react 生命周期

初始化  componentDidMount

更新  componentDidUpdate

销毁 componentWillUnmount

电子时钟：

class Clock extends React.Component{
    constructor(props){
        super(props)
        this.state = {
            date: new Date()
        }
    }

    componentDidMount() {
        this.timer = setInterval( () =>{
            this.setState({
                date: new Date()
            })
        },1000)
    }

    componentDidUpdate(currentProps, currentState){
         console.log(currentState);
    }

    componentWillUnmount(){
        clearInterval(this.timer)
    }
    render(){
        return(
            <div className="clock">
                <h1>{this.state.date.toLocaleTimeString()}</h1>
            </div>
        )
    }
}

export default Clock