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- 2-4 Resolve
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- 2-5 Plugins
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- 2-6 DevServer
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- 2-7 其它配置项
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- 2-8 整体配置结构
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- 2-9 多种配置类型
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- 2-10 配置总结
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- 3-1 使用 ES6 语言
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- 3-2 使用 TypeScript 语言
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- 3-3 使用 Flow 检查器
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- 3-4 使用 SCSS
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- 3-5 使用 PostCSS
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- 3-6 使用 React 框架
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- 3-7 使用 Vue 框架
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- 3-8 使用 Angular2 框架
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- 3-9 为单页应用生成 HTML
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- 3-10 管理多个单页应用
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- 3-11 构建同构应用
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- 3-12 构建 Electron 应用
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- 3-13 构建 Npm 模块
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- 3-14 构建离线应用
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- 3-15 搭配 Npm Script
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- 3-16 检查代码
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- 3-17 通过 Node.js API 启动 Webpack
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- 3-18 使用 Webpack Dev Middleware
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- 3-19 加载图片
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- 3-20 加载SVG
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- 3-21 加载 Source Map
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- 3-22 实战总结
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- 第4章 优化
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- <li class="chapter " data-level="1.6.1" data-path="4-1缩小文件搜索范围.html">
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- 4-1 缩小文件搜索范围
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- 4-2 使用 DllPlugin
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- 4-3 使用 HappyPack
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- 4-4 使用 ParallelUglifyPlugin
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- 4-5 使用自动刷新
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- 4-6 开启模块热替换
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- 4-7 区分环境
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- 4-8 压缩代码
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- <h1 id="4-6-开启模块热替换">4-6 开启模块热替换</h1>
- <p>要做到实时预览,除了在<a href="4-5使用自动刷新.html">4-5使用自动刷新</a>中介绍的刷新整个网页外,DevServer 还支持一种叫做模块热替换(Hot Module Replacement)的技术可在不刷新整个网页的情况下做到超灵敏的实时预览。
- 原理是当一个源码发生变化时,只重新编译发生变化的模块,再用新输出的模块替换掉浏览器中对应的老模块。</p>
- <p>模块热替换技术的优势有:</p>
- <ul>
- <li>实时预览反应更快,等待时间更短。</li>
- <li>不刷新浏览器能保留当前网页的运行状态,例如在使用 Redux 来管理数据的应用中搭配模块热替换能做到代码更新时 Redux 中的数据还保持不变。</li>
- </ul>
- <p>总的来说模块热替换技术很大程度上的提高了开发效率和体验。</p>
- <h2 id="模块热替换的原理">模块热替换的原理</h2>
- <p>模块热替换的原理和自动刷新原理类似,都需要往要开发的网页中注入一个代理客户端用于连接 DevServer 和网页,
- 不同在于模块热替换独特的模块替换机制。</p>
- <p>DevServer 默认不会开启模块热替换模式,要开启该模式,只需在启动时带上参数 <code>--hot</code>,完整命令是 <code>webpack-dev-server --hot</code>。</p>
- <blockquote>
- <p>除了通过在启动时带上 <code>--hot</code> 参数,还可以通过接入 Plugin 实现,相关代码如下:</p>
