React UseMemo源码分析

useMemo

useMemo 是 React 提供的内置 Hooks，主要作用就是缓存，如果依赖项没有变化，Memo 方法不会再次执行，计算量比较高的方法可以使用，从而提高用户体验。本文将通过一个例子跟踪 Memo 的创建、更新流程。

App.js

//App.js
import { useState } from 'react';
import { createTodos } from './utils.js';
import TodoList from './TodoList.js';

const todos = createTodos();

export default function App() {
  const [tab, setTab] = useState('all');
  const [isDark, setIsDark] = useState(false);
  return (
    <>
      <button onClick={() => setTab('all')}>
        All
      </button>
      <button onClick={() => setTab('active')}>
        Active
      </button>
      <button onClick={() => setTab('completed')}>
        Completed
      </button>
      <br />
      <label>
        <input
          type="checkbox"
          checked={isDark}
          onChange={e => setIsDark(e.target.checked)}
        />
        Dark mode
      </label>
      <hr />
      <TodoList
        todos={todos}
        tab={tab}
        theme={isDark ? 'dark' : 'light'}
      />
    </>
  );
}

//TodoList.js
import List from './List.js';
import { filterTodos } from './utils.js'
import {useMemo} from 'react'

export default function TodoList({ todos, theme, tab }) {
  const visibleTodos = useMemo(
    () => filterTodos(todos, tab),
    [todos, tab]
  );
  return (
    <div className={theme}>
      <p><b>Note: <code>List</code> is artificially slowed down!</b></p>
      <List items={visibleTodos} />
    </div>
  );
}


/*
 *  filename List.js
 * 
 */
import { memo } from 'react';

const List = memo(function List({ items }) {
  console.log('[ARTIFICIALLY SLOW] Rendering <List /> with ' + items.length + ' items');
  let startTime = performance.now();
  while (performance.now() - startTime < 500) {
    // Do nothing for 500 ms to emulate extremely slow code
  }

  return (
    <ul>
      {items.map(item => (
        <li key={item.id}>
          {item.completed ?
            <s>{item.text}</s> :
            item.text
          }
        </li>
      ))}
    </ul>
  );
});

export default List;

/*
 *  filename utils.js
 * 
 */
export function createTodos() {
    const todos = [];
    for (let i = 0; i < 50; i++) {
      todos.push({
        id: i,
        text: "Todo " + (i + 1),
        completed: Math.random() > 0.5
      });
    }
    return todos;
  }
  
  export function filterTodos(todos, tab) {
    return todos.filter(todo => {
      if (tab === 'all') {
        return true;
      } else if (tab === 'active') {
        return !todo.completed;
      } else if (tab === 'completed') {
        return todo.completed;
      }
    });
  }