react原生中的ref是什么?我应该在什么时候使用ref？

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.myRef = null;
  }

  render() {
    return <div ref={(element) => { this.myRef = element; }}>Hello, World!</div>;
  }
}

class MyComponent extends React.Component {
  myRef = React.createRef();

  render() {
    return <div ref={this.myRef}>Hello, World!</div>;
  }
}

import React, { useRef } from 'react';

function TextInputWithFocusButton() {
  const inputEl = useRef(null);

  const onButtonClick = () => {
    // `current` points to the mounted text input element
    inputEl.current.focus();
  };

  return (
    <>
      <input ref={inputEl} type="text" />
      <button onClick={onButtonClick}>Focus the input</button>
    </>
  );
}

export default TextInputWithFocusButton;