.Net观察者模式

// 客户端（Client）
static void Main(string[] args)
{
    //我们有了三位储户，都是武林高手，也比较有钱
    Depositor huangFeiHong = new BeiJingDepositor("黄飞鸿", 3000);
    Depositor fangShiYu = new BeiJingDepositor("方世玉", 1300);
    Depositor hongXiGuan = new BeiJingDepositor("洪熙官", 2500);

    BankMessageSystem beijingBank = new BeiJingBankMessageSystem();
    //这三位开始订阅银行短信业务
    beijingBank.Add(huangFeiHong);
    beijingBank.Add(fangShiYu);
    beijingBank.Add(hongXiGuan);

    //黄飞鸿取100块钱
    huangFeiHong.GetMoney(100);
    beijingBank.Notify();

    //黄飞鸿和方世玉都取了钱
    huangFeiHong.GetMoney(200);
    fangShiYu.GetMoney(200);
    beijingBank.Notify();

    //他们三个都取了钱
    huangFeiHong.GetMoney(320);
    fangShiYu.GetMoney(4330);
    hongXiGuan.GetMoney(332);
    beijingBank.Notify();
}

//银行短信系统抽象接口，是被观察者--该类型相当于抽象主体角色Subject
public abstract class BankMessageSystem
{
    protected IList<Depositor> observers;

    //构造函数初始化观察者列表实例
    protected BankMessageSystem()
    {
        observers = new List<Depositor>();
    }

    //增加预约储户
    public abstract void Add(Depositor depositor);

    //删除预约储户
    public abstract void Delete(Depositor depositor);

    //通知储户
    public void Notify()
    {
        foreach (Depositor depositor in observers)
        {
            if (depositor.AccountIsChanged)
            {
                depositor.Update(depositor.Balance, depositor.OperationDateTime);
                //账户发生了变化，并且通知了，储户的账户就认为没有变化
                depositor.AccountIsChanged = false;
            }
        }
    }
}

//北京银行短信系统，是被观察者--该类型相当于具体主体角色ConcreteSubject
public sealed class BeiJingBankMessageSystem : BankMessageSystem
{
    //增加预约储户
    public override void Add(Depositor depositor)
    {
        //应该先判断该用户是否存在，存在不操作，不存在则增加到储户列表中，这里简化了
        observers.Add(depositor);
    }

    //删除预约储户
    public override void Delete(Depositor depositor)
    {
        //应该先判断该用户是否存在，存在则删除，不存在无操作，这里简化了
        observers.Remove(depositor);
    }
}

//储户的抽象接口--相当于抽象观察者角色（Observer）
public abstract class Depositor
{
    //状态数据
    private string _name;
    private int _balance;
    private int _total;
    private bool _isChanged;

    //初始化状态数据
    protected Depositor(string name, int total)
    {
        this._name = name;
        this._balance = total;//存款总额等于余额
        this._isChanged = false;//账户未发生变化
    }

    //储户的名称，假设可以唯一区别的
    public string Name
    {
        get { return _name; }
        private set { this._name = value; }
    }

    public int Balance
    {
        get { return this._balance; }
    }

    //取钱
    public void GetMoney(int num)
    {
        if (num <= this._balance && num > 0)
        {
            this._balance = this._balance - num;
            this._isChanged = true;
            OperationDateTime = DateTime.Now;
        }
    }

    //账户操作时间
    public DateTime OperationDateTime { get; set; }

    //账户是否发生变化
    public bool AccountIsChanged
    {
        get { return this._isChanged; }
        set { this._isChanged = value; }
    }

    //更新储户状态
    public abstract void Update(int currentBalance, DateTime dateTime);
}

//北京的具体储户--相当于具体观察者角色ConcreteObserver
public sealed class BeiJingDepositor : Depositor
{
    public BeiJingDepositor(string name, int total) : base(name, total) { }

    public override void Update(int currentBalance, DateTime dateTime)
    {
        Console.WriteLine(Name + ":账户发生了变化，变化时间是" + dateTime.ToString() + ",当前余额是" + currentBalance.ToString());
    }
}