옵저버 패턴 (Observer Pattern)

옵저버 패턴은 객체의 상태 변화를 관찰하는 옵저버들을 두어 변화가 있을 때마다 객체가 옵저버들에게 메소드 등으로 통지하는 행동 디자인 패턴이다. 변화가 있는 객체에서 옵저버 리스트를 두어 변화가 있을 때 리스트에 있는 옵저버들을 호출하게 된다.

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장단점

장점

• 객체와 옵저버간 결합이 느슨해진다.
• 직관적인 이해가 쉽다.

단점

• 멀티스레드 환경에서 등록/취소 작업과 실행 작업이 겹칠 수 있어 결과가 비틀어질 수 있다.
• 너무 많은 옵저버가 있는 경우 시간이 느려지기도 하고 상태 관리가 힘들다.

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코드 예시

	public interface IObserver
    {
        void Update(ISubject subject);
    }

    public interface ISubject
    {
        void Attach(IObserver observer);

        void Detach(IObserver observer);

        void Notify();
    }

    public class Subject : ISubject
    {
        public int State { get; set; } = -0;

        private List<IObserver> _observers = new List<IObserver>();

        public void Attach(IObserver observer)
        {
            Console.WriteLine("Subject: Attached an observer.");
            this._observers.Add(observer);
        }

        public void Detach(IObserver observer)
        {
            this._observers.Remove(observer);
            Console.WriteLine("Subject: Detached an observer.");
        }

		// 변화 업데이트
        public void Notify()
        {
            Console.WriteLine("Subject: Notifying observers...");

            foreach (var observer in _observers)
            {
                observer.Update(this);
            }
        }

        public void SomeBusinessLogic()
        {
            Console.WriteLine("\nSubject: I'm doing something important.");
            this.State = new Random().Next(0, 10);

            Thread.Sleep(15);

            Console.WriteLine("Subject: My state has just changed to: " + this.State);
            this.Notify();
        }
    }

    class ConcreteObserverA : IObserver
    {
        public void Update(ISubject subject)
        {            
            if ((subject as Subject).State < 3)
            {
                Console.WriteLine("ConcreteObserverA: Reacted to the event.");
            }
        }
    }

    class ConcreteObserverB : IObserver
    {
        public void Update(ISubject subject)
        {
            if ((subject as Subject).State == 0 || (subject as Subject).State >= 2)
            {
                Console.WriteLine("ConcreteObserverB: Reacted to the event.");
            }
        }
    }
    
    class Program
    {
        static void Main(string[] args)
        {
            var subject = new Subject();
            var observerA = new ConcreteObserverA();
            subject.Attach(observerA);

            var observerB = new ConcreteObserverB();
            subject.Attach(observerB);

            subject.SomeBusinessLogic();
            subject.SomeBusinessLogic();

            subject.Detach(observerB);

            subject.SomeBusinessLogic();
        }
    }

출력
Subject: Attached an observer.
Subject: Attached an observer.

Subject: I'm doing something important.
Subject: My state has just changed to: 2
Subject: Notifying observers...
ConcreteObserverA: Reacted to the event.
ConcreteObserverB: Reacted to the event.

Subject: I'm doing something important.
Subject: My state has just changed to: 1
Subject: Notifying observers...
ConcreteObserverA: Reacted to the event.
Subject: Detached an observer.

Subject: I'm doing something important.
Subject: My state has just changed to: 5
Subject: Notifying observers...