状态模式

一个对象有状态变化，每次变化都会触发一个逻辑，不能总是用if...else来控制。比如交通信号红绿灯，收藏，关注，，

举例：

//状态单独抽象出来
class State{
    constructor(color){
        this.color = color;
    }
    handle(context){
        console.log(`turn to ${this.color} light`);
        context.setState(this);
    }
}

//主体单独抽象出来
class Context{
    constructor(){
        this.state = null;
    }
    getState(){
        return this.state;
    }
    setState(state){
        this.state = state;
    }
}

let context = new Context();

let green = new State('green');
let yellow = new State('yellow');
let red = new State('red');

//绿灯亮了，状态的切换和获取是分离的
//状态的切换
green.handle(context);
//状态的获取
var state = context.getState();
console.log(state);
//红灯亮了
red.handle(context);
var state = context.getState();
console.log(state);
//黄灯亮了
yellow.handle(context);
var state = context.getState();
console.log(state);

// turn to green light
// State { color: 'green' }
// turn to red light
// State { color: 'red' }
// turn to yellow light
// State { color: 'yellow' }