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2018-06-26-TheRoadtolearnReactPart4.html
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<h1 id="The-Road-to-learn-React书籍学习笔记-第四章"><a href="#The-Road-to-learn-React书籍学习笔记-第四章" class="headerlink" title="The Road to learn React书籍学习笔记(第四章)"></a>The Road to learn React书籍学习笔记(第四章)</h1><p><a href="https://github.com/LbhFront-end/react-pratice/tree/master/my-first-react-app"><img src="https://img.shields.io/badge/%E4%BB%A3%E7%A0%81%E8%AF%A6%E6%83%85%E8%A7%81%E6%88%91%E7%9A%84github-%E7%82%B9%E6%88%91-blue.svg" alt="The Road to learn React 代码"></a></p>
<h2 id="高级React组件"><a href="#高级React组件" class="headerlink" title="高级React组件"></a>高级React组件</h2><p>本章将重点介绍高级 React 组件的实现。我们将了解什么是高阶组件以及如何实现它们。此外，我们还将深入探讨 React 中更高级的主题，并用它实现复杂的交互功能。</p>
<h2 id="引用-DOM-元素"><a href="#引用-DOM-元素" class="headerlink" title="引用 DOM 元素"></a>引用 <code>DOM</code> 元素</h2><p>有时候我们需要在 <code>React</code> 与 <code>DOM</code> 节点进行交互。 <code>ref</code> 属性可以让我们访问元素中的一个节点，通常，访问 <code>DOM</code> 节点是 <code>React</code> 中的一个反模式，因为我们应该遵循它的声明式编程和单向数据流。当我们引入第一搜索组件时，就已经了解刀这些问题，但是在某些情况下，我们仍然要访问 <code>DOM</code> 节点。官方文档提到了三种情况<br>使用 <code>DOM API</code>(<code>focus</code>事件，媒体播放等)<br>调用命令式 <code>DOM</code> 节点动画<br>与需要 <code>DOM</code> 节点的第三方库集成</p>
<p>让我们通过 Search 组件这个例子看一下。当应用程序第一次渲染时，<code>input</code> 字段应该被聚焦。这是需要访问 <code>DOM API</code> 的一种用例。本章将展示渲染时聚焦 input 字段是如何工作的，但由于这个功能对于应用程序并不是很有用，所以我们将在本章之后省略这些更改。尽管如此，你仍然可以为自己的应用程序保留它。</p>
<p>通常，<code>无状态组件</code>和 <code>ES6 类组件</code>中都可以使用 <code>ref</code> 属性。在聚焦 <code>input</code> 字段的用例中，我们就需要一个生命周期方法。这就是为什么接下来会先在 ES6 类组件中展示如何使用 <code>ref</code> 属性。</p>
<p>第一步是将无状态组件重构为 ES6 类组件。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line">class Search extends Component {</span><br><span class="line"> render() {</span><br><span class="line"> const {</span><br><span class="line"> value,</span><br><span class="line"> onChange,</span><br><span class="line"> onSubmit,</span><br><span class="line"> children</span><br><span class="line"> } = this.props;</span><br><span class="line"></span><br><span class="line"> return (</span><br><span class="line"> <form onSubmit={onSubmit}></span><br><span class="line"> <input</span><br><span class="line"> type="text"</span><br><span class="line"> value={value}</span><br><span class="line"> onChange={onChange}</span><br><span class="line"> /></span><br><span class="line"> <button type="submit"></span><br><span class="line"> {children}</span><br><span class="line"> </button></span><br><span class="line"> </form></span><br><span class="line"> );</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>ES6 类组件的this对象可以帮助我们通过 <code>ref</code> 属性引用 <code>DOM</code> 节点。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line">class Search extends Component {</span><br><span class="line"> render() {</span><br><span class="line"> const {</span><br><span class="line"> value,</span><br><span class="line"> onChange,</span><br><span class="line"> onSubmit,</span><br><span class="line"> children</span><br><span class="line"> } = this.props;</span><br><span class="line"></span><br><span class="line"> return (</span><br><span class="line"> <form onSubmit={onSubmit}></span><br><span class="line"> <input</span><br><span class="line"> type="text"</span><br><span class="line"> value={value}</span><br><span class="line"> onChange={onChange}</span><br><span class="line"> ref={(node) => { this.input = node; }}</span><br><span class="line"> /></span><br><span class="line"> <button type="submit"></span><br><span class="line"> {children}</span><br><span class="line"> </button></span><br><span class="line"> </form></span><br><span class="line"> );</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>现在，你可以通过使用 <code>this</code> 对象、适当的生命周期方法和 <code>DOM API</code> 