1. 简介

观察者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

1.1 作用

观察者模式定义的是一种一对多的关系：

• 一是主题类，多是观察者类
• 当主题类的状态发生改变时，会通知与之对应的观察者类，去相应地更新状态
• 支持动态增加、删除观察者类对象

解决的问题

• 如下图所示，每次到货都要发邮件给各家各户
  

• 用观察者模式解决问题：商店浪费资源去通知没有需求的客户
  

优缺点

1.2使用场景

• 当应用中一些对象必须观察其他对象时，可用观察者模式，但仅能在有限的复杂度或特定情况下使用
• 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. 实现

2.1 UML 图

2.2 代码

Program：

namespace Observer
{
    class Program
    {
        static void Main(string[] args)
        {
            Subject subject = new Subject();
            ObserverA observerA = new ObserverA();
            ObserverB observerB = new ObserverB();

            subject.Attach(observerA);
            subject.Attach(observerB);

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

            subject.Detach(observerA);

            subject.SomeBusinessLogic();
        }
    }
}

IObserver：

namespace Observer
{
    interface IObserver
    {
        public void Update(ISubject subject) { }
    }
}

ObserverA：

using System;

namespace Observer
{
    class ObserverA: IObserver
    {
        public void Update(ISubject subject)
        {
            if ((subject as Subject).state > 2)
            {
                Console.WriteLine("ObserverAAA Update ... state: " + (subject as Subject).state.ToString());
            }
        }
    }
}

ObserverB：

using System;

namespace Observer
{
    class ObserverB:IObserver
    {
        public void Update(ISubject subject)
        {
            if((subject as Subject).state > 0 && (subject as Subject).state < 3)
            {
                Console.WriteLine("ObserverBBB Update ... state: " + (subject as Subject).state.ToString());
            }
        }
    }
}

ISubject：

namespace Observer
{
    interface ISubject
    {
        public void Attach(IObserver observer) { }
        public void Detach(IObserver observer) { }
        public void Notify() { }
    }
}

Subject：

using System;
using System.Collections.Generic;
using System.Threading;

namespace Observer
{
    class Subject: ISubject
    {
        public int state;
        private List<IObserver> observerList = new List<IObserver>();

        public void Attach(IObserver observer) 
        {
            if (observer != null)
            {
                observerList.Add(observer);
            }
        }

        public void Detach(IObserver observer) 
        {
            if (observer != null)
            {
                observerList.Remove(observer);
            }
        }

        public void Notify()
        {
            foreach (var observer in observerList)
            {
                observer.Update(this);
            }
        }

        public void SomeBusinessLogic()
        {
            Console.WriteLine("Subject do SomeBusinessLogic");
            state = new Random().Next(0, 10);

            Thread.Sleep(15);
            Console.WriteLine("Subject SomeBusinessLogic state: " + state.ToString());

            Notify();
        }
    }
}

F5 运行结果：

Subject do SomeBusinessLogic
Subject SomeBusinessLogic state: 8
ObserverAAA Update … state: 8
Subject do SomeBusinessLogic
Subject SomeBusinessLogic state: 0
Subject do SomeBusinessLogic
Subject SomeBusinessLogic state: 1
ObserverBBB Update … state: 1

参考：观察者模式