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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>闭包</code> <code>Fiber</code> <code>链表</code></p><p>Hooks 是 React 16.8 的新增特性。它可以让你在不编写 class 的情况下使用 state 以及其他的 React 特性。</p><p>Hooks 主要是利用闭包来保存状态，使用链表保存一系列 Hooks，将链表中的第一个 Hook 与 Fiber 关联。在 Fiber 树更新时，就能从 Hooks 中计算出最终输出的状态和执行相关的副作用。</p><p>使用 Hooks 的注意事项：</p><ul><li>不要在循环，条件或嵌套函数中调用 Hooks。</li><li>只在 React 函数中调用 Hooks。</li></ul><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-简化实现">1. 简化实现<a href="#1-简化实现" class="hash-link" aria-label="Direct link to 1. 简化实现" title="Direct link to 1. 简化实现"></a></h3><p><a href="https://codesandbox.io/s/hopeful-williams-id7ey?file=/src/index.js" target="_blank" rel="noopener noreferrer">React Hooks 模拟实现</a></p><p>该示例是一个 React Hooks 接口的简化模拟实现，可以实际运行观察。其中 <code>react.js</code> 文件模拟实现了 <code>useState</code> 和 <code>useEffect</code> 接口，其基本原理和 react 实际实现类似。</p><h3 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="2-对比分析">2. 对比分析<a href="#2-对比分析" class="hash-link" aria-label="Direct link to 2. 对比分析" title="Direct link to 2. 对比分析"></a></h3><h4 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="21-状态-hook">2.1 状态 Hook<a href="#21-状态-hook" class="hash-link" aria-label="Direct link to 2.1 状态 Hook" title="Direct link to 2.1 状态 Hook"></a></h4><p>模拟的 useState 实现中，通过闭包，将 state 保存在 <code>memoizedState[cursor]</code>。 memoizedState 是一个数组，可以按顺序保存 hook 多次调用产生的状态。</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword" style="color:#00009f">let</span><span class="token plain"> memoizedState </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">[</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token keyword" style="color:#00009f">let</span><span class="token plain"> cursor </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">0</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token keyword" style="color:#00009f">function</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">useState</span><span class="token punctuation" style="color:#393A34">(</span><span class="token parameter">initialValue</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 初次调用时，传入的初始值作为 state，后续使用闭包中保存的 state</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword" style="color:#00009f">let</span><span class="token plain"> state </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> memoizedState</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">cursor</span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">??</span><span class="token plain"> initialValue</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 对游标进行闭包缓存，使得 setState 调用时，操作正确的对应状态</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> _cursor </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> cursor</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> </span><span class="token function-variable function" style="color:#d73a49">setState</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token parameter">newValue</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token arrow operator" style="color:#393A34">=></span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">memoizedState</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">_cursor</span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> newValue</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 游标自增，为接下来调用的 hook 使用时，引用 memoizedState 中的新位置</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> cursor </span><span class="token operator" style="color:#393A34">+=</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">1</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword control-flow" style="color:#00009f">return</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">state</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> setState</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg class="copyButtonIcon_y97N" viewBox="0 0 24 24"><path d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg class="copyButtonSuccessIcon_LjdS" viewBox="0 0 24 24"><path d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>实际的 useState 实现经过多方面的<a href="https://overreacted.io/why-do-hooks-rely-on-call-order/" target="_blank" rel="noopener noreferrer">综合考虑</a>，React 最终选择将 Hooks 设计为顺序结构，这也是 Hooks 不能条件调用的原因。</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword" style="color:#00009f">function</span><span class="token plain"> mountState</span><span class="token operator" style="color:#393A34"><</span><span class="token constant" style="color:#36acaa">S</span><span class="token operator" style="color:#393A34">></span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">initialState</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token arrow operator" style="color:#393A34">=></span><span class="token plain"> </span><span class="token constant" style="color:#36acaa">S</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">|</span><span class="token plain"> </span><span class="token constant" style="color:#36acaa">S</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">)</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">[</span><span class="token constant" style="color:#36acaa">S</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token maybe-class-name">Dispatch</span><span class="token operator" style="color:#393A34"><</span><span class="token maybe-class-name">BasicStateAction</span><span class="token operator" style="color:#393A34"><</span><span class="token constant" style="color:#36acaa">S</span><span class="token operator" style="color:#393A34">>></span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 创建 Hook，并将当前 Hook 添加到 Hooks 链表中</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> hook </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">mountWorkInProgressHook</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 如果初始值是函数，则调用函数取得初始值</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword control-flow" style="color:#00009f">if</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">typeof</span><span class="token plain"> initialState </span><span class="token operator" style="color:#393A34">===</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"function"</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> initialState </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">initialState</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> hook</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">memoizedState</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> hook</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">baseState</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> initialState</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 创建一个链表来存放更新对象</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> queue </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">hook</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">queue</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">pending</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token keyword null nil" style="color:#00009f">null</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">dispatch</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token keyword null nil" style="color:#00009f">null</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">lastRenderedReducer</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> basicStateReducer</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">lastRenderedState</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> initialState</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// dispatch 用于修改状态，并将此次更新添加到更新对象链表中</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">dispatch</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token maybe-class-name">Dispatch</span><span class="token operator" style="color:#393A34"><</span><span