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浅谈React中key的作用

作者:CaptainDrake

React中key用于唯一标识元素,提升列表渲染性能并确保状态一致性,key帮助React高效比较新旧节点,本文主要介绍了React 中 key 的作用,感兴趣的可以了解一下

key 概念

在 React 中,key 用于识别哪些元素是变化、添加或删除的。

在列表渲染中,key 尤其重要,因为它能提高渲染性能和确保组件状态的一致性。

key 的作用

1)唯一性标识:

React 通过 key 唯一标识列表中的每个元素。当列表发生变化(增删改排序)时,React 会通过 key 快速判断:

如果没有 key,React 会默认使用数组索引(index)作为标识,这在动态列表中会导致 性能下降状态错误

2)保持组件状态:

使用 key 能确保组件在更新过程中状态的一致性。不同的 key 会使 React 认为它们是不同的组件实例,因而会创建新的组件实例,而不是重用现有实例。这对于有状态的组件尤为重要。

// 如果初始列表是 [A, B],用索引 index 作为 key:
<ul>
  {items.map((item, index) => (
    <li key={index}>{item}</li>
  ))}
</ul>

// 在头部插入新元素变为 [C, A, B] 时:
// React 会认为 key=0 → C(重新创建)
// key=1 → A(复用原 key=0 的 DOM,但状态可能残留)
// 此时,原本属于 A 的输入框状态可能会错误地出现在 C 中。

3)高效的 Diff 算法:

在列表中使用 key 属性,React 可以通过 Diff 算法快速比较新旧元素,确定哪些元素需要重新渲染,哪些元素可以复用。这减少了不必要的 DOM 操作,从而提高渲染性能。

源码解析

以下是 React 源码中与 key 相关的关键部分:

1)生成 Fiber树

在生成 Fiber 树时,React 使用 key 来匹配新旧节点。

src/react/packages/react-reconciler/src/ReactChildFiber.js

    // * 协调子节点,构建新的子fiber结构,并且返回新的子fiber
  function reconcileChildFibers(
    returnFiber: Fiber,
    currentFirstChild: Fiber | null, // 老fiber的第一个子节点
    newChild: any,
    lanes: Lanes,
  ): Fiber | null {
    // This indirection only exists so we can reset `thenableState` at the end.
    // It should get inlined by Closure.
    thenableIndexCounter = 0;
    const firstChildFiber = reconcileChildFibersImpl(
      returnFiber,
      currentFirstChild,
      newChild,
      lanes,
      null, // debugInfo
    );

    thenableState = null;
    // Don't bother to reset `thenableIndexCounter` to 0 because it always gets
    // set at the beginning.
    return firstChildFiber;
  }
  
  function reconcileChildrenArray(
    returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    newChildren: Array<any>,
    lanes: Lanes,
    debugInfo: ReactDebugInfo | null,
  ): Fiber | null {
    let resultingFirstChild: Fiber | null = null; // 存储新生成的child
    let previousNewFiber: Fiber | null = null;

    let oldFiber = currentFirstChild;
    let lastPlacedIndex = 0;
    let newIdx = 0;
    let nextOldFiber = null;
    // ! 1. 从左边往右遍历,比较新老节点,如果节点可以复用,继续往右,否则就停止
    for (; oldFiber !== null && newIdx < newChildren.length; newIdx++) {
      if (oldFiber.index > newIdx) {
        nextOldFiber = oldFiber;
        oldFiber = null;
      } else {
        nextOldFiber = oldFiber.sibling;
      }
      const newFiber = updateSlot(
        returnFiber,
        oldFiber,
        newChildren[newIdx],
        lanes,
        debugInfo,
      );
      if (newFiber === null) {
        // TODO: This breaks on empty slots like null children. That's
        // unfortunate because it triggers the slow path all the time. We need
        // a better way to communicate whether this was a miss or null,
        // boolean, undefined, etc.
        if (oldFiber === null) {
          oldFiber = nextOldFiber;
        }
        break;
      }
      if (shouldTrackSideEffects) {
        if (oldFiber && newFiber.alternate === null) {
          // We matched the slot, but we didn't reuse the existing fiber, so we
          // need to delete the existing child.
          deleteChild(returnFiber, oldFiber);
        }
      }
      lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
      if (previousNewFiber === null) {
        // TODO: Move out of the loop. This only happens for the first run.
        resultingFirstChild = newFiber;
      } else {
        // TODO: Defer siblings if we're not at the right index for this slot.
        // I.e. if we had null values before, then we want to defer this
        // for each null value. However, we also don't want to call updateSlot
        // with the previous one.
        previousNewFiber.sibling = newFiber;
      }
      previousNewFiber = newFiber;
      oldFiber = nextOldFiber;
    }