在组件挂载的时候来聚焦 <code>input</code> 字段</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br></pre></td><td class="code"><pre><span class="line">lass Search extends Component {</span><br><span class="line"> componentDidMount() {</span><br><span class="line"> if(this.input) {</span><br><span class="line"> this.input.focus();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> render() {</span><br><span class="line"> const {</span><br><span class="line"> value,</span><br><span class="line"> onChange,</span><br><span class="line"> onSubmit,</span><br><span class="line"> children</span><br><span class="line"> } = this.props;</span><br><span class="line"></span><br><span class="line"> return (</span><br><span class="line"> <form onSubmit={onSubmit}></span><br><span class="line"> <input</span><br><span class="line"> type="text"</span><br><span class="line"> value={value}</span><br><span class="line"> onChange={onChange}</span><br><span class="line"> ref={(node) => { this.input = node; }}</span><br><span class="line"> /></span><br><span class="line"> <button type="submit"></span><br><span class="line"> {children}</span><br><span class="line"> </button></span><br><span class="line"> </form></span><br><span class="line"> );</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>当应用程序渲染时，input 字段应该被聚焦。这就是ref属性的基本用法。</p>
<p>但是我们怎样在没有this对象的无状态组件中访问ref属性呢？接下来我们在无状态组件中演示。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br></pre></td><td class="code"><pre><span class="line">const Search = ({</span><br><span class="line"> value,</span><br><span class="line"> onChange,</span><br><span class="line"> onSubmit,</span><br><span class="line"> children</span><br><span class="line"> }) => {</span><br><span class="line"> let input;</span><br><span class="line"> return (</span><br><span class="line"> <form onSubmit={onSubmit}></span><br><span class="line"> <input</span><br><span class="line"> type="text"</span><br><span class="line"> value={value}</span><br><span class="line"> onChange={onChange}</span><br><span class="line"> ref={(node) => input = node}</span><br><span class="line"> /></span><br><span class="line"> <button type="submit"></span><br><span class="line"> {children}</span><br><span class="line"> </button></span><br><span class="line"> </form></span><br><span class="line"> );</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h2 id="加载"><a href="#加载" class="headerlink" title="加载"></a>加载</h2><p>现在让我们回到应用程序。当向 <code>Hacker News API</code> 发起搜索请求时，我们想要显示一个加载指示符。</p>
<p>请求是异步的，此时应该向用户展示某些事情即将发生的某种反馈。让我们在 <code>src／App.js</code> 中定义一个可重用的 <code>Loading</code> 组件。</p>
<blockquote>
<p>const Loading = () =><br>\<div>Loading …\</div></p>
</blockquote>
<p>现在你将需要一个属性来存储加载状态。根据加载状态可以决定稍后是否显示所加载的组件。</p>
<p>src/App.js</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line">class App extends Component {</span><br><span class="line"> constructor(props) {</span><br><span class="line"> super(props);</span><br><span class="line"></span><br><span class="line"> this.state = {</span><br><span class="line"> results: null,</span><br><span class="line"> searchKey: '',</span><br><span class="line"> searchTerm: DEFAULT_QUERY,</span><br><span class="line"> error: null,</span><br><span class="line"> isLoading: false,</span><br><span class="line"> };</span><br><span class="line"></span><br><span class="line"> ...</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">...</span><br><span class="line"></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>isLoading 的初始值是 false。在 App 组件挂载完成之前，无需加载任何东西。</p>
<p>当发起请求时，将加载状态 (isLoading) 设置为 true。最终，请求会成功，那时可以将加载状态 (isLoading) 设置为 false。</p>