class="token maybe-class-name">BasicStateAction</span><span class="token operator" style="color:#393A34"><</span><span class="token constant" style="color:#36acaa">S</span><span class="token operator" style="color:#393A34">>></span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">queue</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">dispatch</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">dispatchAction</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">bind</span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword null nil" style="color:#00009f">null</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> currentlyRenderingFiber</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> queue</span><span class="token punctuation" style="color:#393A34">)</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> any</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword control-flow" style="color:#00009f">return</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">hook</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">memoizedState</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> dispatch</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg class="copyButtonIcon_y97N" viewBox="0 0 24 24"><path d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg class="copyButtonSuccessIcon_LjdS" viewBox="0 0 24 24"><path d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><h4 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="21-副作用-hook">2.1 副作用 Hook<a href="#21-副作用-hook" class="hash-link" aria-label="Direct link to 2.1 副作用 Hook" title="Direct link to 2.1 副作用 Hook"></a></h4><p>模拟的 useEffect 实现，同样利用了 memoizedState 闭包来存储依赖数组。依赖数组进行浅比较，默认的比较算法是 <code>Object.is</code>。</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword" style="color:#00009f">function</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">useEffect</span><span class="token punctuation" style="color:#393A34">(</span><span class="token parameter">cb</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> depArray</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> oldDeps </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> memoizedState</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">cursor</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword" style="color:#00009f">let</span><span class="token plain"> hasChange </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token boolean" style="color:#36acaa">true</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword control-flow" style="color:#00009f">if</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">oldDeps</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 对比传入的依赖数组与闭包中保存的旧依赖数组，采用浅比较算法</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> hasChange </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> depArray</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">some</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">(</span><span class="token parameter">dep</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> i</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token arrow operator" style="color:#393A34">=></span><span class="token plain"> </span><span class="token operator" style="color:#393A34">!</span><span class="token known-class-name class-name">Object</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">is</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">dep</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> oldDeps</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">i</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword control-flow" style="color:#00009f">if</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">hasChange</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">cb</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> memoizedState</span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">cursor</span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> depArray</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> cursor</span><span class="token operator" style="color:#393A34">++</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg class="copyButtonIcon_y97N" viewBox="0 0 24 24"><path d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg class="copyButtonSuccessIcon_LjdS" viewBox="0 0 24 24"><path d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>实际的 useEffect 实现：</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword" style="color:#00009f">function</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">mountEffect</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token function-variable function" style="color:#d73a49">create</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token arrow operator" style="color:#393A34">=></span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token arrow operator" style="color:#393A34">=></span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">void</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">|</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">void</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">deps</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token known-class-name class-name">Array</span><span class="token operator" style="color:#393A34"><</span><span class="token plain">mixed</span><span class="token operator" style="color:#393A34">></span><span class="token plain"> </span><span class="token operator" style="color:#393A34">|</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">void</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">|</span><span class="token plain"> </span><span class="token keyword null nil" style="color:#00009f">null</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">)</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">void</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword control-flow" style="color:#00009f">return</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">mountEffectImpl</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token maybe-class-name">UpdateEffect</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">|</span><span class="token plain"> </span><span class="token maybe-class-name">PassiveEffect</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// fiberFlags</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token maybe-class-name">HookPassive</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// hookFlags</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> create</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> deps</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">}</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token keyword" style="color:#00009f">function</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">mountEffectImpl</span><span class="token punctuation" style="color:#393A34">(</span><span class="token parameter">fiberFlags</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> hookFlags</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> create</span><span class="token parameter punctuation" style="color:#393A34">,</span><span class="token parameter"> deps</span><span class="token punctuation" style="color:#393A34">)</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">void</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 创建hook</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> hook </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">mountWorkInProgressHook</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> nextDeps </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> deps </span><span class="token operator" style="color:#393A34">===</span><span class="token plain"> </span><span class="token keyword nil" style="color:#00009f">undefined</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">?