main.js

import React, { Component } from "react";

class Counter extends Component {
  state = { count: 1 };
  onClick = () => this.setState(({ count }) => ({ count: count + 1 }));
  render() {
    return (
      <button onClick={this.onClick}>
        {this.state.count}
      </button>
    );
  }
}

class Demo extends Component {
  componentDidMount() {
    this.props.setVariables({ foo: 43 });
  }
  componentWillReceiveProps({ setVariables }) {
    setVariables({ foo: 44 });
  }
  render() {
    return (
      <div>
        <Counter />
      </div>
    );
  }
}

class App extends Component {
  state = { variables: { foo: 42 } };
  setVariables = variables => this.setState({ variables });
  render() {
    return (
      <div>
        <Demo
          setVariables={this.setVariables}
          variables={this.state.variables}
        />
      </div>
    );
  }
}

export default App;