观察者模式

一、定义：

　　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。此种模式通常被用来实时事件处理系统。其中两个重要对象是观察者和主题，要想主题对象发生改变时，能通知到所有观察者角色，则自然主题角色必须引用观察者，从而完成观察模式。

 　　特点：

　　1、定义了一对多的关系。

　　2、主题用一个共同的接口来更新观察者。

　　3、观察者和主题用松耦合的方式结合，不需要知道实现细节，只需要实现接口就可以。

二、UML类图：

　　

三、基本代码：　　

    class Program
    {
        static void Main(string[] args)
        {
            ConcreteSubject cs = new ConcreteSubject();

            cs.Attach(new ConcreteObserver(cs, "x"));
            cs.Attach(new ConcreteObserver(cs, "y"));

            cs.SubjectState = "abc";
            cs.Notify();

            Console.Read();
        }
    }

    abstract class Subject
    {
        private IList<Observer> observers = new List<Observer>();

        public void Attach(Observer observer)
        {
            observers.Add(observer);
        }

        public void Detach(Observer observer)
        {
            observers.Remove(observer);
        }

        public void Notify()
        {
            foreach (Observer ob in observers)
            {
                ob.Update();
            }
        }
    }

    class ConcreteSubject : Subject
    {
        private string subjectState;
        public string SubjectState
        {
            get { return subjectState; }
            set { subjectState = value; }
        }
    }

    abstract class Observer
    {
        public abstract void Update();
    }

    class ConcreteObserver : Observer
    {
        private string name;
        private string observerState;
        private ConcreteSubject subject;
        public ConcreteSubject Subject
        {
            get { return subject; }
            set { subject = value; }
        }

        public ConcreteObserver(ConcreteSubject subject, string name)
        {
            this.subject = subject;
            this.name = name;
        }

        public override void Update()
        {
            observerState = subject.SubjectState;
            Console.WriteLine("观察者{0}的新状态是{1}", name, observerState);
        }  
    }

class Program
    {
        static void Main(string[] args)
        {
            ConcreteSubject cs = new ConcreteSubject();

            cs.Attach(new ConcreteObserver(cs, "x"));
            cs.Attach(new ConcreteObserver(cs, "y"));

            cs.SubjectState = "abc";
            cs.Notify();

            Console.Read();
        }
    }

    abstract class Subject
    {
        private IList<Observer> observers = new List<Observer>();

        public void Attach(Observer observer)
        {
            observers.Add(observer);
        }

        public void Detach(Observer observer)
        {
            observers.Remove(observer);
        }

        public void Notify()
        {
            foreach (Observer ob in observers)
            {
                ob.Update();
            }
        }
    }

    class ConcreteSubject : Subject
    {
        private string subjectState;
        public string SubjectState
        {
            get { return subjectState; }
            set { subjectState = value; }
        }
    }

    abstract class Observer
    {
        public abstract void Update();
    }

    class ConcreteObserver : Observer
    {
        private string name;
        private string observerState;
        private ConcreteSubject subject;
        public ConcreteSubject Subject
        {
            get { return subject; }
            set { subject = value; }
        }

        public ConcreteObserver(ConcreteSubject subject, string name)
        {
            this.subject = subject;
            this.name = name;
        }

        public override void Update()
        {
            observerState = subject.SubjectState;
            Console.WriteLine("观察者{0}的新状态是{1}", name, observerState);
        }  
    }

四、适用场景：

　　当一个对象的改变需要同时改变其他对象时，而且不知道具体有多少对象有待改变时，应该考虑使用观察者模式。

　　举例说明：

　　

　　实例代码：　

interface ISubject
    {
        void Attach(Observer observer);
        void Detach(Observer observer);
        void Notify();
        string SubjectState { get; set; }
    }

    class Boss : ISubject
    {
        private IList<Observer> observers = new List<Observer>();
        private string bossAction;

        public string SubjectState
        {
            get { return bossAction; }
            set { bossAction = value; }
        }

        public void Attach(Observer observer)
        {
            observers.Add(observer);
        }

        public void Detach(Observer observer)
        {
            observers.Remove(observer);
        }

        public void Notify()
        {
            foreach (Observer ob in observers)
            {
                ob.Update();
            }
        }
    }

    abstract class Observer
    {
        protected string name;
        protected ISubject subject;

        public Observer(string name, ISubject subject)
        {
            this.name = name;
            this.subject = subject;
        }

        public abstract void Update();
    }

    class StockObserver : Observer
    {
        public StockObserver (string name,ISubject subject):base(name,subject )
        {}

        public override void Update()
        {
            Console.WriteLine("{0},{1} 关闭股票，继续工作", subject.SubjectState, name);
        }
    }

    class NBAObserver : Observer
    {
        public NBAObserver (string name,ISubject subject):base(name,subject)
        {}

        public override void Update()
        {
            Console.WriteLine("{0},{1} 关闭NBA，继续工作", subject.SubjectState, name);
        }
    }

