Observer Pattern

ObserverPattern1.cs

using System;
using System.Collections.Generic;

namespace design_pattern
{
    class Program
    {
        static void Main(string[] args)
        {
            var people1 = new People("Alvin");
            var people2 = new People("Lydia");
            var people3 = new People("Pekola");

            var trafficLight = new TrafficLight();
            //訂閱
            //按下有聲號誌鈕，但Pekola戴著耳機沒有聽到
            trafficLight.RegisterObserver(people1);
            trafficLight.RegisterObserver(people2);


            //綠燈
            trafficLight.SetLightState(TrafficLightState.Green);
            // Alvin過馬路囉！
            // Lydia過馬路囉！

            //紅燈
            trafficLight.SetLightState(TrafficLightState.Red);
            // Alvin紅燈停！
            // Lydia紅燈停！

            //Pekola拿下了耳機
            trafficLight.RegisterObserver(people3);
            trafficLight.SetLightState(TrafficLightState.Green);
            // Alvin過馬路囉！
            // Lydia過馬路囉！
            // Pekola過馬路囉！
        }

        public interface ISubject
        {
            public void RegisterObserver(IObserver observer);
            public void RemoveObserver(IObserver observer);
            public void NotifyObserver();
        }

        // 觀察者介面 - 讓Subject更新Observer狀態
        public interface IObserver
        {
            public void Update(TrafficLightState trafficLightState);
        }

        public enum TrafficLightState
        {
            Default = 0,
            Red = 1,
            Yellow = 2,
            Green = 3,
        }

        // 紅綠燈實作ISubject
        public class TrafficLight : ISubject
        {
            //儲存所有訂閱的觀察者
            private List<IObserver> _observers;
            private TrafficLightState _lightState;

            private void SetLightState()
            {
                NotifyObserver();
            }

            public TrafficLightState GetLightState => _lightState;

            public void SetLightState(TrafficLightState lightState)
            {
                _lightState = lightState;
                NotifyObserver();
            }

            public TrafficLight()
            {
                _observers = new List<IObserver>();
            }

            public void RegisterObserver(IObserver observer)
            {
                _observers.Add(observer);
            }

            public void RemoveObserver(IObserver observer)
            {
                _observers.Remove(observer);
            }

            public void NotifyObserver()
            {
                foreach (var observer in _observers)
                {
                    observer.Update(_lightState);
                }
                // 也可以這樣寫
                // _observers.ForEach(observer => observer.Update(_lightState));
            }
        }

        public class People : IObserver
        {
            private readonly string _name;

            public People(string name)
            {
                _name = name;
            }

            public void Update(TrafficLightState trafficLightState)
            {
                if (trafficLightState == TrafficLightState.Green)
                    Walk();
                if (trafficLightState == TrafficLightState.Red)
                    Stop();
            }

            public void Walk()
            {
                Console.WriteLine($"{_name}過馬路囉！");
            }

            public void Stop()
            {
                Console.WriteLine($"{_name}紅燈停！");
            }
        }
    }
}