State

一个 State 对象封装一个与状态相关的行为, 运用有限状态机 (Finite State Machines) 根据 Object State 改变 Object Behavior, object set state as its member, state set object as its method parameter:

• 每一种状态和它对应的行为之间的关系局部化:
  • 状态具体行为封装在状态类中, 便于阅读和管理代码.
  • 状态切换规则分布在状态类中, 有效地消除了大量条件分支语句.
• 符合开放封闭原则.

State Use Case

• Networking protocol stack.
• Game player logic.
• Workflow management.
• Finite state machines.

class Switch {
  constructor() {
    this.state = new OffState()
  }

  on() {
    this.state.on(this)
  }

  off() {
    this.state.off(this)
  }
}

class State {
  constructor() {
    if (this.constructor === State)
      throw new Error('abstract!')
  }

  on(sw) {
    console.log('Light is already on.')
  }

  off(sw) {
    console.log('Light is already off.')
  }
}

class OnState extends State {
  constructor() {
    super()
    console.log('Light turned on.')
  }

  off(sw) {
    console.log('Turning light off...')
    sw.state = new OffState()
  }
}

class OffState extends State {
  constructor() {
    super()
    console.log('Light turned off.')
  }

  on(sw) {
    console.log('Turning light on...')
    sw.state = new OnState()
  }
}

const button = new Switch()
button.on()
button.off()