react18中的计算属性及useMemo的性能优化技巧

react18里面的计算属性和使用useMemo来提升组件性能的方法

计算属性

实现效果
代码实现
函数式组件极简洁的实现，就这样

import { useState } from "react";
function FullName() {
  const [firstName, setFirstName] = useState("");
  const [lastName, setLastName] = useState("");
  const fullName = firstName + " " + lastName;
  return (
    <>
      <h1>fullname:{fullName}</h1>

      <label>
        First Name :
        <input
          type="text"
          value={firstName}
          onChange={(e) => setFirstName(e.target.value)}
        />
      </label>

      <br />
      <label>
        Last Name :
        <input
          type="text"
          value={lastName}
          onChange={(e) => setLastName(e.target.value)}
        />
      </label>
    </>
  );
}

export default FullName;

计算大的方法缓存

子组件会因为props和state的变化而重新渲染，这其中如果有很大计算量的方法的话，就会严重拖慢页面加载速度。像下面这种就是因为一个无关的更新，导致计算一会在执行，计算量大的话性能损耗很大
请添加图片描述

import { useState, useMemo } from "react";
function TodoList({ todos, filter }) {
  const [newTodo, setNewTodo] = useState("");
  const visibleTodos = getFilteredTodos(todos, filter);
  return (
    <>
      <ul>
        {visibleTodos.map((todo) => {
          return <li>{todo.name}</li>;
        })}
      </ul>
      {newTodo}
      <br />
      <input value={newTodo} onChange={(e) => setNewTodo(e.target.value)} />
    </>
  );
}

function getFilteredTodos(todos, filter) {
  console.log("🚀 ~ getFilteredTodos ~ todos:", todos);
  return todos.filter((todo) => {
    return filter.call(null, todo);
  });
}
export default TodoList;

优化代码

import { useState, useMemo } from "react";
function TodoList({ todos, filter }) {
  const [newTodo, setNewTodo] = useState("");
  const visibleTodos = useMemo(
    () => getFilteredTodos(todos, filter),
    [todos, filter]
  );
  return (
    <>
      <ul>
        {visibleTodos.map((todo) => {
          return <li>{todo.name}</li>;
        })}
      </ul>
      {newTodo}
      <br />
      <input value={newTodo} onChange={(e) => setNewTodo(e.target.value)} />
    </>
  );
}

function getFilteredTodos(todos, filter) {
  console.log("🚀 ~ getFilteredTodos ~ todos:", todos);
  return todos.filter((todo) => {
    return filter.call(null, todo);
  });
}
export default TodoList;