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react组件实现按需加载.html
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<!DOCTYPE html>
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2018-08-15
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<p>随着前端应用体积的扩大，资源加载的优化是我们必须要面对的问题，动态代码加载就是其中的一个方案，webpack 提供了符合 ECMAScript 提案 的 <code>import()</code> 语法 ，让我们来实现动态地加载模块（注：require.ensure 与 import() 均为 webpack 提供的代码动态加载方案，在 webpack 2.x 中，require.ensure 已被 import 取代）。</p>
<h2 id="React-lazy-是什么"><a href="#React-lazy-是什么" class="headerlink" title="React.lazy 是什么"></a>React.lazy 是什么</h2><p>在 React 16.6 版本中，新增了 React.lazy 函数，它能让你像渲染常规组件一样处理动态引入的组件，配合 webpack 的 Code Splitting ，只有当组件被加载，对应的资源才会导入 ，从而达到懒加载的效果。</p>
<h2 id="使用-React-lazy"><a href="#使用-React-lazy" class="headerlink" title="使用 React.lazy"></a>使用 React.lazy</h2><figure class="highlight js"><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></pre></td><td class="code"><pre><span class="line"><span class="comment">// 不使用 React.lazy</span></span><br><span class="line"><span class="keyword">import</span> OtherComponent <span class="keyword">from</span> <span class="string">'./OtherComponent'</span>;</span><br><span class="line"><span class="comment">// 使用 React.lazy</span></span><br><span class="line"><span class="keyword">const</span> OtherComponent = React.lazy(<span class="function"><span class="params">()</span> =></span> <span class="keyword">import</span>(<span class="string">'./OtherComponent'</span>))</span><br></pre></td></tr></table></figure>
<p>React.lazy 接受一个函数作为参数，这个函数需要调用 import() 。它需要返回一个 Promise，该 Promise 需要 resolve 一个 defalut export 的 React 组件。</p>
<p>在控制台打印可以看到，React.lazy 方法返回的是一个 lazy 组件的对象，类型是 react.lazy，并且 lazy 组件具有 _status 属性，与 Promise 类似它具有 Pending、Resolved、Rejected 三个状态，分别代表组件的加载中、已加载、和加载失败三种状态。</p>
<blockquote>
<p>需要注意的一点是，React.lazy 需要配合 Suspense 组件一起使用，在 Suspense 组件中渲染 React.lazy 异步加载的组件。如果单独使用 React.lazy，React 会给出错误提示。其中在 Suspense 组件中，
fallback 是一个必需的占位属性，如果没有这个属性的话也是会报错的。</p>
</blockquote>
<figure class="highlight js"><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></pre></td><td class="code"><pre><span class="line"><span class="comment">// 延迟加载回调 不需要加载效果设置为null maxDuration可以设置最大持续时间 最新版本中已移除了这个属性</span></span><br><span class="line"><span class="keyword">const</span> SuspenseComponent = <span class="function">(<span class="params">Component</span>) =></span> <span class="function">(<span class="params">props</span>) =></span> {</span><br><span class="line"> <span class="keyword">return</span> (</span><br><span class="line"> <Suspense fallback={<span class="built_in">Math</span>.random() >= <span class="number">0.5</span> ? <span class="xml"><span class="tag"><<span class="name">Loading</span> /></span></span> : <span class="xml"><span class="tag"><<span class="name">SkeletonView</span> /></span></span>}></span><br><span class="line"> <Component {...props}><<span class="regexp">/Component></span></span><br><span class="line"><span class="regexp"> </</span>Suspense></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">component: SuspenseComponent(OtherComponent),</span><br></pre></td></tr></table></figure>
<hr>
<a id="more"></a>
<h2 id="Webpack-动态加载"><a href="#Webpack-动态加载" class="headerlink" title="Webpack 动态加载"></a>Webpack 动态加载</h2><p>上面使用了 <code>import()</code> 语法，webpack 检测到这种语法会自动代码分割。使用这种动态导入语法代替以前的静态引入，可以让组件在渲染的时候，再去加载组件对应的资源。这个异步加载流程的实现机制是怎么样
呢？</p>
<figure class="highlight js"><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><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br></pre></td><td class="code"><pre><span class="line">__webpack_require__.e = <span class="function"><span class="keyword">function</span> <span class="title">requireEnsure</span>(<span class="params">chunkId</span>) </span>{</span><br><span class="line"> <span class="comment">// installedChunks 是在外层代码中定义的对象，可以用来缓存了已加载 chunk</span></span><br><span class="line"> <span class="keyword">var</span> installedChunkData = installedChunks[chunkId]</span><br><span class="line"> <span class="comment">// 判断 installedChunkData 是否为 0：表示已加载 </span></span><br><span class="line"> <span