オブジェクト指向プログラミングにおける golang 関数の設計パターン

package main

import "fmt"

type Singleton struct{}

var instance *Singleton

func GetInstance() *Singleton {
  if instance == nil {
    instance = &Singleton{}
  }

  return instance
}

func main() {
  s1 := GetInstance()
  s2 := GetInstance()

  if s1 == s2 {
    fmt.Println("Same instance")
  }
}

package main

import "fmt"

type Item interface {
  GetName() string
}

type Book struct {
  name string
}

func (b *Book) GetName() string {
  return b.name
}

type Movie struct {
  name string
}

func (m *Movie) GetName() string {
  return m.name
}

type Factory struct {
  itemType string
}

func NewFactory(itemType string) *Factory {
  return &Factory{itemType: itemType}
}

func (f *Factory) CreateItem(name string) Item {
  switch f.itemType {
  case "book":
    return &Book{name}
  case "movie":
    return &Movie{name}
  }

  return nil
}

func main() {
  factory := NewFactory("book")
  item := factory.CreateItem("The Hitchhiker's Guide to the Galaxy")
  fmt.Println(item.GetName())
}

package main

import "fmt"

type Observable interface {
  AddObserver(observer Observer)
  RemoveObserver(observer Observer)
  NotifyObservers()
}

type Observer interface {
  Update()
}

type ConcreteObservable struct {
  observers []Observer
  state     int
}

func NewConcreteObservable() *ConcreteObservable {
  return &ConcreteObservable{
    observers: make([]Observer, 0),
  }
}

func (o *ConcreteObservable) AddObserver(observer Observer) {
  o.observers = append(o.observers, observer)
}

func (o *ConcreteObservable) RemoveObserver(observer Observer) {
  for i, obs := range o.observers {
    if obs == observer {
      o.observers = append(o.observers[:i], o.observers[i+1:]...)
      break
    }
  }
}

func (o *ConcreteObservable) NotifyObservers() {
  for _, observer := range o.observers {
    observer.Update()
  }
}

func (o *ConcreteObservable) SetState(state int) {
  o.state = state
  o.NotifyObservers()
}

type ConcreteObserver struct {
  name string
}

func NewConcreteObserver(name string) *ConcreteObserver {
  return &ConcreteObserver{name}
}

func (o *ConcreteObserver) Update() {
  fmt.Printf("Observer %s notified\n", o.name)
}

func main() {
  observable := NewConcreteObservable()

  observer1 := NewConcreteObserver("observer1")
  observer2 := NewConcreteObserver("observer2")

  observable.AddObserver(observer1)
  observable.AddObserver(observer2)

  observable.SetState(1)
  observable.SetState(2)
}