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padding-top--md padding-bottom--lg"><div class="row"><div class="col docItemCol_3FnS"><div class="docItemContainer_33ec"><article><div class="tocCollapsible_1PrD tocMobile_3Hoh"><button type="button" class="clean-btn tocCollapsibleButton_2O1e">On this page</button></div><div class="markdown"><header><h1>如何减少白屏的时间</h1></header><h2><a aria-hidden="true" tabindex="-1" class="anchor anchor__h2 anchorWithStickyNavbar_31ik" id="回答关键点"></a>回答关键点<a class="hash-link" href="#回答关键点" title="Direct link to heading">#</a></h2><p><code>资源优化</code> <code>预加载</code> <code>服务端渲染</code> <code>性能监控指标</code> <code>HTTP/2</code></p><p>前端性能优化是前端开发中一个重要环节，它包括很多内容，其中页面的白屏时间是用户最初接触到的部分，白屏时间过长会显著影响用户的留存率和转换率。</p><p>我们以一个 APP 内嵌 Webview 打开页面作为例子，来分析页面打开过程以及可优化的方向：</p><ol start="0"><li>前置条件<ul><li>性能监控指标</li></ul></li><li>APP 内点击打开页面</li><li>DNS 解析<ul><li>预解析</li><li>域名收敛</li></ul></li><li>TCP 连接<ul><li>预连接</li></ul></li><li>发送并响应请求<ul><li>HTTP/2</li></ul></li><li>浏览器解析页面<ul><li>服务端渲染</li></ul></li><li>加载资源并渲染页面<ul><li>骨架屏</li><li>资源优化</li><li>资源预加载</li></ul></li><li>请求接口，获取数据并渲染<ul><li>接口预加载</li><li>接口合并</li></ul></li></ol><h2><a aria-hidden="true" tabindex="-1" class="anchor anchor__h2 anchorWithStickyNavbar_31ik" id="知识点深入"></a>知识点深入<a class="hash-link" href="#知识点深入" title="Direct link to heading">#</a></h2><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithStickyNavbar_31ik" id="1-前端性能监控指标"></a>1. 前端性能监控指标<a class="hash-link" href="#1-前端性能监控指标" title="Direct link to heading">#</a></h3><p>性能优化的前置条件是性能有测量标准并可以被监控。常用的性能监控指标有以下几块。</p><p>Navigation Timing API：</p><ul><li>responseStart - fetchStart：收到首字节的耗时</li><li>domContentLoadedEventEnd - fetchStart：HTML 加载完成耗时</li><li>loadEventStart - fetchStart：页面完全加载耗时</li><li>domainLookupEnd - domainLookupStart：DNS 解析耗时</li><li>connectEnd - connectStart：TCP 连接耗时</li><li>responseStart - requestStart：Time to First Byte（TTFB）</li><li>responseEnd - responseStart：数据传输耗时</li><li>domInteractive - responseEnd：DOM 解析耗时</li><li>loadEventStart - domContentLoadedEventEnd：资源加载耗时（页面中同步加载的资源)</li></ul><p>Lighthouse Performance：</p><ul><li>FP（First Paint）：首次绘制</li><li>FCP（First Contentful Paint）：首次内容绘制</li><li>FMP（First Meaningful Paint）：首次有效绘制</li><li>LCP（Largest Contentful Paint）：最大可见元素绘制</li><li>TTI（Time to Interactive）：可交互时间</li><li>TTFB（Time to First Byte）：浏览器接收第一个字节的时间</li></ul><p>除了上面之外，UC 内核也有一套性能监控指标：</p><ul><li>T0：Blink 收到 HTTP Head 的时间。</li><li>T1：首屏有内容显示的时间。</li><li>T2：首屏全部显示出来的时间。</li></ul><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithStickyNavbar_31ik" id="2-dns-解析优化"></a>2. DNS 解析优化<a class="hash-link" href="#2-dns-解析优化" title="Direct link to heading">#</a></h3><p>DNS 解析优化是性能优化重要的一环，DNS 的作用是根据域名获取对应的 IP 地址，获取之后后续的 HTTP 流程才能进行下去。</p><p>DNS 解析是一个开销较大的过程，一次 DNS 解析通常需要耗费几十到上百毫秒，而在移动端网络或其他弱网环境下 DNS 解析延迟会更加严重，对 DNS 解析优化则可以减少这一步骤的耗时。</p><h4><a aria-hidden="true" tabindex="-1" class="anchor anchor__h4 anchorWithStickyNavbar_31ik" id="21-dns-预解析"></a>2.1 DNS 预解析<a class="hash-link" href="#21-dns-预解析" title="Direct link to heading">#</a></h4><p>我们可以通过 DNS 预解析的方式提前获取 IP 地址，以缩短后续请求的响应时间。</p><p>前端可以通过 <code>dns-prefetch</code> 预解析，具体方式如下：</p><div class="codeBlockContainer_K1bP"><div class="codeBlockContent_hGly html"><pre tabindex="0" class="prism-code language-html codeBlock_23N8 thin-scrollbar" style="color:#393A34;background-color:#f6f8fa"><code class="codeBlockLines_39YC"><span class="token-line" style="color:#393A34"><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">link</span><span class="token tag" style="color:#00009f"> </span><span class="token tag attr-name" style="color:#00a4db">rel</span><span class="token tag attr-value punctuation attr-equals" style="color:#393A34">=</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag attr-value" style="color:#e3116c">dns-prefetch</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag" style="color:#00009f"> </span><span