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<code>时间复杂度O(n)</code></p><p>现代网站大多具有复杂布局，大量的节点和交互操作等特征，直接操作 DOM 方法不当带来的性能问题不可忽视。虚拟 DOM 的本质是 JavaScript 对象，它可以代表 DOM 的一部分特征，是 DOM 的抽象简化版本。通过预先操作虚拟 DOM，在某个时机找出和真实 DOM 之间的差异部分并重新渲染，来提升操作真实 DOM 的性能和效率。</p><p>为达到这个目的，还需要关注两个问题：什么时候重新渲染，怎么高效选择重新渲染的范围。找出需要重新渲染的范围，就是 Diff 的过程。React 和 Vue 的 Diff 算法思路基本一致，只对同层节点进行比较，利用唯一标识符对节点进行区分。</p><h2><a aria-hidden="true" tabindex="-1" class="anchor anchor__h2 anchorWithHideOnScrollNavbar_3R7-" 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 anchorWithHideOnScrollNavbar_3R7-" id="1-diff-算法"></a>1. Diff 算法<a class="hash-link" href="#1-diff-算法" title="Direct link to heading">#</a></h3><p>两棵树的比对和更新，涉及到树编辑距离（Tree Editing Distance）算法：将一棵树转化为另一棵树的最小操作成本。操作类型包括：删除、插入、修改。时间复杂度为 O(n^3)。</p><p>为了降低时间复杂度，React 和 Vue 的思路是基于以下两个假设条件，缩减递归迭代规模，将 Diff 算法的时间复杂度降低为 O(n)：</p><ol><li>相同类型的组件产生相同的 DOM 结构，反之亦然。所以不同类型组件的结构不需要进一步递归 Diff。</li><li>同一层级的一组节点，可以通过唯一标识符进行区分。</li></ol><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithHideOnScrollNavbar_3R7-" id="2-react-reconciliation"></a>2. React Reconciliation<a class="hash-link" href="#2-react-reconciliation" title="Direct link to heading">#</a></h3><p>在 React 中，将虚拟 DOM 和真实 DOM 进行比对然后同步的过程被称为 Reconciliation（调和），Fiber 是 React 16 中新的调和引擎。它的主要目标是实现虚拟 DOM 的增量渲染。</p><p>Diff 的大致过程是，当对比两棵虚拟 DOM 树时，React 先对比根元素。依据根元素的类型不同，会有不同的操作：</p><ol><li><p><strong>不同类型的元素</strong></p><p>如果元素的类型不同，React 会抛弃旧树并建立新树。如以下情况，会导致完全重建：</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 comment" style="color:#999988;font-style:italic"><!-- old --></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">button</span><span class="token tag" style="color:#00009f"> </span><span class="token tag attr-name" style="color:#00a4db">class</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">bg-blue-100</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain">HZFE</span><span class="token tag punctuation" style="color:#393A34"></</span><span class="token tag" style="color:#00009f">button</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block">
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</span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token comment" style="color:#999988;font-style:italic"><!-- new --></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">div</span><span class="token tag" style="color:#00009f"> </span><span class="token tag attr-name" style="color:#00a4db">class</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">bg-blue-100</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain">HZFE</span><span class="token tag punctuation" style="color:#393A34"></</span><span class="token tag" style="color:#00009f">div</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></li><li><p><strong>相同类型的元素</strong></p><p>如果元素是两个相同类型的 React DOM 元素时，React 会查看两者的属性，保留 DOM 节点，只更新改变的属性。如以下情况，React 只更新颜色样式。</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 comment" style="color:#999988;font-style:italic"><!-- old --></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">button</span><span class="token tag" style="color:#00009f"> </span><span class="token tag attr-name" style="color:#00a4db">class</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">bg-blue-100 text-center</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain">HZFE</span><span class="token tag punctuation" style="color:#393A34"></</span><span class="token tag" style="color:#00009f">button</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block">
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</span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token comment" style="color:#999988;font-style:italic"><!-- new --></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">button</span><span class="token tag" style="color:#00009f"> </span><span class="token tag attr-name" style="color:#00a4db">class</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">bg-red-100 text-center</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain">HZFE</span><span class="token tag punctuation" style="color:#393A34"></</span><span class="token tag" style="color:#00009f">button</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></li></ol><p>在元素类型相同的情况下，比对完元素后，会递归元素的子元素。默认情况下，React 会同时迭代新老两个子元素列表。对于列表的更新，React 建议在列表项中标识 key 属性。避免以下低效场景：</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 comment" style="color:#999988;font-style:italic"><!-- bad --></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token comment" style="color:#999988;font-style:italic"><!-- React 不会意识到可以保留<li>HZFE</li>和<li>Front-End</li>子树的完整，而是重写每个元素 --></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block">
