A.jsx

// @flow

import React, {Component} from 'react';
import type {ComponentType} from 'react';


import B from './B'

class A extended Component<any> {
  ref = {}
  refB = (c: ComponentType) => {thif.ref.B = c}
  getBState = (): any => ref.B.wrappedInstance.state
  render(){
    return(<B ref={this.refB}>)
  }
}