- <pre><code class="lang-js"><span class="hljs-keyword">const</span> HotModuleReplacementPlugin = <span class="hljs-built_in">require</span>(<span class="hljs-string">'webpack/lib/HotModuleReplacementPlugin'</span>);
- <span class="hljs-built_in">module</span>.exports = {
- entry:{
- <span class="hljs-comment">// 为每个入口都注入代理客户端</span>
- main:[<span class="hljs-string">'webpack-dev-server/client?http://localhost:8080/'</span>, <span class="hljs-string">'webpack/hot/dev-server'</span>,<span class="hljs-string">'./src/main.js'</span>],
- },
- plugins: [
- <span class="hljs-comment">// 该插件的作用就是实现模块热替换,实际上当启动时带上 `--hot` 参数,会注入该插件,生成 .hot-update.json 文件。</span>
- <span class="hljs-keyword">new</span> HotModuleReplacementPlugin(),
- ],
- devServer:{
- <span class="hljs-comment">// 告诉 DevServer 要开启模块热替换模式</span>
- hot: <span class="hljs-literal">true</span>,
- }
- };
- </code></pre>
- <p>在启动 Webpack 时带上参数 <code>--hot</code> 其实就是自动为你完成以上配置。</p>
- </blockquote>
- <p>启动后日志如下:</p>
- <pre><code>> webpack-dev-server --hot
- Project is running at http://localhost:8080/
- webpack output is served from /
- webpack: wait until bundle finished: /
- webpack: wait until bundle finished: /bundle.js
- Hash: fe62ac6b753c1d98961b
- Version: webpack 3.5.5
- Time: 3563ms
- Asset Size Chunks Chunk Names
- bundle.js 1.11 MB 0 [emitted] [big] main
- bundle.js.map 1.33 MB 0 [emitted] main
- [50] (webpack)/hot/log.js 1.04 kB {0} [built]
- [118] multi (webpack)-dev-server/client?http://localhost:8080 webpack/hot/dev-server ./main.js 52 bytes {0} [built]
- [119] (webpack)-dev-server/client?http://localhost:8080 5.83 kB {0} [built]
- [120] ./node_modules/url/url.js 23.3 kB {0} [built]
- [126] ./node_modules/strip-ansi/index.js 161 bytes {0} [built]
- [128] ./node_modules/loglevel/lib/loglevel.js 6.74 kB {0} [built]
- [129] (webpack)-dev-server/client/socket.js 856 bytes {0} [built]
- [161] (webpack)-dev-server/client/overlay.js 3.6 kB {0} [built]
- [166] (webpack)/hot nonrecursive ^\.\/log$ 170 bytes {0} [built]
- [168] (webpack)/hot/dev-server.js 1.61 kB {0} [built]
- [169] (webpack)/hot/log-apply-result.js 1.31 kB {0} [built]
- [170] ./main.js 2.35 kB {0} [built]
- + 262 hidden modules
- </code></pre><p>可以看出 <code>bundle.js</code> 代理客户端相关的代码包含九个文件:</p>
- <pre><code> [119] (webpack)-dev-server/client?http://localhost:8080 5.83 kB {0} [built]
- [120] ./node_modules/url/url.js 23.3 kB {0} [built]
- [126] ./node_modules/strip-ansi/index.js 161 bytes {0} [built]
- [128] ./node_modules/loglevel/lib/loglevel.js 6.74 kB {0} [built]
- [129] (webpack)-dev-server/client/socket.js 856 bytes {0} [built]
- [161] (webpack)-dev-server/client/overlay.js 3.6 kB {0} [built]
- [166] (webpack)/hot nonrecursive ^\.\/log$ 170 bytes {0} [built]
- [168] (webpack)/hot/dev-server.js 1.61 kB {0} [built]
- [169] (webpack)/hot/log-apply-result.js 1.31 kB {0} [built]
- </code></pre><p>相比于自动刷新的代理客户端,多出了后三个用于模块热替换的文件,也就是说代理客户端更大了。</p>
- <p>修改源码 <code>main.css</code> 文件后,新输出了如下日志:</p>
- <pre><code>webpack: Compiling...
- Hash: 18f81c959118f6230623
- Version: webpack 3.5.5
- Time: 551ms
- Asset Size Chunks Chunk Names
- bundle.js 1.11 MB 0 [emitted] [big] main
- 0.ea11a51f97f2b52bca7d.hot-update.js 353 bytes 0 [emitted] main
- ea11a51f97f2b52bca7d.hot-update.json 43 bytes [emitted]
- bundle.js.map 1.33 MB 0 [emitted] main
- 0.ea11a51f97f2b52bca7d.hot-update.js.map 577 bytes 0 [emitted] main
- [68] ./node_modules/css-loader!./main.css 217 bytes {0} [built]
- [166] (webpack)/hot nonrecursive ^\.\/log$ 170 bytes {0} [built]
- + 275 hidden modules
- webpack: Compiled successfully.