    // !2.1 新节点没了,(老节点还有)。则删除剩余的老节点即可
    // 0 1 2 3 4
    // 0 1 2 3
    if (newIdx === newChildren.length) {
      // We've reached the end of the new children. We can delete the rest.
      deleteRemainingChildren(returnFiber, oldFiber);
      if (getIsHydrating()) {
        const numberOfForks = newIdx;
        pushTreeFork(returnFiber, numberOfForks);
      }
      return resultingFirstChild;
    }
    // ! 2.2 (新节点还有),老节点没了
    // 0 1 2 3 4
    // 0 1 2 3 4 5
    if (oldFiber === null) {
      // If we don't have any more existing children we can choose a fast path
      // since the rest will all be insertions.
      for (; newIdx < newChildren.length; newIdx++) {
        const newFiber = createChild(
          returnFiber,
          newChildren[newIdx],
          lanes,
          debugInfo,
        );
        if (newFiber === null) {
          continue;
        }
        lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
        if (previousNewFiber === null) {
          // TODO: Move out of the loop. This only happens for the first run.
          resultingFirstChild = newFiber;
        } else {
          previousNewFiber.sibling = newFiber;
        }
        previousNewFiber = newFiber;
      }
      if (getIsHydrating()) {
        const numberOfForks = newIdx;
        pushTreeFork(returnFiber, numberOfForks);
      }
      return resultingFirstChild;
    }

    // !2.3 新老节点都还有节点,但是因为老fiber是链表,不方便快速get与delete,
    // !   因此把老fiber链表中的节点放入Map中,后续操作这个Map的get与delete
    // 0 1|   4 5
    // 0 1| 7 8 2 3
    // Add all children to a key map for quick lookups.
    const existingChildren = mapRemainingChildren(returnFiber, oldFiber);

    // Keep scanning and use the map to restore deleted items as moves.
    for (; newIdx < newChildren.length; newIdx++) {
      const newFiber = updateFromMap(
        existingChildren,
        returnFiber,
        newIdx,
        newChildren[newIdx],
        lanes,
        debugInfo,
      );
      if (newFiber !== null) {
        if (shouldTrackSideEffects) {
          if (newFiber.alternate !== null) {
            // The new fiber is a work in progress, but if there exists a
            // current, that means that we reused the fiber. We need to delete
            // it from the child list so that we don't add it to the deletion
            // list.
            existingChildren.delete(
              newFiber.key === null ? newIdx : newFiber.key,
            );
          }
        }
        lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
        if (previousNewFiber === null) {
          resultingFirstChild = newFiber;
        } else {
          previousNewFiber.sibling = newFiber;
        }
        previousNewFiber = newFiber;
      }
    }

     // !3. 如果是组件更新阶段,此时新节点已经遍历完了,能复用的老节点都用完了,
    // ! 则最后查找Map里是否还有元素,如果有,则证明是新节点里不能复用的,也就是要被删除的元素,此时删除这些元素就可以了
    if (shouldTrackSideEffects) {
      // Any existing children that weren't consumed above were deleted. We need
      // to add them to the deletion list.
      existingChildren.forEach(child => deleteChild(returnFiber, child));
    }

    if (getIsHydrating()) {
      const numberOfForks = newIdx;
      pushTreeFork(returnFiber, numberOfForks);
    }
    return resultingFirstChild;
  }

在 reconcileChildFibers 中的关键使用:

顶层“单个元素”分支(如 reconcileSingleElement):先在兄弟链表里按 key 查找可复用的老 Fiber;若 key 相同再比类型,复用成功则删除其他老兄弟,否则删到尾并新建。

  function reconcileSingleElement(
    returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    element: ReactElement,
    lanes: Lanes,
    debugInfo: ReactDebugInfo | null,
  ): Fiber {
    const key = element.key;
    let child = currentFirstChild;
    // 检查老的fiber单链表中是否有可以复用的节点
    while (child !== null) {
      if (child.key === key) {
        ...
        if (child.elementType === elementType || ... ) {
          deleteRemainingChildren(returnFiber, child.sibling);
          const existing = useFiber(child, element.props);
          ...
          return existing;
        }
        deleteRemainingChildren(returnFiber, child);
        break;
      } else {
        deleteChild(returnFiber, child);
      }
    }
    ...
  }

