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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>响应变化</code> <code>属性</code> <code>侦听</code></p><p>computed 是模板表达式的声明式描述，会<strong>创建新的响应式数据</strong>。而 watch 是响应式数据的<strong>自定义侦听器</strong>，用于<strong>响应数据的变化</strong>。除此之外，computed 还具有<strong>可缓存，可依赖多个属性，getter 函数无副作用</strong>等特点。watch 则更适用于<strong>异步或开销大的操作</strong>。</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-实现原理">1. 实现原理<a href="#1-实现原理" class="hash-link" aria-label="Direct link to 1. 实现原理" title="Direct link to 1. 实现原理"></a></h3><p>在了解 Vue 数据双向绑定的基础上，computed 等同于为属性设置 <strong>getter 函数</strong>（也可设置 <a href="https://cn.vuejs.org/v2/guide/computed.html#%E8%AE%A1%E7%AE%97%E5%B1%9E%E6%80%A7%E7%9A%84-setter" target="_blank" rel="noopener noreferrer">setter</a>），而 watch 等同于为属性的 <strong>setter 设置回调函数、监听深度 deep 及响应速度 immediate</strong>。下面简单讲解下两者的实现原理，具体细节可以参考<a href="https://github.com/vuejs/vue" target="_blank" rel="noopener noreferrer">源码</a>。</p><h4 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="11-computed-原理">1.1 computed 原理<a href="#11-computed-原理" class="hash-link" aria-label="Direct link to 1.1 computed 原理" title="Direct link to 1.1 computed 原理"></a></h4><p>主要分为四个阶段</p><p><img loading="lazy" src="https://user-images.githubusercontent.com/12165373/125889331-b97687a3-2037-4bb9-9d4e-8405f5ed271f.png" alt="computed四个阶段" class="img_ev3q"></p><ol><li><strong>初始化</strong>：为 computed 属性创建 lazy watcher（此处 watcher 指双向绑定中的监听器，下同）。</li><li><strong>首次模板渲染</strong>：渲染 watcher 检测到 computed 属性时，会调用 computed 属性的 getter 方法，而 computed 属性的 getter 方法会调用依赖属性的 getter，从而形成链式调用，同时保存引用关系用于更新。取得计算结果后 lazy watcher 会将结果缓存，并返回给渲染 watcher 进行模板渲染。</li><li><strong>多次模板渲染</strong>：直接取 lazy watcher 中的缓存值给到渲染 watcher 进行渲染。</li><li><strong>依赖属性更新</strong>：根据首次模板渲染阶段构建的依赖关系向上通知 lazy watcher 进行重新计算，缓存计算结果并通知渲染 watcher 重新渲染更新页面。</li></ol><h4 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="12-watch-原理">1.2 watch 原理<a href="#12-watch-原理" class="hash-link" aria-label="Direct link to 1.2 watch 原理" title="Direct link to 1.2 watch 原理"></a></h4><p>watch 本质上是为每个监听属性 setter 创建了一个 watcher，当被监听的属性更新时，调用传入的回调函数。常见的配置选项有 deep 和 immediate，对应原理如下：</p><ol><li><strong>deep</strong>：深度监听对象，为对象的每一个属性创建一个 watcher，从而确保对象的每一个属性更新时都会触发传入的回调函数。主要原因在于对象属于引用类型，单个属性的更新并不会触发对象 setter，因此引入 deep 能够很好地解决监听对象的问题。同时也会引入判断机制，确保在多个属性更新时回调函数仅触发一次，避免性能浪费。</li><li><strong>immediate</strong>：在初始化时直接调用回调函数，可以通过在 created 阶段手动调用回调函数实现相同的效果。</li></ol><h3 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="2-适用场景">2. 适用场景<a href="#2-适用场景" class="hash-link" aria-label="Direct link to 2. 适用场景" title="Direct link to 2. 适用场景"></a></h3><ul><li>computed：需要处理复杂逻辑的模板表达式。</li><li>watch：需要执行异步或开销较大的操作。</li></ul><p>从表现上看，computed 会创建<strong>新的属性</strong>，而 watch 可以通过<strong>将属性设置在 data 中，再监听依赖属性变化，调用 handler 方法更新属性</strong>的方式达到同样的效果。因此不难得出 computed 的使用场景可以被 watch 覆盖这一结论。但在具体的使用上还是优先考虑 computed，因为相同场景下 watch 所需的代码量和性能开销一般来说会比 computed 大，具体可以参照 <a href="https://cn.vuejs.org/v2/guide/computed.html#%E8%AE%A1%E7%AE%97%E5%B1%9E%E6%80%A7-vs-%E4%BE%A6%E5%90%AC%E5%B1%9E%E6%80%A7" target="_blank" rel="noopener noreferrer">computed vs watched</a>。在 computed 无法满足需求的情况下再考虑使用 watch，也可以有效避免 watch 滥用，提升性能。</p><h3 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="3-vue3-与-vue2-区别">3. Vue3 与 Vue2 区别<a href="#3-vue3-与-vue2-区别" class="hash-link" aria-label="Direct link to 3. Vue3 与 Vue2 区别" title="Direct link to 3. Vue3 与 Vue2 区别"></a></h3><p><strong>Vue3 中 computed 和 watch 的原理以及在 Options API 中的使用方式和 Vue2 保持一致。但在 Vue3 的新特性组合式（Composition）API 中，使用方式和功能相比 Vue2 有了明显差别</strong>。使用方式由原来在组件中声明改为直接从 Vue 中导入使用，各自的调用方式和参数也发生了改变，功能更加多样，同时 Vue3 还引入了 watchEffect 作为 watch 的补充，以求用更简洁的代码来覆盖更广的使用场景。具体使用参考<a href="https://v3.cn.vuejs.org/guide/reactivity-computed-watchers.html" target="_blank" rel="noopener noreferrer">官方文档</a>。</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://cn.vuejs.org/v2/guide/computed.html" target="_blank" rel="noopener noreferrer">Vuejs computed</a></li><li><a href="https://github.com/vuejs/vue" target="_blank" rel="noopener noreferrer">Vuejs 源码</a></li><li><a href="https://v3.cn.vuejs.org/guide/composition-api-introduction.html" target="_blank" rel="noopener noreferrer">Vue3 组合式 api</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/frame-vue-data-binding"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">框架：Vue 的数据绑定机制</div></a><a class="pagination-nav__link pagination-nav__link--next" href="/awesome-interview/book1/js-closures"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">基础：闭包的作用和原理</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-实现原理" class="table-of-contents__link toc-highlight">1. 实现原理</a><ul><li><a href="#11-computed-原理" class="table-of-contents__link toc-highlight">1.1 computed 原理</a></li><li><a href="#12-watch-原理" class="table-of-contents__link toc-highlight">1.2 watch 原理</a></li></ul></li><li><a href="#2-适用场景" class="table-of-contents__link toc-highlight">2. 适用场景</a></li><li><a href="#3-vue3-与-vue2-区别" class="table-of-contents__link toc-highlight">3. Vue3 与 Vue2 区别</a></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 class="chatbox-container" style="height:0"><div style="flex-grow:1;padding:0 10%"><iframe src="https://interview-book-ai.hzfe.org" height="100%" width="100%"></iframe></div></div></div></div></main></div></div></div>
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