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class="breadcrumbs__link">模拟题一</span><meta itemprop="position" content="1"></li><li itemscope="" itemprop="itemListElement" itemtype="https://schema.org/ListItem" class="breadcrumbs__item breadcrumbs__item--active"><span class="breadcrumbs__link" itemprop="name">浏览器：浏览器的重排重绘</span><meta itemprop="position" content="2"></li></ul></nav><div class="tocCollapsible_ETCw theme-doc-toc-mobile tocMobile_ITEo"><button type="button" class="clean-btn tocCollapsibleButton_TO0P">On this page</button></div><div class="theme-doc-markdown markdown"><h1>浏览器的重排重绘</h1><h2 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="相关问题">相关问题<a href="#相关问题" class="hash-link" aria-label="Direct link to 相关问题" title="Direct link to 相关问题"></a></h2><ul><li>如何提升页面渲染性能</li><li>如何减少页面重排重绘</li><li>哪些行为会引起重排/重绘</li></ul><h2 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="回答关键点">回答关键点<a href="#回答关键点" class="hash-link" aria-label="Direct link to 回答关键点" title="Direct link to 回答关键点"></a></h2><p><code>渲染性能</code> <code>Layout</code> <code>Paint</code></p><p>浏览器渲染大致分为四个阶段，其中在解析 HTML 后，会依次进入 Layout 和 Paint 阶段。<strong>样式或节点的更改，以及对布局信息的访问等</strong>，都有可能导致重排和重绘。而重排和重绘的过程在<strong>主线程</strong>中进行，这意味着不合理的重排重绘会导致<strong>渲染卡顿，用户交互滞后</strong>等性能问题。</p><h2 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="知识点深入">知识点深入<a href="#知识点深入" class="hash-link" aria-label="Direct link to 知识点深入" title="Direct link to 知识点深入"></a></h2><h3 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="1-什么是重排重绘">1. 什么是重排重绘<a href="#1-什么是重排重绘" class="hash-link" aria-label="Direct link to 1. 什么是重排重绘" title="Direct link to 1. 什么是重排重绘"></a></h3><p><img loading="lazy" src="https://user-images.githubusercontent.com/17002181/126033732-d5002255-1c88-4dee-9371-da166aacdca9.png" alt="image" class="img_ev3q"></p><ol><li>Parse HTML：相关引擎分别解析文档和样式表以及脚本，生成 DOM 和 CSSOM ，最终合成为 Render 树。</li><li>Layout：浏览器通过 Render 树中的信息，以递归的形式计算出每个节点的尺寸大小和在页面中的具体位置。</li><li>Paint：浏览器将 Render 树中的节点转换成在屏幕上绘制实际像素的指令，这个过程发生在多个图层上。</li><li>Composite：浏览器将所有层按照一定顺序合并为一个图层并绘制在屏幕上。</li></ol><p>图中所示步骤为浏览器渲染的关键路径。浏览器从获取文档、样式、脚本等内容，到最终渲染结果到屏幕上，通常需要经过如图所示的步骤。而 DOM 或 CSSOM 被修改，会导致浏览器重复执行图中的步骤。重排和重绘，本质上指的就是触发 Layout 和 Paint 的过程，且重排必定导致重绘。</p><h4 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="引起重排重绘的常见操作">引起重排/重绘的常见操作<a href="#引起重排重绘的常见操作" class="hash-link" aria-label="Direct link to 引起重排/重绘的常见操作" title="Direct link to 引起重排/重绘的常见操作"></a></h4><ol><li>外观有变化时，会导致<strong>重绘</strong>。相关的样式属性如 <code>color</code> <code>opacity</code> 等。</li><li>布局结构或节点内容变化时，会导致<strong>重排</strong>。相关的样式属性如 <code>height</code> <code>float</code> <code>position</code> 等。<ul><li>盒子尺寸和类型。</li><li>定位方案（正常流、浮动和绝对定位）。</li><li>文档树中元素之间的关系。</li><li>外部信息（如视口大小等）。</li></ul></li><li>获取布局信息时，会导致<strong>重排</strong>。相关的方法属性如 <code>offsetTop</code> <code>getComputedStyle</code> 等。</li></ol><h3 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="2-如何减少重排重绘">2. 如何减少重排重绘<a href="#2-如何减少重排重绘" class="hash-link" aria-label="Direct link to 2. 如何减少重排重绘" title="Direct link to 2. 如何减少重排重绘"></a></h3><p><img loading="lazy" src="https://user-images.githubusercontent.com/17002181/126033739-3e689aab-fcd8-4928-9e6d-6a047ab2a8c9.png" alt="image" class="img_ev3q"></p><h4 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="意义">意义<a href="#意义" class="hash-link" aria-label="Direct link to 意义" title="Direct link to 意义"></a></h4><p>大多数显示器的刷新率是 60FPS（frames per second）。