react 获取 input 的值

class App extends Component {
  state = {
    username: ‘张三‘
  };
  // 用户名
  getUserName(e){
    console.log(e.target.value);
    this.setState({
      username: e.target.value
    });
    console.log(this.state.username); // setState为异步,存在延迟
  }

  render() {
    return (
      <div>
        <input type="text" onChange={this.getUserName.bind(this)} />
      </div>
    );
  }
}