React源码解析之React.createRef()/forwardRef()

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

作用：
获取目标element的DOM实例

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 <div ref={this.father}>
      this is div
    </div>
  }
  
}

源码：

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

// an immutable object with a single mutable value
//可修改value的 不可变的对象
//没见过这种写法 :RefObject
export function createRef(): RefObject {
  //初始化ref对象，属性current初始值为null
  const refObject = {
    current: null,
  };
  if (__DEV__) {
    Object.seal(refObject);
  }
  return refObject;
}

解析：
源码比较简单，就是返回了带有current属性的refObject

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

作用：
从父组件中获取子组件是FunctionComponent的DOM实例

使用：

import React from 'react'
//funciton component是没有dom实例的，因为它是PureComponent，所以没有this，
// 所以不能通过createRef()来拿到实例

//将Father的father传给子组件，并绑定子组件的DOM实例，从而能在父组件拿到子组件的DOM实例
const Child=React.forwardRef((props,ref)=>{
  return <div ref={ref}>child div</div>
})

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

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

  render(){
    return <Child ref={this.father} />
  }

}

源码：

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

import warningWithoutStack from 'shared/warningWithoutStack';

export default function forwardRef<Props, ElementType: React$ElementType>(
  render: (props: Props, ref: React$Ref<ElementType>) => 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，组件内部的$$typeof是REACT_FORWARD_REF_TYPE
    $$typeof: REACT_FORWARD_REF_TYPE,
    //render即包装的FunctionComponent，ClassComponent是不用forwardRef的
    render,
  };
}

解析：
（1）不看__DEV__的话，返回的也是一个Object，也就是说，Child被forwardRef包裹后，React.forwardRef(Child)的$$typeof是REACT_FORWARD_REF_TYPE

注意：
一旦在Father组件中，用JSX引用了Child组件，那么就是React.createElement(React.forwardRef(Child))，又包裹了一层，此时的$$typeof`是`REACT_ELEMENT_TYPE`，`type`是`React.forwardRef(Child)`，`type`里面的`$$typeof是REACT_FORWARD_REF_TYPE

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

（2）关于forward在高阶组件的用法，请参考：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 (<input type="text" ref={inputRef}/>)
}

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 (<Child ref={rref}/>)
}

注意：
① antdPro中使用的话，我试了是不好用dva的connect包裹的，issue上作者也没回答，就关闭了：https://github.com/ant-design/ant-design-pro/issues/3123

② useImperativeMethods已经重命名为useImperativeHandle，传送门：https://github.com/facebook/react/pull/14565

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（完）