Boss boss = new Boss();
Observer s1 = new StockObserver("张三", boss);
Observer s2 = new NBAObserver("李四", boss);
boss.Attach(s1);
boss.Attach(s2);
boss.SubjectState = "老板回来了";
boss.Notify();

五、观察者模式的缺点及解决方法：

　　在具体前台、同事和老板的实例中，使用依赖倒转原则，但是“抽象通知者”还是依赖“抽象观察者”，即如果没有抽象观察者这样的接口，通知功能就完不成。另外就是每个具体观察者，它不一定是“更新”的方法要调用，可能是其他的方法。解决方法：通知者和观察者之间根本互相不知道，由客户端决定通知谁。对于前台的实例，决定使用委托的方式来实现由客户端来决定具体通知谁的功能。

　　实例说明：

   interface ISubject
    {
        void Attach(Observer observer);
        void Detach(Observer observer);
        void Notify();
        string SubjectState { get; set; }
    }

    class Boss : ISubject
    {
        private IList<Observer> observers = new List<Observer>();
        private string bossAction;

        public string SubjectState
        {
            get { return bossAction; }
            set { bossAction = value; }
        }

        public void Attach(Observer observer)
        {
            observers.Add(observer);
        }

        public void Detach(Observer observer)
        {
            observers.Remove(observer);
        }

        public void Notify()
        {
            foreach (Observer ob in observers)
            {
                ob.Update();
            }
        }
    }

    abstract class Observer
    {
        protected string name;
        protected ISubject subject;

        public Observer(string name, ISubject subject)
        {
            this.name = name;
            this.subject = subject;
        }

        public abstract void Update();
    }

    class StockObserver : Observer
    {
        public StockObserver (string name,ISubject subject):base(name,subject )
        {}

        public override void Update()
        {
            Console.WriteLine("{0},{1} 关闭股票，继续工作", subject.SubjectState, name);
        }
    }

    class NBAObserver : Observer
    {
        public NBAObserver (string name,ISubject subject):base(name,subject)
        {}

        public override void Update()
        {
            Console.WriteLine("{0},{1} 关闭NBA，继续工作", subject.SubjectState, name);
        }
    }

interface ISubject
    {
        void Notify();
        string SubjectState { get; set; }
    }

    public delegate void DelegateHandler();

    class Boss : ISubject
    {
        public event DelegateHandler Update;

        public string SubjectState { get; set; }

        public void Notify()
        {
            Update();
        }
    }

    abstract class Observer
    {
        protected string name;
        protected ISubject subject;

        public Observer(string name, ISubject subject)
        {
            this.name = name;
            this.subject = subject;
        }

        public abstract void Update();
    }

    class StockObserver : Observer
    {
        public StockObserver(string name, ISubject subject)
            : base(name, subject)
        { }

        public override void Update()
        {
            Console.WriteLine("{0},{1} 关闭股票，继续工作", subject.SubjectState, name);
        }
    }

    class NBAObserver : Observer
    {
        public NBAObserver(string name, ISubject subject)
            : base(name, subject)
        { }

        public override void Update()
        {
            Console.WriteLine("{0},{1} 关闭NBA，继续工作", subject.SubjectState, name);
        }
    }

Boss boss = new Boss();
StockObserver s1 = new StockObserver("张三", boss);
NBAObserver s2 = new NBAObserver("李四", boss);

boss.Update += new DelegateHandler(s1.Update);
boss.Update += new DelegateHandler(s2.Update);
boss.SubjectState = "老板回来了";
boss.Notify();

六、总结：

　　观察者模式所做的工作其实就是在解除耦合。让耦合的双方都依赖于抽象，而不是依赖于具体，从而使各自的变化都不会影响到另一边的变化。