C# 观察者模式讲解和代码示例

using System; using System.Collections.Generic; using System.Threading; namespace RefactoringGuru.DesignPatterns.Observer.Conceptual { public interface IObserver { // Receive update from subject void Update(ISubject subject); } public interface ISubject { // Attach an observer to the subject. void Attach(IObserver observer); // Detach an observer from the subject. void Detach(IObserver observer); // Notify all observers about an event. void Notify(); } // The Subject owns some important state and notifies observers when the // state changes. public class Subject : ISubject { // For the sake of simplicity, the Subject's state, essential to all // subscribers, is stored in this variable. 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.). 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. 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). 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(); } } } 

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...