一、简介

状态模式一般用来实现状态机，而状态机常用在游戏、工作流引擎等系统开发中。不过，状态机的实现方式有多种，除了状态模式，比较常用的还有分支逻辑法和查表法。该模式允许对象内部状态改变使改变它的行为。

二、适用场景

• 行为随状态而改变
• 条件、分支语句的替代者

三、UML类图

四、案例

交通信号灯的红黄绿三色的变化。

package main

import (
	"fmt"
)

type TrafficLightState interface {
	Handle(trafficLight *TrafficLight)
}

type RedState struct {
	Name string
}

func NewRedState() *RedState {
	return &RedState{Name: "Red"}
}

func (r *RedState) Handle(trafficLight *TrafficLight) {
	fmt.Println("Red light: Stopped")
	trafficLight.SetState(NewGreenState())
}

type YellowState struct {
	Name string
}

func NewYellowState() *YellowState {
	return &YellowState{Name: "Yellow"}
}

func (y *YellowState) Handle(trafficLight *TrafficLight) {
	fmt.Println("Yellow light: Be prepared to stop")
	trafficLight.SetState(NewRedState())
}

type GreenState struct {
	Name string
}

func NewGreenState() *GreenState {
	return &GreenState{Name: "Green"}
}

func (g *GreenState) Handle(trafficLight *TrafficLight) {
	fmt.Println("Green light: Go")
	trafficLight.SetState(NewYellowState())
}

type TrafficLight struct {
	State TrafficLightState
}

func NewTrafficLight() *TrafficLight {
	return &TrafficLight{State: NewRedState()}
}

func (t *TrafficLight) SetState(state TrafficLightState) {
	t.State = state
}

func (t *TrafficLight) Change() {
	t.State.Handle(t)
}

func main() {
	trafficLight := NewTrafficLight()
	for i := 0; i < 10; i++ {
		trafficLight.Change()
	}
}