设计模式-观察者 2022-03-03

观察者模式(Observer Pattern)是一种行为型设计模式，允许你定义一种订阅机制，可在对象事件发生时通知多个”观察”该对象的其他对象。

解释

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拥有一些值得关注的状态的对象通常被称为目标，由于它要将自身的状态改变通知给其他对象，我们也将其称为发布者(publisher)。所有希望关注发布者状态变化的其他对象被称为订阅者(subscribers)。

观察者模式建议你为发布者类添加订阅机制，让每个对象都能订阅或取消订阅发布者事件流。该机制包括:

1. 一个用于存储订阅者对象引用的列表成员变量
2. 几个用于添加或删除该列表中订阅者的公有方法

无论何时发生了重要的发布者事件，它都要遍历订阅者并调用其对象的特定通知方法。

观察者模式 UML

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适用场景

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1. 当一个对象状态的改变需要改变其他对象，或实际对象是事先未知的或动态变化的时，可使用观察者模式
2. 当应用中的一些对象必须观察其他对象时，可使用该模式。但仅能在有限时间内或特定情况下使用

代码示例

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using System;
namespace ObserverPatternDemo 
{
    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();
            // output

            // 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...
        }
    }
}