react中怎么改变state的值

import React from 'react'

export default  class ClickS extends React.Component {

  constructor () {

    super()

    this.state= {

      msg: '123'

    }

  }

  render () {

    return <div>

      <button onClick={()=>this.show()}>按钮</button>

      <h2>{this.state.msg}</h2>

    </div>

  }

  show () {

    console.log(this)

    this.setState({

      msg: '222'

    })

  }

}

<button onClick={this.show.bind(this)}>按钮</button>

show () {

  console.log(this)

  this.setState({

    msg: '222'

  }, () => {

    console.log(this.state.msg) // 更新后的值222

  })

  console.log(this.state.msg) // 123

}