理想情况下，浏览器需要在 1/60 秒内完成渲染阶段并交付一帧。这样用户就会看到一个交互流畅的页面。</p><p>在交互阶段，页面更新（一般是通过执行 JavaScript 来触发）通常会触发重排和重绘。为了提升浏览器渲染效率，应当尽可能减少重绘重排（跳过 Layout/Paint 步骤），从而降低浏览器渲染耗费的时间，将内容尽快渲染到屏幕上。</p><h4 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="解决方案">解决方案<a href="#解决方案" class="hash-link" aria-label="Direct link to 解决方案" title="Direct link to 解决方案"></a></h4><ol><li>对 DOM 进行批量写入和读取（通过虚拟 DOM 或者 DocumentFragment 实现）。</li><li>避免对样式频繁操作，了解常用样式属性触发 Layout / Paint / Composite 的<a href="https://csstriggers.com/" target="_blank" rel="noopener noreferrer">机制</a>，合理使用样式。</li><li>合理利用特殊样式属性（如 transform: translateZ(0) 或者 will-change），将渲染层提升为合成层，开启 GPU 加速，提高页面性能。</li><li>使用变量对布局信息（如 clientTop）进行缓存，避免因频繁读取布局信息而触发重排和重绘。</li></ol><p>另外，可以借助 DevTools Performance 面板来查看产生重排重绘任务占用主线程的情况和调用代码。</p><h2 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="参考资料">参考资料<a href="#参考资料" class="hash-link" aria-label="Direct link to 参考资料" title="Direct link to 参考资料"></a></h2><ol><li><a href="https://developers.google.com/web/fundamentals/performance/critical-rendering-path/render-tree-construction?hl=zh-cn" target="_blank" rel="noopener noreferrer">渲染树构建、布局及绘制</a></li><li><a href="https://developers.google.com/web/fundamentals/performance/rendering/avoid-large-complex-layouts-and-layout-thrashing" target="_blank" rel="noopener noreferrer">避免大型、复杂的布局和布局抖动</a></li><li><a href="https://csstriggers.com/" target="_blank" rel="noopener noreferrer">CSS 属性触发布局、绘制及合成的数据</a></li><li><a href="https://gist.github.com/paulirish/5d52fb081b3570c81e3a" target="_blank" rel="noopener noreferrer">What forces layout / reflow</a></li></ol></div></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Docs pages navigation"><a class="pagination-nav__link pagination-nav__link--prev" href="/awesome-interview/book1/browser-cross-origin"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">浏览器：浏览器跨域</div></a><a class="pagination-nav__link pagination-nav__link--next" href="/awesome-interview/book1/engineer-webpack-workflow"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">工程化：webpack 工作流程</div></a></nav></div></div><div class="col col--3"><div class="tableOfContents_bqdL thin-scrollbar theme-doc-toc-desktop"><ul class="table-of-contents table-of-contents__left-border"><li><a href="#相关问题" class="table-of-contents__link toc-highlight">相关问题</a></li><li><a href="#回答关键点" class="table-of-contents__link toc-highlight">回答关键点</a></li><li><a href="#知识点深入" class="table-of-contents__link toc-highlight">知识点深入</a><ul><li><a href="#1-什么是重排重绘" class="table-of-contents__link toc-highlight">1. 什么是重排重绘</a><ul><li><a href="#引起重排重绘的常见操作" class="table-of-contents__link toc-highlight">引起重排/重绘的常见操作</a></li></ul></li><li><a href="#2-如何减少重排重绘" class="table-of-contents__link toc-highlight">2. 如何减少重排重绘</a><ul><li><a href="#意义" class="table-of-contents__link toc-highlight">意义</a></li><li><a href="#解决方案" class="table-of-contents__link toc-highlight">解决方案</a></li></ul></li></ul></li><li><a href="#参考资料" class="table-of-contents__link toc-highlight">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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