摘要：一作用獲取目標的實例使用源碼可修改的不可變的對象沒見過這種寫法初始化對象，屬性初始值為解析源碼比較簡單，就是返回了帶有屬性的二作用從父組件中獲取子組件是的實例使用是沒有實例的，因為它是，所以沒有，所以不能通過來拿到實例將的傳給子組件，并綁定

一、React.createRef()
GitHub：
https://github.com/AttackXiaoJinJin/reactExplain/blob/master/react16.8.6/packages/react/src/ReactCreateRef.js

作用：
獲取目標element的DOM實例

使用：

import React from "react"

export default class Father extends  React.Completed{
  constructor(props){
    super(props)
    this.father=React.createRef()
  }

  componentDidMount(){
    this.father.current.value="hahhaha"
  }

  render(){
    return 

      this is div
    
  }
  
}

源碼：

import type {RefObject} from "shared/ReactTypes";

// an immutable object with a single mutable value
//可修改value的 不可變的對象
//沒見過這種寫法 :RefObject
export function createRef(): RefObject {
  //初始化ref對象，屬性current初始值為null
  const refObject = {
    current: null,
  };
  if (__DEV__) {
    Object.seal(refObject);
  }
  return refObject;
}

解析：
源碼比較簡單，就是返回了帶有current屬性的refObject

二、React.forwardRef()
GitHub：
https://github.com/AttackXiaoJinJin/reactExplain/blob/master/react16.8.6/packages/react/src/forwardRef.js

作用：
從父組件中獲取子組件是FunctionComponent的DOM實例

使用：

import React from "react"
//funciton component是沒有dom實例的，因為它是PureComponent，所以沒有this，
// 所以不能通過createRef()來拿到實例

//將Father的father傳給子組件，并綁定子組件的DOM實例，從而能在父組件拿到子組件的DOM實例
const Child=React.forwardRef((props,ref)=>{
  return 
child div
})

export default class Father extends  React.Completed{
  constructor(props){
    super(props)
    this.father=React.createRef()
  }

  componentDidMount(){
    this.father.current.value="hahhaha"
  }

  render(){
    return 
  }

}

源碼：

import {REACT_FORWARD_REF_TYPE, REACT_MEMO_TYPE} from "shared/ReactSymbols";

import warningWithoutStack from "shared/warningWithoutStack";

export default function forwardRef(
  render: (props: Props, ref: React$Ref) => React$Node,
) {
  //__DEV__可不看
  if (__DEV__) {
    if (render != null && render.$$typeof === REACT_MEMO_TYPE) {
      warningWithoutStack(
        false,
        "forwardRef requires a render function but received a `memo` " +
          "component. Instead of forwardRef(memo(...)), use " +
          "memo(forwardRef(...)).",
      );
    } else if (typeof render !== "function") {
      warningWithoutStack(
        false,
        "forwardRef requires a render function but was given %s.",
        render === null ? "null" : typeof render,
      );
    } else {
      warningWithoutStack(
        // Do not warn for 0 arguments because it could be due to usage of the "arguments" object
        render.length === 0 || render.length === 2,
        "forwardRef render functions accept exactly two parameters: props and ref. %s",
        render.length === 1
          ? "Did you forget to use the ref parameter?"
          : "Any additional parameter will be undefined.",
      );
    }

    if (render != null) {
      warningWithoutStack(
        render.defaultProps == null && render.propTypes == null,
        "forwardRef render functions do not support propTypes or defaultProps. " +
          "Did you accidentally pass a React component?",
      );
    }
  }

  return {
    //被forwardRef包裹后，組件內部的$$typeof是REACT_FORWARD_REF_TYPE
    $$typeof: REACT_FORWARD_REF_TYPE,
    //render即包裝的FunctionComponent，ClassComponent是不用forwardRef的
    render,
  };
}

解析：
（1）不看__DEV__的話，返回的也是一個Object，也就是說，Child被forwardRef包裹后，React.forwardRef(Child)的$$typeof是REACT_FORWARD_REF_TYPE

注意：
一旦在Father組件中，用JSX引用了Child組件，那么就是React.createElement(React.forwardRef(Child))，又包裹了一層，此時的$$typeof`是`REACT_ELEMENT_TYPE`，`type`是`React.forwardRef(Child)`，`type`里面的`$$typeof是REACT_FORWARD_REF_TYPE

const ReactElement = function(type,...) {
  const element = {
    $$typeof: REACT_ELEMENT_TYPE,
    type: type,
  };
}

（2）關于forward在高階組件的用法，請參考：https://reactjs.org/docs/react-api.html#reactforwardref

（3）如何在antdPro/FunctionComponent中使用：
子：

const Child = (props,ref) => {
  const inputRef = React.useRef();
  React.useImperativeHandle(ref, () => ({
    focus: () => {
      // inputRef.current.focus();
      inputRef.current.value="aaaa"
    }
  }));

  return ()
}

export default React.forwardRef(Child)

父：

import Child from "./Child";
const Father=(props)=> {
   const rref= React.useRef(null)
   useEffect(() => {
    //console.log(rref.current,"rref33")
    rref.current.focus()
  }, []);
return ()
}

注意：
① antdPro中使用的話，我試了是不好用dva的connect包裹的，issue上作者也沒回答，就關閉了：https://github.com/ant-design/ant-design-pro/issues/3123

② useImperativeMethods已經重命名為useImperativeHandle，傳送門：https://github.com/facebook/react/pull/14565

（完）