</span> <span class="time">2022/12/14, 19:36:42</span></div></div> <div class="page-nav-wapper"><div class="page-nav-centre-wrap"><a href="/blog/pages/b7ec27/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">高级指引</div></a> <a href="/blog/pages/82baa3/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">使用State Hook</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
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