class="token tag attr-name" style="color:#00a4db">href</span><span class="token tag attr-value punctuation attr-equals" style="color:#393A34">=</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag attr-value" style="color:#e3116c">https://hzfe.org/</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag" style="color:#00009f"> </span><span class="token tag punctuation" style="color:#393A34">/></span></span></code></pre><button type="button" aria-label="Copy code to clipboard" class="copyButton_Ue-o clean-btn">Copy</button></div></div><h4><a aria-hidden="true" tabindex="-1" class="anchor anchor__h4 anchorWithStickyNavbar_31ik" id="22-域名收敛"></a>2.2 域名收敛<a class="hash-link" href="#22-域名收敛" title="Direct link to heading">#</a></h4><p>域名收敛的目的是减少页面中域名的数量，从而减少所需的 DNS 解析次数，最终减少页面的 DNS 解析过程的耗时，加快页面加载速度。</p><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithStickyNavbar_31ik" id="3-tcp-连接优化"></a>3. TCP 连接优化<a class="hash-link" href="#3-tcp-连接优化" title="Direct link to heading">#</a></h3><p>前端可以通过 <code>preconnect</code> 在请求发送前预先执行一些操作，这些操作包括 DNS 解析，TCP 握手 和 TLS 协商。具体方式如下：</p><div class="codeBlockContainer_K1bP"><div class="codeBlockContent_hGly html"><pre tabindex="0" class="prism-code language-html codeBlock_23N8 thin-scrollbar" style="color:#393A34;background-color:#f6f8fa"><code class="codeBlockLines_39YC"><span class="token-line" style="color:#393A34"><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">link</span><span class="token tag" style="color:#00009f"> </span><span class="token tag attr-name" style="color:#00a4db">href</span><span class="token tag attr-value punctuation attr-equals" style="color:#393A34">=</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag attr-value" style="color:#e3116c">https://hzfe.org</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag" style="color:#00009f"> </span><span class="token tag attr-name" style="color:#00a4db">rel</span><span class="token tag attr-value punctuation attr-equals" style="color:#393A34">=</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag attr-value" style="color:#e3116c">preconnect</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag" style="color:#00009f"> </span><span class="token tag punctuation" style="color:#393A34">/></span></span></code></pre><button type="button" aria-label="Copy code to clipboard" class="copyButton_Ue-o clean-btn">Copy</button></div></div><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithStickyNavbar_31ik" id="4-请求优化"></a>4. 请求优化<a class="hash-link" href="#4-请求优化" title="Direct link to heading">#</a></h3><p>通过使用 HTTP/2 协议，可以依赖 HTTP/2 的多路复用、首部压缩、二进制分帧和服务端推送等特性，从而加快整体请求的响应速度，加快页面的渲染展示。</p><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithStickyNavbar_31ik" id="5-页面解析优化"></a>5. 页面解析优化<a class="hash-link" href="#5-页面解析优化" title="Direct link to heading">#</a></h3><p>浏览器获取 HTML 文件后，需要对 HTML 解析，然后才能开始渲染页面，这个过程中页面也是处于白屏状态。通过对这一过程进行优化可以加快页面的渲染展示。</p><h4><a aria-hidden="true" tabindex="-1" class="anchor anchor__h4 anchorWithStickyNavbar_31ik" id="51-服务端渲染server-side-rendering"></a>5.1 服务端渲染（Server-Side Rendering）<a class="hash-link" href="#51-服务端渲染server-side-rendering" title="Direct link to heading">#</a></h4><p>目前流行的前后端分离的开发模式，由于前端需要等待 JS 文件和接口加载完成之后才能渲染页面，导致白屏时间变长。服务端渲染是指在服务端将页面的渲染逻辑处理好，然后将处理好的 HTML 直接返回给前端展示。这样即可减少页面白屏的时间。</p><h4><a aria-hidden="true" tabindex="-1" class="anchor anchor__h4 anchorWithStickyNavbar_31ik" id="52-预渲染"></a>5.2 预渲染<a class="hash-link" href="#52-预渲染" title="Direct link to heading">#</a></h4><p>除了服务端渲染之外，还可以在前端打包时使用 prerender-spa-plugin 之类的插件进行简单的预渲染，减少页面白屏的时间。</p><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithStickyNavbar_31ik" id="6-资源加载优化和页面渲染优化"></a>6. 资源加载优化和页面渲染优化<a class="hash-link" href="#6-资源加载优化和页面渲染优化" title="Direct link to heading">#</a></h3><p>浏览器解析 HTML 的同时会加载相关的资源，通过对资源的加载过程进行优化也可以减少页面的白屏时间。