- </code></pre><p>DevServer 新生成了一个用于替换老模块的补丁文件 <code>0.ea11a51f97f2b52bca7d.hot-update.js</code>,同时在浏览器开发工具中也能看到请求这个补丁的抓包:</p>
- <p><img src="img/4-6hot-patch.png" alt="图4.5.3 模块热替换模式下的补丁"></p>
- <p>可见补丁中包含了 <code>main.css</code> 文件新编译出来 CSS 代码,网页中的样式也立刻变成了源码中描述的那样。</p>
- <p>但当你修改 <code>main.js</code> 文件时,会发现模块热替换没有生效,而是整个页面被刷新了,为什么修改 <code>main.js</code> 文件时会这样呢?</p>
- <p>Webpack 为了让使用者在使用了模块热替换功能时能灵活地控制老模块被替换时的逻辑,可以在源码中定义一些代码去做相应的处理。</p>
- <p>把的 <code>main.js</code> 文件改为如下:</p>
- <pre><code class="lang-js"><span class="hljs-keyword">import</span> React <span class="hljs-keyword">from</span> <span class="hljs-string">'react'</span>;
- <span class="hljs-keyword">import</span> { render } <span class="hljs-keyword">from</span> <span class="hljs-string">'react-dom'</span>;
- <span class="hljs-keyword">import</span> { AppComponent } <span class="hljs-keyword">from</span> <span class="hljs-string">'./AppComponent'</span>;
- <span class="hljs-keyword">import</span> <span class="hljs-string">'./main.css'</span>;
- render(<span class="xml"><span class="hljs-tag"><<span class="hljs-name">AppComponent</span>/></span></span>, <span class="hljs-built_in">window</span>.document.getElementById(<span class="hljs-string">'app'</span>));
- <span class="hljs-comment">// 只有当开启了模块热替换时 module.hot 才存在</span>
- <span class="hljs-keyword">if</span> (<span class="hljs-built_in">module</span>.hot) {
- <span class="hljs-comment">// accept 函数的第一个参数指出当前文件接受哪些子模块的替换,这里表示只接受 ./AppComponent 这个子模块</span>
- <span class="hljs-comment">// 第2个参数用于在新的子模块加载完毕后需要执行的逻辑</span>
- <span class="hljs-built_in">module</span>.hot.accept([<span class="hljs-string">'./AppComponent'</span>], () => {
- <span class="hljs-comment">// 新的 AppComponent 加载成功后重新执行下组建渲染逻辑</span>
- render(<span class="xml"><span class="hljs-tag"><<span class="hljs-name">AppComponent</span>/></span></span>, <span class="hljs-built_in">window</span>.document.getElementById(<span class="hljs-string">'app'</span>));
- });
- }
- </code></pre>
- <p>其中的 <code>module.hot</code> 是当开启模块热替换后注入到全局的 API,用于控制模块热替换的逻辑。</p>
- <p>现在修改 <code>AppComponent.js</code> 文件,把 <code>Hello,Webpack</code> 改成 <code>Hello,World</code>,你会发现模块热替换生效了。
- 但是当你编辑 <code>main.js</code> 时,你会发现整个网页被刷新了。为什么修改这两个文件会有不一样的表现呢?</p>
- <p>当子模块发生更新时,更新事件会一层层往上传递,也就是从 <code>AppComponent.js</code> 文件传递到 <code>main.js</code> 文件,
- 直到有某层的文件接受了当前变化的模块,也就是 <code>main.js</code> 文件中定义的 <code>module.hot.accept(['./AppComponent'], callback)</code>,
- 这时就会调用 <code>callback</code> 函数去执行自定义逻辑。如果事件一直往上抛到最外层都没有文件接受它,就会直接刷新网页。</p>
- <p>那为什么没有地方接受过 <code>.css</code> 文件,但是修改所有的 <code>.css</code> 文件都会触发模块热替换呢?
- 原因在于 <code>style-loader</code> 会注入用于接受 CSS 的代码。</p>
- <blockquote>
- <p>请不要把模块热替换技术用于线上环境,它是专门为提升开发效率生的。</p>
- </blockquote>
- <h2 id="优化模块热替换">优化模块热替换</h2>
- <p>在发生模块热替换时,你会在浏览器的控制台中看到类似这样的日志:</p>
- <p><img src="img/4-6hmr-log.png" alt="图4.5.4 模块热替换浏览器日志"></p>
- <p>其中的 <code>Updated modules: 68</code> 是指 ID 为68的模块被替换了,这对开发者来说很不友好,因为开发者不知道 ID 和模块之间的对应关系,最好是把替换了的模块的名称输出出来。
- Webpack 内置的 NamedModulesPlugin 插件可以解决该问题,修改 Webpack 配置文件接入该插件:</p>
- <pre><code class="lang-js"><span class="hljs-keyword">const</span> NamedModulesPlugin = <span class="hljs-built_in">require</span>(<span class="hljs-string">'webpack/lib/NamedModulesPlugin'</span>);
- <span class="hljs-built_in">module</span>.exports = {
- plugins: [
- <span class="hljs-comment">// 显示出被替换模块的名称</span>
- <span class="hljs-keyword">new</span> NamedModulesPlugin(),
- ],
- };
- </code></pre>
- <p>重启构建后你会发现浏览器中的日志更加友好了:</p>
- <p><img src="img/4-6hmr-log-named.png" alt="图4.5.4 现实出被替换模块的浏览器日志"></p>
- <p>除此之外,模块热替换还面临着和自动刷新一样的性能问题,因为它们都需要监听文件变化和注入客户端。
- 要优化模块热替换的构建性能,思路和在<a href="4-5使用自动刷新.html">4-5 使用自动刷新</a>中提到的很类似:监听更少的文件,忽略掉 <code>node_modules</code> 目录下的文件。
- 但是其中提到的关闭默认的 inline 模式手动注入代理客户端的优化方法不能用于在使用模块热替换的情况下,
- 原因在于模块热替换的运行依赖在每个 Chunk 中都包含代理客户端的代码。</p>
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