import React, {Component} from 'react';

export default class Parent extends Component {
    render() {
        return(
            <div>
                <Child onRef={this.onRef} />
                <button onClick={this.click} >click</button>
            </div>
        )
    }
    onRef = (ref) => {
        this.child = ref
    }
    click = (e) => {
        this.child.myName()
    }
}

class Child extends Component {
    componentDidMount(){
        this.props.onRef(this)
    }
    myName = () => console.log("123)
    render() {
        return (
               <div></div>
               )
    }
}