对 state 保留或重置

核心概念，当同一个组件在渲染树中同样的位置加载时，组件中的状态会被保留

本文使用 react 官方示例

不同位置渲染相同的组件

import { useState } from "react";

export default function App() {
  const counter = <Counter />;
  return (
    <div>
      {counter}
      {counter}
    </div>
  );
}

function Counter() {
  const [score, setScore] = useState(0);
  const [hover, setHover] = useState(false);

  let className = "counter";
  if (hover) {
    className += " hover";
  }

  return (
    <div
      className={className}
      onPointerEnter={() => setHover(true)}
      onPointerLeave={() => setHover(false)}
    >
      <h1>{score}</h1>
      <button onClick={() => setScore(score + 1)}>加一</button>
    </div>
  );
}

同一组件在不同位置渲染，它们有独立的状态

同一位置渲染同一个组件

import { useState } from "react";

export default function App() {
  const [isFancy, setIsFancy] = useState(false);
  return (
    <div>
      {isFancy ? <Counter isFancy={true} /> : <Counter isFancy={false} />}
      <label>
        <input
          type="checkbox"
          checked={isFancy}
          onChange={(e) => {
            setIsFancy(e.target.checked);
          }}
        />
        使用好看的样式
      </label>
    </div>
  );
}

function Counter({ isFancy }) {
  const [score, setScore] = useState(0);
  const [hover, setHover] = useState(false);

  let className = "counter";
  if (hover) {
    className += " hover";
  }
  if (isFancy) {
    className += " fancy";
  }

  return (
    <div
      className={className}
      onPointerEnter={() => setHover(true)}
      onPointerLeave={() => setHover(false)}
    >
      <h1>{score}</h1>
      <button onClick={() => setScore(score + 1)}>加一</button>
    </div>
  );
}

当切换样式时，虽然通过 condition ? expressionA : expressionB 分发了 Counter 组件，但是是基于同一个位置的同一组件渲染，状态被保留下来

渲染同一个组件时，如何重置状态

渲染在不同的位置

import { useState } from "react";

export default function Scoreboard() {
  const [isPlayerA, setIsPlayerA] = useState(true);
  return (
    <div>
      {isPlayerA && <Counter person="Taylor" />}
      {!isPlayerA && <Counter person="Sarah" />}
      <button
        onClick={() => {
          setIsPlayerA(!isPlayerA);
        }}
      >
        下一位玩家！
      </button>
    </div>
  );
}

function Counter({ person }) {
  const [score, setScore] = useState(0);
  const [hover, setHover] = useState(false);

  let className = "counter";
  if (hover) {
    className += " hover";
  }

  return (
    <div
      className={className}
      onPointerEnter={() => setHover(true)}
      onPointerLeave={() => setHover(false)}
    >
      <h1>
        {person} 的分数：{score}
      </h1>
      <button onClick={() => setScore(score + 1)}>加一</button>
    </div>
  );
}

在组件上使用不同的 key 值

import { useState } from "react";

export default function Scoreboard() {
  const [isPlayerA, setIsPlayerA] = useState(true);
  return (
    <div>
      {isPlayerA ? (
        <Counter key="Taylor" person="Taylor" />
      ) : (
        <Counter key="Sarah" person="Sarah" />
      )}
      <button
        onClick={() => {
          setIsPlayerA(!isPlayerA);
        }}
      >
        下一位玩家！
      </button>
    </div>
  );
}

function Counter({ person }) {
  const [score, setScore] = useState(0);
  const [hover, setHover] = useState(false);

  let className = "counter";
  if (hover) {
    className += " hover";
  }

  return (
    <div
      className={className}
      onPointerEnter={() => setHover(true)}
      onPointerLeave={() => setHover(false)}
    >
      <h1>
        {person} 的分数：{score}
      </h1>
      <button onClick={() => setScore(score + 1)}>加一</button>
    </div>
  );
}