</p><h4><a aria-hidden="true" tabindex="-1" class="anchor anchor__h4 anchorWithStickyNavbar_31ik" id="61-骨架屏"></a>6.1 骨架屏<a class="hash-link" href="#61-骨架屏" title="Direct link to heading">#</a></h4><p>骨架屏是在需要等待加载内容的位置提供一些图形组合占位，提前给用户描述页面的基础结构，等待数据加载完成之后，再替换成实际的内容。</p><p>骨架屏可以在数据加载前，提前渲染页面，缩短白屏时间，提升用户体验。</p><h4><a aria-hidden="true" tabindex="-1" class="anchor anchor__h4 anchorWithStickyNavbar_31ik" id="62-静态资源优化"></a>6.2 静态资源优化<a class="hash-link" href="#62-静态资源优化" title="Direct link to heading">#</a></h4><p>静态资源的优化主要分为两个方向：减小资源大小，加快资源加载速度。</p><p><strong>减小资源大小</strong></p><ul><li>Gzip 压缩文件</li><li>JS 文件拆分，动态加载</li></ul><p><strong>加快资源加载速度</strong></p><ul><li>CDN（Content Delivery Network）</li><li>HTTP/2</li></ul><h4><a aria-hidden="true" tabindex="-1" class="anchor anchor__h4 anchorWithStickyNavbar_31ik" id="63-资源预加载"></a>6.3 资源预加载<a class="hash-link" href="#63-资源预加载" title="Direct link to heading">#</a></h4><p><strong>prefetch</strong></p><p>前端可以使用 prefetch 来指定提前获取之后需要使用到的资源，浏览器将会在空闲的时候加载资源，例如:</p><div class="codeBlockContainer_K1bP"><div class="codeBlockContent_hGly html"><pre tabindex="0" class="prism-code language-html codeBlock_23N8 thin-scrollbar" style="color:#393A34;background-color:#f6f8fa"><code class="codeBlockLines_39YC"><span class="token-line" style="color:#393A34"><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">link</span><span class="token tag" style="color:#00009f"> </span><span class="token tag attr-name" style="color:#00a4db">rel</span><span class="token tag attr-value punctuation attr-equals" style="color:#393A34">=</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag attr-value" style="color:#e3116c">prefetch</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag" style="color:#00009f"> </span><span class="token tag attr-name" style="color:#00a4db">href</span><span class="token tag attr-value punctuation attr-equals" style="color:#393A34">=</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag attr-value" style="color:#e3116c">https://hzfe.org/index.js</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag" style="color:#00009f"> </span><span class="token tag attr-name" style="color:#00a4db">as</span><span class="token tag attr-value punctuation attr-equals" style="color:#393A34">=</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag attr-value" style="color:#e3116c">script</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag" style="color:#00009f"> </span><span class="token tag punctuation" style="color:#393A34">/></span></span></code></pre><button type="button" aria-label="Copy code to clipboard" class="copyButton_Ue-o clean-btn">Copy</button></div></div><p><strong>preload</strong></p><p>前端可以使用 preload 来指定提前获取之后需要使用到的资源，浏览器将会立即加载对应资源，在解析到对应资源时即可立即执行，例如:</p><div class="codeBlockContainer_K1bP"><div class="codeBlockContent_hGly html"><pre tabindex="0" class="prism-code language-html codeBlock_23N8 thin-scrollbar" style="color:#393A34;background-color:#f6f8fa"><code class="codeBlockLines_39YC"><span class="token-line" style="color:#393A34"><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">link</span><span class="token tag" style="color:#00009f"> </span><span class="token tag attr-name" style="color:#00a4db">rel</span><span class="token tag attr-value punctuation attr-equals" style="color:#393A34">=</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag attr-value" style="color:#e3116c">preload</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag" style="color:#00009f"> </span><span class="token tag attr-name" style="color:#00a4db">href</span><span class="token tag attr-value punctuation attr-equals" style="color:#393A34">=</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag attr-value" style="color:#e3116c">https://hzfe.org/index.js</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag" style="color:#00009f"> </span><span