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</span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token comment" style="color:#999988;font-style:italic"><!-- old --></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">ul</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">li</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain">HZFE</span><span class="token tag punctuation" style="color:#393A34"></</span><span class="token tag" style="color:#00009f">li</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">li</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain">Front-End</span><span class="token tag punctuation" style="color:#393A34"></</span><span class="token tag" style="color:#00009f">li</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token tag punctuation" style="color:#393A34"></</span><span class="token tag" style="color:#00009f">ul</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token comment" style="color:#999988;font-style:italic"><!-- new --></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">ul</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">li</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain">Back-End</span><span class="token tag punctuation" style="color:#393A34"></</span><span class="token tag" style="color:#00009f">li</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">li</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain">HZFE</span><span class="token tag punctuation" style="color:#393A34"></</span><span class="token tag" style="color:#00009f">li</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">li</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain">Front-End</span><span class="token tag punctuation" style="color:#393A34"></</span><span class="token tag" style="color:#00009f">li</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token tag punctuation" style="color:#393A34"></</span><span class="token tag" style="color:#00009f">ul</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block">
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</span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token comment" style="color:#999988;font-style:italic"><!-- good --></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token comment" style="color:#999988;font-style:italic"><!-- 子列表项有稳定且在兄弟节点中唯一的 key 属性， --></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token comment" style="color:#999988;font-style:italic"><!-- React 使用 key 从新老树中匹配对应节点比较，提高 Diff 效率。 --></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block">
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</span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token comment" style="color:#999988;font-style:italic"><!-- old --></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">ul</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">li</span><span class="token tag" style="color:#00009f"> </span><span class="token tag attr-name" style="color:#00a4db">key</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">2016</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain">HZFE</span><span class="token tag punctuation" style="color:#393A34"></</span><span class="token tag" style="color:#00009f">li</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">li</span><span class="token tag" style="color:#00009f"> </span><span class="token tag attr-name" style="color:#00a4db">key</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">2017</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain">Front-End</span><span class="token tag punctuation" style="color:#393A34"></</span><span class="token tag" style="color:#00009f">li</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token tag punctuation" style="color:#393A34"></</span><span class="token tag" style="color:#00009f">ul</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token comment" style="color:#999988;font-style:italic"><!