设计模式学习笔记——观察者模式（Observer）

1.特点：一对多依赖关系，一端变化、多端得到通知并自动更新。（目标与观察者可实现弱耦合）

2.概念：有时被称作发布/订阅模式，观察者模式定义了一种一对多的依赖关系，让多个观察者对象同时监听某一个主题对象。这个主题对象在状态发生变化时，会通知所有观察者对象，使它们能够自动更新自己。

3.类图：

4.代码实现

/// <summary>
    /// 抽象主题类
    /// </summary>
    public abstract class Subject
    {
        private IList<Observer> observers = new List<Observer>();
 
        /// <summary>
        /// 增加观察者
        /// </summary>
        /// <param name="observer"></param>
        public void Attach(Observer observer)
        {
            observers.Add(observer);
        }
 
        /// <summary>
        /// 移除观察者
        /// </summary>
        /// <param name="observer"></param>
        public void Detach(Observer observer)
        {
            observers.Remove(observer);
        }
 
        /// <summary>
        /// 向观察者（们）发出通知
        /// </summary>
        public void Notify()
        {
            foreach (Observer o in observers)
            {
                o.Update();
            }
        }
    }
 
    /// <summary>
    /// 抽象观察者类，为所有具体观察者定义一个接口，在得到通知时更新自己
    /// </summary>
    public abstract class Observer
    {
        public abstract void Update();
    }
 
    /// <summary>
    /// 具体观察者或具体通知者，将有关状态存入具体观察者对象；在具体主题的内部状态改变时，给所有登记过的观察者发出通知。具体主题角色通常用一个具体子类实现。
    /// </summary>
    public class ConcreteSubject : Subject
    {
        private string subjectState;
 
        /// <summary>
        /// 具体观察者的状态
        /// </summary>
        public string SubjectState
        {
            get { return subjectState; }
            set { subjectState = value; }
        }
    }
 
    /// <summary>
    /// 具体观察者，实现抽象观察者角色所要求的更新接口，已是本身状态与主题状态相协调
    /// </summary>
    public class ConcreteObserver : Observer
    {
        private string observerState;
        private string name;
        private ConcreteSubject subject;
 
        /// <summary>
        /// 具体观察者用一个具体主题来实现
        /// </summary>
        public ConcreteSubject Subject
        {
            get { return subject; }
            set { subject = value; }
        }
 
        public ConcreteObserver(ConcreteSubject subject, string name)
        {
            this.subject = subject;
            this.name = name;
        }
 
        /// <summary>
        /// 实现抽象观察者中的更新操作
        /// </summary>
        public override void Update()
        {
            observerState = subject.SubjectState;
            Console.WriteLine("The observer's state of {0} is {1}", name, observerState);
        }
    }

class Program
    {
        static void Main(string[] args)
        {
            // 具体主题角色通常用具体自来来实现
            ConcreteSubject subject = new ConcreteSubject();
 
            subject.Attach(new ConcreteObserver(subject, "Observer A"));
            subject.Attach(new ConcreteObserver(subject, "Observer B"));
            subject.Attach(new ConcreteObserver(subject, "Observer C"));
 
            subject.SubjectState = "Ready";
            subject.Notify();
 
            Console.Read();
        }
    }