class="keyword">if</span> (installedChunkData === <span class="number">0</span>) {</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function"><span class="keyword">function</span>(<span class="params">resolve</span>) </span>{</span><br><span class="line"> resolve()</span><br><span class="line"> })</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">if</span> (installedChunkData) {</span><br><span class="line"> <span class="keyword">return</span> installedChunkData[<span class="number">2</span>]</span><br><span class="line"> } </span><br><span class="line"> <span class="comment">// 如果 chunk 还未加载，则构造对应的 Promsie 并缓存在 installedChunks 对象中</span></span><br><span class="line"> <span class="keyword">var</span> promise = <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function"><span class="keyword">function</span>(<span class="params">resolve, reject</span>) </span>{</span><br><span class="line"> installedChunkData = installedChunks[chunkId] = [resolve, reject]</span><br><span class="line"> })</span><br><span class="line"> installedChunkData[<span class="number">2</span>] = promise</span><br><span class="line"> <span class="comment">// 构造 script 标签</span></span><br><span class="line"> <span class="keyword">var</span> head = <span class="built_in">document</span>.getElementsByTagName(<span class="string">"head"</span>)[<span class="number">0</span>]</span><br><span class="line"> <span class="keyword">var</span> script = <span class="built_in">document</span>.createElement(<span class="string">"script"</span>)</span><br><span class="line"> script.type = <span class="string">"text/javascript"</span></span><br><span class="line"> script.charset = <span class="string">"utf-8"</span></span><br><span class="line"> script.async = <span class="literal">true</span></span><br><span class="line"> script.timeout = <span class="number">120000</span></span><br><span class="line"> <span class="keyword">if</span> (__webpack_require__.nc) {</span><br><span class="line"> script.setAttribute(<span class="string">"nonce"</span>, __webpack_require__.nc)</span><br><span class="line"> }</span><br><span class="line"> script.src =</span><br><span class="line"> __webpack_require__.p +</span><br><span class="line"> <span class="string">"static/js/"</span> +</span><br><span class="line"> ({ <span class="string">"0"</span>: <span class="string">"alert"</span> }[chunkId] || chunkId) +</span><br><span class="line"> <span class="string">"."</span> +</span><br><span class="line"> { <span class="string">"0"</span>: <span class="string">"620d2495"</span> }[chunkId] +</span><br><span class="line"> <span class="string">".chunk.js"</span></span><br><span class="line"> <span class="keyword">var</span> timeout = setTimeout(onScriptComplete, <span class="number">120000</span>)</span><br><span class="line"> script.onerror = script.onload = onScriptComplete</span><br><span class="line"> <span class="function"><span class="keyword">function</span> <span class="title">onScriptComplete</span>(<span class="params"></span>) </span>{</span><br><span class="line"> script.onerror = script.onload = <span class="literal">null</span></span><br><span class="line"> clearTimeout(timeout)</span><br><span class="line"> <span class="keyword">var</span> chunk = installedChunks[chunkId]</span><br><span class="line"> <span class="comment">// 如果 chunk !== 0 表示加载失败</span></span><br><span class="line"> <span class="keyword">if</span> (chunk !== <span class="number">0</span>) {</span><br><span class="line"> <span class="comment">// 返回错误信息</span></span><br><span class="line"> <span class="keyword">if</span> (chunk) {</span><br><span class="line"> chunk[<span class="number">1</span>](<span class="keyword">new</span> <span class="built_in">Error</span>(<span class="string">"Loading chunk "</span> + chunkId + <span class="string">" failed."</span>))</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">// 将此 chunk 的加载状态重置为未加载状态</span></span><br><span class="line"> installedChunks[chunkId] = <span class="literal">undefined</span></span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> head.appendChild(script)</span><br><span class="line"> <span class="comment">// 返回 fulfilled 的 Promise</span></span><br><span class="line"> <span class="keyword">return</span> promise</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>结合上面的代码来看，webpack 通过创建 script 标签来实现动态加载的，找出依赖对应的 chunk 信息，然后生成 script 标签来动态加载 chunk，每个 chunk 都有对应的状态：未加载 、 加载中、已加载 。</p>