<p>src/App.js</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br></pre></td><td class="code"><pre><span class="line">class App extends Component {</span><br><span class="line"></span><br><span class="line">...</span><br><span class="line"></span><br><span class="line">setSearchTopStories(result) {</span><br><span class="line"> ...</span><br><span class="line"></span><br><span class="line"> this.setState({</span><br><span class="line"> results: {</span><br><span class="line"> ...results,</span><br><span class="line"> [searchKey]: { hits: updatedHits, page }</span><br><span class="line"> },</span><br><span class="line"> isLoading: false</span><br><span class="line"> });</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> fetchSearchTopStories(searchTerm, page = 0) {</span><br><span class="line"> this.setState({ isLoading: true });</span><br><span class="line"></span><br><span class="line"> fetch(`${PATH_BASE}${PATH_SEARCH}?${PARAM_SEARCH}${searchTerm}&${PARAM_PAGE}\</span><br><span class="line"> ${page}&${PARAM_HPP}${DEFAULT_HPP}`)</span><br><span class="line"> .then(response => response.json())</span><br><span class="line"> .then(result => this.setSearchTopStories(result))</span><br><span class="line"> .catch(e => this.setState({ error: e }));</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> ...</span><br><span class="line"></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>最后一步，我们将在应用程序中使用 Loading 组件。基于加载状态 (isLoading) 的条件来决定渲染 Loading 组件或 Button 组件。后者为一个用于获取更多数据的按钮。</p>
<p>src/App.js</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br></pre></td><td class="code"><pre><span class="line">class App extends Component {</span><br><span class="line"></span><br><span class="line"> ...</span><br><span class="line"></span><br><span class="line"> render() {</span><br><span class="line"> const {</span><br><span class="line"> searchTerm,</span><br><span class="line"> results,</span><br><span class="line"> searchKey,</span><br><span class="line"> error,</span><br><span class="line"> isLoading</span><br><span class="line"> } = this.state;</span><br><span class="line"></span><br><span class="line"> ...</span><br><span class="line"></span><br><span class="line"> return (</span><br><span class="line"> <div className="page"></span><br><span class="line"> ...</span><br><span class="line"> <div className="interactions"></span><br><span class="line"> { isLoading</span><br><span class="line"> ? <Loading /></span><br><span class="line"> : <Button</span><br><span class="line"> onClick={() => this.fetchSearchTopStories(searchKey, page + 1)}></span><br><span class="line"> More</span><br><span class="line"> </Button></span><br><span class="line"> }</span><br><span class="line"> </div></span><br><span class="line"> </div></span><br><span class="line"> );</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>由于我们在componentDidMount（）中发起请求，Loading 组件会在应用程序启动的时候显示。此时，因为列表是空的，所以不显示 Table 组件。当响应数据从 Hacker News API 返回时，返回的数据会通过 Table 组件显示出来，加载状态 (isLoading) 设置为 false，然后 Loading 组件消失。同时，出现了可以获取更多的数据的“More”按钮。一旦点击按钮，获取更多的数据，该按钮将消失，加载组件会重新出现。</p>
<h2 id="高阶组件"><a href="#高阶组件" class="headerlink" title="高阶组件"></a>高阶组件</h2><p>高阶组件（HOC）是 React 中的一个高级概念。HOC 与高阶函数是等价的。它接受任何输入 - 多数时候是一个组件，也可以是可选参数 - 并返回一个组件作为输出。返回的组件是输入组件的增强版本，并且可以在JSX中使用。</p>
<p>HOC可用于不同的情况，比如：准备属性，管理状态或更改组件的表示形式。其中一种情况是将 HOC 用于帮助实现条件渲染。想象一下现在有一个 List 组件，由于列表可以为空或无，那么它可以渲染一个列表或者什么也不渲染。当没有列表的时候，HOC 可以屏蔽掉这个不显示任何内容的列表。另一方面，这个简单的 List 组件不再需要关心列表存不存在，它只关心渲染列表。</p>
<h2 id="高级排序"><a href="#高级排序" class="headerlink" title="高级排序"></a>高级排序</h2><p>我们已经实现了客户端和服务器端搜索交互。因为我们已经拥了 Table 组件，所以增强 Table 组件的交互性是有意义的。那接下来，我们为 Table 组件加入根据列标题进行排序的功能如何？</p>
<p>你自己写一个排序函数，但是一般这种情况，我个人更喜欢使用第三方工具库。lodash就是这些工具库之一，当然你也可以选择适用于你的任何第三方库。让我们安装 lodash 并使用。</p>
<p>参考链接：<a href="https://leanpub.com/the-road-to-learn-react-chinese">https://leanpub.com/the-road-to-learn-react-chinese</a></p>
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