-- new --></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">ul</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">li</span><span class="token tag" style="color:#00009f"> </span><span class="token tag attr-name" style="color:#00a4db">key</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">2015</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain">Back-End</span><span class="token tag punctuation" style="color:#393A34"></</span><span class="token tag" style="color:#00009f">li</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">li</span><span class="token tag" style="color:#00009f"> </span><span class="token tag attr-name" style="color:#00a4db">key</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">2016</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain">HZFE</span><span class="token tag punctuation" style="color:#393A34"></</span><span class="token tag" style="color:#00009f">li</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token tag punctuation" style="color:#393A34"><</span><span class="token tag" style="color:#00009f">li</span><span class="token tag" style="color:#00009f"> </span><span class="token tag attr-name" style="color:#00a4db">key</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">2017</span><span class="token tag attr-value punctuation" style="color:#393A34">"</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain">Front-End</span><span class="token tag punctuation" style="color:#393A34"></</span><span class="token tag" style="color:#00009f">li</span><span class="token tag punctuation" style="color:#393A34">></span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token tag punctuation" style="color:#393A34"></</span><span class="token tag" style="color:#00009f">ul</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 anchorWithHideOnScrollNavbar_3R7-" id="2-vue2x-diff"></a>2. Vue2.x Diff<a class="hash-link" href="#2-vue2x-diff" title="Direct link to heading">#</a></h3><p>Vue 的 Diff 算法和 React 的类似，只在同一层次进行比较，不进行跨层比较。如果两个元素被判定为不相同，则不继续递归比较。在 Diff 子元素的过程中，采用双端比较的方法，设立 4 个指针：</p><ul><li>oldStartIdx 指向旧子元素列表中，从左边开始 Diff 的元素索引。初始值：第一个元素的索引。</li><li>newStartIdx 指向新子元素列表中，从左边开始 Diff 的元素索引。初始值：第一个元素的索引。</li><li>oldEndIdx 指向旧子元素列表中，从右边开始 Diff 的元素索引。初始值：最后一个元素的索引。</li><li>newEndIdx 指向新子元素列表中，从右边开始 Diff 的元素索引。初始值：最后一个元素的索引。</li></ul><p><img src="https://user-images.githubusercontent.com/17002181/130326547-7cdcbc06-b400-43d3-89ce-e73934e38bdf.png" alt="image"></p><p>Vue 同时遍历新老子元素虚拟 DOM 列表，并采用头尾比较。一般有 4 种情况：</p><ol><li><p><strong>当新老 start 指针指向的是相同节点</strong></p><p>复用节点并按需更新。</p><p>新老 start 指针向右移动一位。</p></li><li><p><strong>当新老 end 指针指向的是相同节点</strong></p><p>复用节点并按需更新。</p><p>新老 end 指针向左移动一位。</p></li><li><p><strong>当老 start 指针和新 end 指针指向的是相同节点</strong></p><p>复用节点并按需更新，将节点对应的真实 DOM 移动到子元素列表队尾。</p><p>老 start 指针向右移动一位。</p><p>新 end 指针向左移动一位。</p></li><li><p><strong>当老 end 指针和新 start 指针指向的是相同节点</strong></p><p>复用节点并按需更新，将节点对应的真实 DOM 移动到子元素列表队头。</p><p>老 end 指针向左移动一位。</p><p>新 start 指针向右移动一位。</p></li></ol><p>在不满足以上情况的前提下，会尝试检查新 start 指针指向的节点是否有唯一标识符 key，如果有且能在旧列表中找到拥有相同 key 的相同类型节点，则可复用并按需更新，且移动节点到新的位置。新 start 指针向右移动一位。如果依旧不满足条件，则新增相关节点。</p><p>当新老列表的中任意一个列表的头指针索引大于尾指针索引时，循环遍历结束，按需删除或新增相关节点即可。</p><h2><a aria-hidden="true" tabindex="-1" class="anchor anchor__h2 anchorWithHideOnScrollNavbar_3R7-" 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><ul><li><a href="https://reactjs.org/docs/reconciliation.html" target="_blank" rel="noopener noreferrer">Reconciliation</a></li><li><a href="https://github.com/vuejs/vue/blob/2.6/src/core/vdom/patch.js" target="_blank" rel="noopener noreferrer">patch</a></li></ul></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/frame-react-hooks"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">« 框架：React Hooks 实现原理</div></a></div><div class="pagination-nav__item pagination-nav__item--next"><a class="pagination-nav__link" href="/awesome-interview/book3/js-async"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">基础：JavaScript 异步编程 »</div></a></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="#相关问题" class="table-of-contents__link">相关问题</a></li><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-diff-算法" class="table-of-contents__link">1. Diff 算法</a></li><li><a href="#2-react-reconciliation" class="table-of-contents__link">2. React Reconciliation</a></li><li><a href="#2-vue2x-diff" class="table-of-contents__link">2. Vue2.x Diff</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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