</span><span class="token plain"> </span><span class="token keyword null nil" style="color:#00009f">null</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> deps</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 设置 workInProgress 的副作用标记</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> currentlyRenderingFiber</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">flags</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">|=</span><span class="token plain"> fiberFlags</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// fiberFlags 被标记到 workInProgress</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 创建 Effect, 挂载到 hook.memoizedState 上</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> hook</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">memoizedState</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">pushEffect</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token maybe-class-name">HookHasEffect</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">|</span><span class="token plain"> hookFlags</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// hookFlags 用于创建 effect</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> create</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword nil" style="color:#00009f">undefined</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> nextDeps</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg class="copyButtonIcon_y97N" viewBox="0 0 24 24"><path d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg class="copyButtonSuccessIcon_LjdS" viewBox="0 0 24 24"><path d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><h3 class="anchor anchorWithHideOnScrollNavbar_WYt5" id="3-hooks-如何与-fiber-共同工作">3. Hooks 如何与 Fiber 共同工作<a href="#3-hooks-如何与-fiber-共同工作" class="hash-link" aria-label="Direct link to 3. Hooks 如何与 Fiber 共同工作" title="Direct link to 3. Hooks 如何与 Fiber 共同工作"></a></h3><p>在了解如何工作之前，先看看 Hook 与 Fiber 的部分结构定义：</p><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword module" style="color:#00009f">export</span><span class="token plain"> type </span><span class="token maybe-class-name">Hook</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">memoizedState</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> any</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 最新的状态值</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">baseState</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> any</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 初始状态值</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">baseQueue</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token maybe-class-name">Update</span><span class="token operator" style="color:#393A34"><</span><span class="token plain">any</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> any</span><span class="token operator" style="color:#393A34">></span><span class="token plain"> </span><span class="token operator" style="color:#393A34">|</span><span class="token plain"> </span><span class="token keyword null nil" style="color:#00009f">null</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">queue</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token maybe-class-name">UpdateQueue</span><span class="token operator" style="color:#393A34"><</span><span class="token plain">any</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> any</span><span class="token operator" style="color:#393A34">></span><span class="token plain"> </span><span class="token operator" style="color:#393A34">|</span><span class="token plain"> </span><span class="token keyword null nil" style="color:#00009f">null</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 环形链表，存储的是该 hook 多次调用产生的更新对象</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">next</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token maybe-class-name">Hook</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">|</span><span class="token plain"> </span><span class="token keyword null nil" style="color:#00009f">null</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// next 指针，之下链表中的下一个 Hook</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">;</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg class="copyButtonIcon_y97N" viewBox="0 0 24 24"><path d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg class="copyButtonSuccessIcon_LjdS" viewBox="0 0 24 24"><path d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><div class="language-js codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-js codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword module" style="color:#00009f">export</span><span class="token plain"> type </span><span class="token maybe-class-name">Fiber</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">updateQueue</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> mixed</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 存储 Fiber 节点相关的副作用链表</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">memoizedState</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> any</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 存储 Fiber 节点相关的状态值</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token literal-property property" style="color:#36acaa">flags</span><span class="token operator" style="color:#393A34">:</span><span class="token plain"> </span><span class="token maybe-class-name">Flags</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// 标识当前 Fiber 节点是否有副作用</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">;</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg class="copyButtonIcon_y97N" viewBox="0 0 24 24"><path d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg class="copyButtonSuccessIcon_LjdS" viewBox="0 0 24 24"><path d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>与上节中的模拟实现不同，真实的 Hooks 是一个单链表的结构，React 按 Hooks 的执行顺序依次将 Hook 节点添加到链表中。下面以 useState 和 useEffect 两个最常用的 hook 为例，来分析 Hooks 如何与 Fiber 共同工作。</p><p>在每个<strong>状态 Hook</strong>（如 useState）节点中，会通过 queue 属性上的循环链表记住所有的更新操作，并在 update 阶段依次执行循环链表中的所有更新操作，最终拿到最新的 state 返回。</p><p>状态 Hooks 组成的链表的具体结构如下图所示：</p><p><img loading="lazy" src="https://user-images.githubusercontent.com/4338052/130256374-80a453a4-2084-4bb8-bfba-2344f25100c4.png" alt="State Hook" class="img_ev3q"></p><p>在每个<strong>副作用 Hook</strong>（如 useEffect）节点中，创建 effect 挂载到 Hook 的 memoizedState 中，并添加到环形链表的末尾，该链表会保存到 Fiber 节点的 updateQueue 中，在 commit 阶段执行。</p><p>副作用 Hooks 组成的链表的具体结构如下图所示：</p><p><img loading="lazy" src="https://user-images.githubusercontent.com/4338052/130258719-4a2ad7d5-3a0b-4f9c-b278-1f35a0b96843.png" alt="Effect Hook" class="img_ev3q"></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://overreacted.io/why-do-hooks-rely-on-call-order/" target="_blank" rel="noopener noreferrer">Why Do React Hooks Rely on Call Order?</a></li><li><a href="https://medium.com/@ryardley/react-hooks-not-magic-just-arrays-cd4f1857236e" target="_blank" rel="noopener noreferrer">React hooks: not magic, just arrays</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/book3/engineer-webpack-loader"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">工程化：谈下 webpack loader 的机制</div></a><a class="pagination-nav__link pagination-nav__link--next" href="/awesome-interview/book3/frame-diff"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">框架：常见框架的 Diff 算法</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-简化实现" class="table-of-contents__link toc-highlight">1. 简化实现</a></li><li><a href="#2-对比分析" class="table-of-contents__link toc-highlight">2. 对比分析</a><ul><li><a href="#21-状态-hook" class="table-of-contents__link toc-highlight">2.1 状态 Hook</a></li><li><a href="#21-副作用-hook" class="table-of-contents__link toc-highlight">2.1 副作用 Hook</a></li></ul></li><li><a href="#3-hooks-如何与-fiber-共同工作" class="table-of-contents__link toc-highlight">3. 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