React - 获取父组件中子组件中的ref

Question

I'm not trying to do anything hacky using refs. I just need the ref to the element because the element is a canvas, and to draw on a canvas you need its ref.

class Parent extends Component {
  clickDraw = () => {
    // when button clicked, get the canvas context and draw on it.
    // how?
  }

  render() {
    return (
      <div>
        <button onClick={this.clickDraw}> Draw </button>
        <Child />
      </div>
    );
  }
}


class Child extends Component {
  componentDidMount() {
    const ctx = this.canvas.getContext('2d');
    // draw something on the canvas once it's mounted
    ctx.fillStyle = "#FF0000";
    ctx.fillRect(0,0,150,75);
  }

  render() {
    return (
      <canvas width={300}
              height={500}
              ref={canvasRef => this.canvas = canvasRef}>
      </canvas>
    );
  }
}

=====

Something I tried (which technically works but feels strange) is define the <canvas> in the parent, so in its ref function, this refers to the parent component. Then I pass the <canvas> and this.canvas to the child as two separate props. I return the <canvas> (named this.props.canvasJSX ) in the child's render function, and I use this.canvas (named this.props.canvasRef ) to get its context to draw on it. See below:

class Parent extends Component {
  clickDraw = () => {
    // now I have access to the canvas context and can draw
    const ctx = this.canvas.getContext('2d');
    ctx.fillStyle = "#00FF00";
    ctx.fillRect(0,0,275,250);
  }

  render() {
    const canvas = (
      <canvas width={300}
              height={500}
              ref={canvasRef => this.canvas = canvasRef}>
      </canvas>
    );
    return (
      <div>
        <button onClick={this.clickDraw}> Draw </button>
        <Child canvasJSX={canvas}
               canvasRef={this.canvas} />
      </div>
    );
  }
}


class Child extends Component {
  componentDidMount() {
    const ctx = this.props.canvasRef.getContext('2d');
    // draw something on the canvas once it's mounted
    ctx.fillStyle = "#FF0000";
    ctx.fillRect(0,0,150,75);
  }

  render() {
    return this.props.canvas;
  }
}

Is there a more standard way of achieving this?

Answer 1

You should actually be using the first approach and you can access the child elements refs in the parent

class Parent extends Component {
  clickDraw = () => {
    // when button clicked, get the canvas context and draw on it.
    const ctx = this.childCanvas.canvas.getContext('2d');
    ctx.fillStyle = "#00FF00";
    ctx.fillRect(0,0,275,250);
  }

  render() {
    return (
      <div>
        <button onClick={this.clickDraw}> Draw </button>
        <Child ref={(ip) => this.childCanvas = ip}/>;
      </div>
    );
  }
}


class Child extends Component {
  constructor() {
     super();
     this.canvas = null;
  }
  componentDidMount() {
    const ctx = this.canvas.getContext('2d');
    // draw something on the canvas once it's mounted
    ctx.fillStyle = "#FF0000";
    ctx.fillRect(0,0,150,75);
  }

  render() {
    return (
      <canvas width={300}
              height={500}
              ref={canvasRef => this.canvas = canvasRef}>
      </canvas>
    );
  }
}

You can only use this approach is the child component is declared as a class .

Answer 2

If it cannot be avoided the suggested pattern extracted from the React docs would be:

import React, {Component} from 'react';

const Child = ({setRef}) => <input type="text" ref={setRef} />;

class Parent extends Component {
    constructor(props) {
        super(props);
        this.setRef = this.setRef.bind(this);
    }
    componentDidMount() {
        // Call function on Child dom element
        this.childInput.focus();
    }
    setRef(input) {
        this.childInput = input;
    }
    render() {
        return <Child setRef={this.setRef} />
    }
}

The Parent passes a function as prop bound to Parent 's this . React will call the Child 's ref callback setRef and attach the childInput property to this which as we already noted points to the Parent .