useReducer - keydown 处理程序重新评估每个渲染的初始状态

Question

https://codesandbox.io/s/suspicious-chebyshev-vy4mf2

useReducer 挂钩和 useEffect 以及 keydown 事件处理程序。问题是，在处理键盘按钮按下而导致的每次重新渲染时，组件函数都会重新运行获取初始状态的函数。

可见的错误结果是每次渲染时在控制台中记录的随机数（我希望没有记录）。换句话说，这里的目标是停止在每个渲染上运行 getInitialState 函数。

import { useEffect, useReducer } from "react";
import "./styles.css";

interface IAction {
  type: string;
  payload: string;
}

export default function App() {
  const reducer = (state: string, action: IAction) => {
    switch (action.type) {
      case "KEYDOWN":
        return action.payload;
      default:
        return state;
    }
  };

  const getInitialState = () => {
    const rand = Math.random().toString();
    console.log(rand);
    return rand;
  };

  const [state, dispatch] = useReducer(reducer, getInitialState());

  const keyDownHandler = (e: KeyboardEvent): void => {
    dispatch({
      type: "KEYDOWN",
      payload: e.key
    });
  };

  useEffect(() => {
    document.addEventListener("keydown", keyDownHandler);
    return () => document.removeEventListener("keydown", keyDownHandler);
  }, [state, dispatch]);

  return <div className="App">{state}</div>;
}

Answer 1

当组件发现它需要重新渲染时，它的函数将再次运行。你所做的相当于询问为什么

export default function App() {
  // ...
  const [state, dispatch] = useReducer(reducer, getInitialState());

每次 App 运行时都运行 getInitialState ，这相当于

export default function App() {
  // ...
  const parameterToPass = getInitialState();
  const [state, dispatch] = useReducer(reducer, parameterToPass);

Which should make what's comebclear 。

这并不是说 useReducer 需要再次计算初始值 - 它不需要，它只在第一次运行时使用第二个参数来确定初始状态。您正在调用每次应用程序重新渲染时计算初始状态的函数。

只要您确保计算初始状态的函数没有副作用，请随意忽略它 - 您现在所做的就很好。

另一种选择是将第三个参数传递给 useReducer 作为调用以计算初始状态的函数。

const [state, dispatch] = useReducer(reducer, null, getInitialState);

Answer 2

您在每次渲染时调用 getInitialState，这并不意味着它正在重新初始化。

您可以通过在专为函数使用而设计的第三个参数位置运行初始化程序来避免这种情况。

您还应该在渲染函数之外为减速器创建函数，否则它会比应有的频率更频繁地重新运行。

const { useEffect, useReducer } = React;

interface IAction {
  type: string;
  payload: string;
}
const reducer = (state: string, action: IAction) => {
  switch (action.type) {
    case "KEYDOWN":
      return action.payload;
    default:
      return state;
  }
};
function App() {
  const getInitialState = () => {
    const rand = Math.random().toString();
    console.log(rand);
    return rand;
  };

  const [state, dispatch] = useReducer(reducer, null, getInitialState);

  const keyDownHandler = (e: KeyboardEvent): void => {
    dispatch({
      type: "KEYDOWN",
      payload: e.key
    });
  };

  useEffect(() => {
    document.addEventListener("keydown", keyDownHandler);
    return () => document.removeEventListener("keydown", keyDownHandler);
  }, [state, dispatch]);

  return <div className="App">{state}</div>;
}
const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.0/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.0/umd/react-dom.production.min.js"></script>
    <div id="root"></div>