设计模式(九)-状态模式

状态模式

允许一个对象或者类,在其内部状态修改时改变它的行为.

• 图例

image.png

• 普通代码示例

class Battery{
    constructor(){
        this.amount=100
        this.state='success'
    }
    show(){
        if(this.amount==100){
            console.log('显示绿色')
            this.state='success'
            this.amount=60
        }else if(this.amount==60){
            console.log('显示黄色')
            this.state='warning'
            this.amount=30
        }else{
            console.log('显示红色')
            this.state='danger'
            this.amount=10
        }
    }
}

let batter=new Battery()
batter.show() //显示绿色
batter.show()//显示黄色
batter.show()//显示红色

• 状态模式优化版

//定义单独状态类,把显示的逻辑委托给了状态对象，内部还需要维护状态变化.
class SuccessState{
   constructor(batter){
      this.batter=batter  
   }
   show(){
       console.log('显示绿色')
       this.batter.amount=60
       return 'success'
   }
}

class WarningState{
    constructor(batter){
        this.batter=batter  
    }
    show(){
        console.log('显示黄色')
        this.batter.amount=30
        return 'warning'
    }
}
class DangerState{
    constructor(batter){
        this.batter=batter  
    }
    show(){
        console.log('显示红色')
        this.batter.amount=10
        return 'danger'
    }
}

class Battery{
    constructor(){
        this.amount=100
        this.state=new SuccessState(this)
    }
    show(){
        this.state.show()
        if(this.amount==100){
            this.state=new SuccessState(this)
        }else if(this.amount==60){
            this.state=new WarningState(this)
        }else{
            this.state=new DangerState(this)
        }
      }
}

let batter=new Battery()
batter.show() //显示绿色
batter.show()//显示黄色
batter.show()//显示红色

• 使用场景

javascript-state-machine库

 const StateMachine=require('javascript-state-machine')
 var fsm = new StateMachine({
    init: 'solid',
    transitions: [
      { name: 'melt', from: 'solid', to: 'liquid' },
      { name: 'freeze', from: 'liquid', to: 'solid' },
      { name: 'vaporize', from: 'liquid', to: 'gas' },
      { name: 'condense', from: 'gas', to: 'liquid' }
    ],
    methods: {
      onMelt: function() { console.log('I melted')},
      onFreeze:  function() { console.log('I froze') },
      onVaporize: function() { console.log('I vaporized') },
      onCondense: function() { console.log('I condensed') }
    }
  });
fsm.freeze()

优点
1.封装了转换规则
2.将所有某个状态相关的行为放到一个类中,并可以方便的增加新的状态.
3.允许状态转换逻辑与状态对象合成一体,而不是某一个巨大的条件语句块.
4.可以让多个对象或者类共享一个状态对象.