状态模式.md

状态模式

状态模式: 将事物内部的每个状态分别封装成类, 内部状态改变会产生不同行为。

优点: 用对象代替字符串记录当前状态, 状态易维护

缺点: 需编写大量状态类对象

场景 demo

某某牌电灯, 按一下按钮打开弱光, 按两下按钮打开强光, 按三下按钮关闭灯光。

// 将状态封装成不同类
const weakLight = function(light) {
  this.light = light
}

weakLight.prototype.press = function() {
  console.log('打开强光')
  this.light.setState(this.light.strongLight)
}

const strongLight = function(light) {
  this.light = light
}

strongLight.prototype.press = function() {
  console.log('关灯')
  this.light.setState(this.light.offLight)
}

const offLight = function(light) {
  this.light = light
}

offLight.prototype.press = function() {
  console.log('打开弱光')
  this.light.setState(this.light.weakLight)
}

const Light = function() {
  this.weakLight = new weakLight(this)
  this.strongLight = new strongLight(this)
  this.offLight = new offLight(this)
  this.currentState = this.offLight          // 初始状态
}

Light.prototype.init = function() {
  const btn = document.createElement('button')
  btn.innerHTML = '按钮'
  document.body.append(btn)
  const self = this
  btn.addEventListener('click', function() {
    self.currentState.press()
  })
}

Light.prototype.setState = function(state) { // 改变当前状态
  this.currentState = state
}

const light = new Light()
light.init()

// 打开弱光
// 打开强光
// 关灯

非面向对象实现的状态模式

借助于 JavaScript 的委托机制, 可以像如下实现状态模式:

const obj = {
  'weakLight': {
    press: function() {
      console.log('打开强光')
      this.currentState = obj.strongLight
    }
  },
  'strongLight': {
    press: function() {
      console.log('关灯')
      this.currentState = obj.offLight
    }
  },
  'offLight': {
    press: function() {
      console.log('打开弱光')
      this.currentState = obj.weakLight
    }
  },
}

const Light = function() {
  this.currentState = obj.offLight
}

Light.prototype.init = function() {
  const btn = document.createElement('button')
  btn.innerHTML = '按钮'
  document.body.append(btn)
  const self = this
  btn.addEventListener('click', function() {
    self.currentState.press.call(self) // 通过 call 完成委托
  })
}

const light = new Light()
light.init()