问PureComponent不呈现

EN

import React from 'react';
import './App.css';
import Displayer from './Components/displayer';
const days=[
  'Monday',
  'Tuesday',
  'Wednesday',
  'Thursday',
  'Friday',
  'Saturday',
  'Sunday'
];

class App extends React.PureComponent{
  constructor(props){
    super(props);
    this.state={day:[]};
    this.increment=this.increment.bind(this);
    this.decrement=this.decrement.bind(this);
  }
  increment(){
    var demo=this.state.day;
    if(this.state.day.length<7)
      demo.push(days[this.state.day.length]);
    this.setState({day:demo});
  }
  decrement(){
    var demo=this.state.day;
    demo.pop();
    this.setState({day:demo});
  }
  render(){
  return (
    <div>
      <Displayer day={this.state.day} increment={this.increment} decrement={this.decrement}/>
    </div>
  );
  }
}

export default App;

import React from 'react';

class Displayer extends React.PureComponent{
    render(){
    var {day,increment,decrement}=this.props;
    return(
        <div>
            <button onClick={increment}>Add Day</button>
            <button onClick={decrement}>Remove Day</button>
            {
                day.map((item,index)=>{
                    return <h1 key={index}>{item}</h1>
                })
            }
        </div>
    )
    }
}

export default Displayer;