2)比较新旧节点

在比较新旧节点时,React 通过 key 来确定节点是否相同:

src/react/packages/react-reconciler/src/ReactChildFiber.js

  function updateSlot(
    returnFiber: Fiber,
    oldFiber: Fiber | null,
    newChild: any,
    lanes: Lanes,
    debugInfo: null | ReactDebugInfo,
  ): Fiber | null {
    // Update the fiber if the keys match, otherwise return null.
    const key = oldFiber !== null ? oldFiber.key : null;

    if (
      (typeof newChild === 'string' && newChild !== '') ||
      typeof newChild === 'number'
    ) {
      // Text nodes don't have keys. If the previous node is implicitly keyed
      // we can continue to replace it without aborting even if it is not a text
      // node.
      if (key !== null) {
        return null;
      }
      return updateTextNode(
        returnFiber,
        oldFiber,
        '' + newChild,
        lanes,
        debugInfo,
      );
    }

    if (typeof newChild === 'object' && newChild !== null) {
      switch (newChild.$$typeof) {
        case REACT_ELEMENT_TYPE: {
          if (newChild.key === key) {
            return updateElement(
              returnFiber,
              oldFiber,
              newChild,
              lanes,
              mergeDebugInfo(debugInfo, newChild._debugInfo),
            );
          } else {
            return null;
          }
        }
        case REACT_PORTAL_TYPE: {
          if (newChild.key === key) {
            return updatePortal(
              returnFiber,
              oldFiber,
              newChild,
              lanes,
              debugInfo,
            );
          } else {
            return null;
          }
        }
        case REACT_LAZY_TYPE: {
          const payload = newChild._payload;
          const init = newChild._init;
          return updateSlot(
            returnFiber,
            oldFiber,
            init(payload),
            lanes,
            mergeDebugInfo(debugInfo, newChild._debugInfo),
          );
        }
      }

      if (isArray(newChild) || getIteratorFn(newChild)) {
        if (key !== null) {
          return null;
        }

        return updateFragment(
          returnFiber,
          oldFiber,
          newChild,
          lanes,
          null,
          mergeDebugInfo(debugInfo, newChild._debugInfo),
        );
      }

      // Usable node types
      //
      // Unwrap the inner value and recursively call this function again.
      if (typeof newChild.then === 'function') {
        const thenable: Thenable<any> = (newChild: any);
        return updateSlot(
          returnFiber,
          oldFiber,
          unwrapThenable(thenable),
          lanes,
          debugInfo,
        );
      }

      if (newChild.$$typeof === REACT_CONTEXT_TYPE) {
        const context: ReactContext<mixed> = (newChild: any);
        return updateSlot(
          returnFiber,
          oldFiber,
          readContextDuringReconcilation(returnFiber, context, lanes),
          lanes,
          debugInfo,
        );
      }

      throwOnInvalidObjectType(returnFiber, newChild);
    }

    if (__DEV__) {
      if (typeof newChild === 'function') {
        warnOnFunctionType(returnFiber, newChild);
      }
      if (typeof newChild === 'symbol') {
        warnOnSymbolType(returnFiber, newChild);
      }
    }

    return null;
  }

实际案例

1)简单列表

假设我们有一个简单的列表:

const items = this.state.items.map(item => 
	<li key={item.id}>{ item.text }</li>
)

在上述代码中,每个

元素都有一个唯一的 key。

如果 items 数组发生变化(如添加或删除元素),React将根据 key 来高效地更新DOM:

2)错误案例演示

import React, { useState } from 'react'

// 错误案例:使用数组索引作为 key,导致组件在插入/重排时状态错乱
// 复现实验:
// 1) 在下方两个输入框分别输入不同文本(对应 A、B)
// 2) 点击“在头部插入 C” → 列表从 [A, B] 变为 [C, A, B]
// 3) 使用 index 作为 key 时:
//    key=0 → C(重新创建)
//    key=1 → A(复用原 key=0 的 DOM,状态可能残留)
//    因此原本属于 A 的输入框状态可能会错误地出现在 C 中

function InputItem({ label }: { label: string }) {
	const [text, setText] = useState<string>('')
	return (
		<div
			style={{ display: 'flex', alignItems: 'center', gap: 8, marginBottom: 8 }}
		>
			<span style={{ width: 80 }}>{label}</span>
			<input
				placeholder="在此输入以观察状态"
				value={text}
				onChange={e => setText(e.target.value)}
			/>
		</div>
	)
}

export default function TestDemo() {
	const [labels, setLabels] = useState<string[]>(['A', 'B'])

	const prependC = () => {
		setLabels(prev => ['C', ...prev])
	}

	return (
		<div style={{ padding: 16 }}>
			<h3>错误示例:使用 index 作为 key(头部插入触发状态错乱)</h3>
			<button onClick={prependC} style={{ marginBottom: 12 }}>
				在头部插入 C
			</button>
			{labels.map((label, index) => (
				// 错误:使用 index 作为 key,头部插入 C 后会发生状态错位
				<InputItem key={index} label={label} />
			))}
		</div>
	)
}

到此这篇关于浅谈React中key的作用的文章就介绍到这了,更多相关React key内容请搜索脚本之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持脚本之家!

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