foo.js

// prop callback
class Compoennt {
  render() {
    return (
      <div>
        <Thing onChange={(val) => {
          this.setState({ val1: val })
        }}/>
        <Thing onChange={(val) => {
          this.setState({ val2: val })
        }}/>

        <div>{this.state.val1 + this.state.val2}</div>
      </div>
    )
  }
}



React.cloneElement(el, { val })

// context
<Thing>
  <Child/>
</Thing>

// render callback
<Thing>
  {(val1) => (
    <Thing>
      {(val2) => (
        val1 + val2
      )}
    </Thing>
  )}
</Thing>

// higher order component
withStuff(Thing)