<h2 id="Suspense"><a href="#Suspense" class="headerlink" title="Suspense"></a>Suspense</h2><p>Suspense 内部主要通过捕获组件的状态去判断如何加载，上面我们提到 React.lazy 创建的动态加载组件具有 Pending、Resolved、Rejected 三种状态，当这个组件的状态为 Pending 时显示的是 Suspense
中 fallback 的内容，只有状态变为 resolve 后才显示组件。</p>
<h2 id="Error-Boundaries-统一处理资源加载失败场景"><a href="#Error-Boundaries-统一处理资源加载失败场景" class="headerlink" title="Error Boundaries 统一处理资源加载失败场景"></a>Error Boundaries 统一处理资源加载失败场景</h2><p>如果遇到网络问题或是组件内部错误，页面的动态资源可能会加载失败，为了优雅降级，可以使用 <a href="https://react.docschina.org/docs/error-boundaries.html" target="_blank" rel="noopener">Error Boundaries</a> 来解决这个问题。
Error Boundaries 是一种组件，如果你在组件中定义了 static getDerivedStateFromError() 或 componentDidCatch() 生命周期函数，它就会成为一个 Error Boundaries 的组件。</p>
<figure class="highlight js"><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></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">ErrorBoundary</span> <span class="keyword">extends</span> <span class="title">React</span>.<span class="title">Component</span> </span>{</span><br><span class="line"> <span class="keyword">constructor</span>(props) {</span><br><span class="line"> <span class="keyword">super</span>(props);</span><br><span class="line"> <span class="keyword">this</span>.state = { <span class="attr">hasError</span>: <span class="literal">false</span> };</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">static</span> getDerivedStateFromError(error) { </span><br><span class="line"> <span class="comment">// 更新 state 使下一次渲染能够显示降级后的 UI</span></span><br><span class="line"> <span class="keyword">return</span> { <span class="attr">hasError</span>: <span class="literal">true</span> }; </span><br><span class="line"> }</span><br><span class="line"> componentDidCatch(error, errorInfo) { </span><br><span class="line"> <span class="comment">// 你同样可以将错误日志上报给服务器</span></span><br><span class="line"> logErrorToMyService(error, errorInfo);</span><br><span class="line"> }</span><br><span class="line"> render() {</span><br><span class="line"> <span class="keyword">if</span> (<span class="keyword">this</span>.state.hasError) { </span><br><span class="line"> <span class="comment">// 你可以自定义降级后的 UI 并渲染 </span></span><br><span class="line"> <span class="keyword">return</span> <span class="xml"><span class="tag"><<span class="name">h1</span>></span>对不起，发生异常，请刷新页面重试<span class="tag"></<span class="name">h1</span>></span></span>; </span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">this</span>.props.children; </span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>你可以在 componentDidCatch 或者 getDerivedStateFromError 中打印错误日志并定义显示错误信息的条件，当捕获到 error 时便可以渲染备用的组件元素，不至于导致页面资源加载失败而出现空白。</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><ErrorBoundary></span><br><span class="line"> <App /></span><br><span class="line"><<span class="regexp">/ErrorBoundary></span></span><br></pre></td></tr></table></figure>
<blockquote>
<p>需要注意的是：错误边界仅可以捕获其子组件的错误，它无法捕获其自身的错误。</p>
</blockquote>
<h2 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h2><p>React.lazy() 和 React.Suspense 的提出为现代 React 应用的性能优化和工程化提供了便捷之路。 React.lazy 可以让我们像渲染常规组件一样处理动态引入的组件，结合 Suspense 可以更优雅地展现组件懒
加载的过渡动画以及处理加载异常的场景。</p>
<blockquote>
<p>注意：React.lazy 和 Suspense 尚不可用于服务器端，如果需要服务端渲染，可遵从官方建议使用 <a href="https://github.com/gregberge/loadable-components" target="_blank" rel="noopener">Loadable Components</a>。loadable支持服务端
懒加载，而且支持库的懒加载。</p>
</blockquote>
<p><a href="https://github.com/cosyer/react-koa2-ssr" target="_blank" rel="noopener">react koa2 ssr教程</a></p>
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<p><span>本文标题:</span><a href="/react%E7%BB%84%E4%BB%B6%E5%AE%9E%E7%8E%B0%E6%8C%89%E9%9C%80%E5%8A%A0%E8%BD%BD.html">react组件实现按需加载</a></p>
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<p><span>发布时间:</span>2018年08月15日 - 10:08</p>
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