using System;
using System.Collections.Generic;
using System.Threading;namespace RefactoringGuru.DesignPatterns.Observer.Conceptual
{// Observer觀察者 也可以叫做訂閱者 subscriberspublic interface IObserver{// Receive update from subject// 接收來自主題的更新void Update(ISubject subject);}// 擁有一些值得關注的狀態的對象通常被稱為目標， 由于它要將自身的狀態改變通知給其他對象， 我們也將其稱為發布者 （publisher）。public interface ISubject{// Attach an observer to the subject.// 加一個觀察者。void Attach(IObserver observer);// Detach an observer from the subject.// 刪一個觀察者。void Detach(IObserver observer);// Notify all observers about an event.// 通知所有觀察者 某一事件 void Notify();}// The Subject owns some important state and notifies observers when the// state changes.// 主題擁有一些重要的狀態，并在狀態發生變化時通知觀察者。// 也可以叫發布者 Publisherpublic class Subject : ISubject{// For the sake of simplicity, the Subject's state, essential to all// subscribers, is stored in this variable.// 為了簡便起見，主題的狀態，對所有訂閱者都至關重要的，存儲在這個變量中。public int State { get; set; } = -0;// List of subscribers. In real life, the list of subscribers can be// stored more comprehensively (categorized by event type, etc.).// 訂閱者列表。在現實生活中，訂閱者列表可以更全面地存儲（按事件類型等分類）。private List<IObserver> _observers = new List<IObserver>();// The subscription management methods.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.");}// Trigger an update in each subscriber.// 在每個訂閱者中觸發更新public void Notify(){Console.WriteLine("Subject: Notifying observers...");foreach (var observer in _observers){observer.Update(this);}}// Usually, the subscription logic is only a fraction of what a Subject// can really do. Subjects commonly hold some important business logic,// that triggers a notification method whenever something important is// about to happen (or after it).// 通常，訂閱邏輯只是主題真正能做的一小部分。主題通常包含一些重要的業務邏輯，每當重要的事情即將發生（或之后）時，就會觸發通知方法。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();}}// Concrete Observers react to the updates issued by the Subject they had// been attached to.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){// The client code.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();}}
}

輸出：

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...

?原文鏈接：

C# 觀察者模式講解和代碼示例

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