class="token tag attr-name" style="color:#00a4db">as</span><span class="token tag attr-value punctuation attr-equals" style="color:#393A34">=</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag attr-value" style="color:#e3116c">script</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag" style="color:#00009f"> </span><span class="token tag punctuation" style="color:#393A34">/></span></span></code></pre><button type="button" aria-label="Copy code to clipboard" class="copyButton_Ue-o clean-btn">Copy</button></div></div><p><strong>quicklink</strong></p><p>quicklink 是 Google 开源的预加载库，quicklink 会判断链接进入视口之后，在闲时预加载。quicklink 实际上加速的是次级页面。</p><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithStickyNavbar_31ik" id="7-接口请求优化"></a>7. 接口请求优化<a class="hash-link" href="#7-接口请求优化" title="Direct link to heading">#</a></h3><p>浏览器在加载完 HTML 和资源之后，一般需要请求接口获取数据之后才会完整渲染页面，对接口请求进行优化也可加快页面的展示。</p><p><strong>接口合并</strong></p><p>过多的接口请求会影响页面初始化时的渲染过程，可以通过增加一层中间层合并部分请求，达到加速页面展示的目的。</p><h2><a aria-hidden="true" tabindex="-1" class="anchor anchor__h2 anchorWithStickyNavbar_31ik" id="扩展阅读"></a>扩展阅读<a class="hash-link" href="#扩展阅读" title="Direct link to heading">#</a></h2><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithStickyNavbar_31ik" id="1-native-相关优化"></a>1. Native 相关优化<a class="hash-link" href="#1-native-相关优化" title="Direct link to heading">#</a></h3><p><strong>WebView 容器预加载</strong></p><p>内嵌在 APP 内的网页白屏时间实际还依赖 APP 的 WebView 初始化时间，所以通过对 APP 的 WebView 容器进行优化也可以减少页面的白屏时间，例如预热 WebView，即在 APP 打开之后的某一时间点，预先加载一个或多个 WebView 容器，在用户点击打开网页时直接使用预热好的 WebView。</p><p><strong>DNS 优化</strong></p><p>APP 可以在打开之后预解析网页所需的一些域名，在打开网页时即可直接使用 DNS 缓存。</p><p><strong>资源预加载</strong></p><p>APP 可以将网页中所需的资源预加载到本地，在网页请求资源时直接拦截并返回本地文件，即可加快网页加载速度，减少白屏时间。</p><p><strong>接口预加载</strong></p><p>APP 可以通过配置文件获取网页需要提前发起请求的接口，在用户进入页面时同步发起请求，即可在页面载入完成之后直接使用，减少白屏时间。</p><h2><a aria-hidden="true" tabindex="-1" class="anchor anchor__h2 anchorWithStickyNavbar_31ik" id="参考资料"></a>参考资料<a class="hash-link" href="#参考资料" title="Direct link to heading">#</a></h2><ol><li><a href="https://developer.mozilla.org/en-US/docs/Web/API/Navigation_timing_API" target="_blank" rel="noopener noreferrer">Navigation Timeing API</a></li></ol></div></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Docs pages navigation"><div class="pagination-nav__item"><a class="pagination-nav__link" href="/awesome-interview/book3/algorithm-binary-tree-k"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">« 算法：二叉搜索树的第 k 个结点</div></a></div><div class="pagination-nav__item pagination-nav__item--next"></div></nav></div></div><div class="col col--3"><div class="tableOfContents_35-E thin-scrollbar"><ul class="table-of-contents table-of-contents__left-border"><li><a href="#回答关键点" class="table-of-contents__link">回答关键点</a></li><li><a href="#知识点深入" class="table-of-contents__link">知识点深入</a><ul><li><a href="#1-前端性能监控指标" class="table-of-contents__link">1. 前端性能监控指标</a></li><li><a href="#2-dns-解析优化" class="table-of-contents__link">2. DNS 解析优化</a></li><li><a href="#3-tcp-连接优化" class="table-of-contents__link">3. TCP 连接优化</a></li><li><a href="#4-请求优化" class="table-of-contents__link">4. 请求优化</a></li><li><a href="#5-页面解析优化" class="table-of-contents__link">5. 页面解析优化</a></li><li><a href="#6-资源加载优化和页面渲染优化" class="table-of-contents__link">6. 资源加载优化和页面渲染优化</a></li><li><a href="#7-接口请求优化" class="table-of-contents__link">7. 接口请求优化</a></li></ul></li><li><a href="#扩展阅读" class="table-of-contents__link">扩展阅读</a><ul><li><a href="#1-native-相关优化" class="table-of-contents__link">1. Native 相关优化</a></li></ul></li><li><a href="#参考资料" class="table-of-contents__link">参考资料</a></li></ul></div></div></div><div class="row"><div class="col"><div class="react-utterences"><div>Loading script...</div></div></div><div class="col col--3"></div></div></div></main></div></div></div>
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