CODESAMPLE

State - Go

State Go

The State pattern is a behavioral pattern that allows an object to alter its behavior when its internal state changes. It encapsulates different states of an object as separate classes (concrete states) and provides a way to transition between them. This avoids complex conditional logic within the object itself.

The Go implementation defines a State interface representing possible states. Concrete state structs (Red, Yellow, Green) implement this interface, each with its own behavior for handling a state transition (e.g., Process). A TrafficLight struct holds the current state and a method Transition to change it. This is idiomatic Go because it leverages interfaces for polymorphism, struct composition for behavior, and avoids direct modification of the TrafficLight’s state from outside, favoring managed transitions.

// traffic_light.go
package main

import "fmt"

// State interface defines the behavior of each state.
type State interface {
	Process(light *TrafficLight)
}

// Red state.
type Red struct{}

func (r *Red) Process(light *TrafficLight) {
	fmt.Println("Red Light: Stop!")
	light.setState(&Yellow{}) // Transition to Yellow
}

// Yellow state.
type Yellow struct{}

func (y *Yellow) Process(light *TrafficLight) {
	fmt.Println("Yellow Light: Caution!")
	light.setState(&Green{}) // Transition to Green
}

// Green state.
type Green struct{}

func (g *Green) Process(light *TrafficLight) {
	fmt.Println("Green Light: Go!")
	light.setState(&Red{}) // Transition to Red
}

// TrafficLight context.
type TrafficLight struct {
	currentState State
}

// NewTrafficLight creates a new TrafficLight in the Red state.
func NewTrafficLight() *TrafficLight {
	return &TrafficLight{currentState: &Red{}}
}

// Transition changes the current state.
func (light *TrafficLight) Transition() {
	light.currentState.Process(light)
}

// setState sets the current state.  Package-level access to maintain control.
func (light *TrafficLight) setState(state State) {
	light.currentState = state
}

func main() {
	light := NewTrafficLight()
	for i := 0; i < 5; i++ {
		light.Transition()
	}
}