.Net 觀察者模式(Observer)

目錄

1. 定義
2. UML
3. 參與者
4. 樣本

 

定義:

定義對象間的一種一對多的依賴關係,以便當一個對象的狀態發生改變時,所有依賴於它的對象都得到通知並自動重新整理。

 

UML:

 

參與者：

主題（Subject）角色：

主題角色把所有對觀察考對象的引用儲存在一個聚集裡，每個主題都可以有任何數量的觀察者。主題提供一個介面，可以增加和刪除觀察者對象，主題角色又叫做被觀察者（Observable）角色。

具體主題（ConcreteSubject）角色：

實現主題這個角色介面的具體類，將有關狀態存入具體現察者對象；在具體主題的內部狀態改變時，給所有登記過的觀察者發出通知。具體主題角色又叫做具體被觀察者角色（Concrete Observable）。

觀察者（Observer）角色：

為所有的具體觀察者定義一個介面，在得到主題的通知時更新自己。這個介面叫做更新介面。抽象觀察者角色一般用一個抽象類別或者一個介面實現。在這個示意性的實現中，更新介面只包含一個方法（即Update()方法），這個方法叫做更新方法。

具體觀察者（ConcreteObserver）角色：

具體現察者角色實現觀察者角色所要求的更新介面，以便使本身的狀態與主題的狀態相協調。如果需要，具體現察者角色可以儲存一個指向具體主題對象的引用。具體觀察者角色通常用一個具體子類實現。

 

樣本：

代碼using System;using System.Collections.Generic; namespace DoFactory.GangOfFour.Observer.Structural{  /// <summary>  /// MainApp startup class for Structural  /// Observer Design Pattern.  /// </summary>  class MainApp  {    /// <summary>    /// Entry point into console application.    /// </summary>    static void Main()    {      // Configure Observer pattern      ConcreteSubject s = new ConcreteSubject();       s.Attach(new ConcreteObserver(s, "X"));      s.Attach(new ConcreteObserver(s, "Y"));      s.Attach(new ConcreteObserver(s, "Z"));       // Change subject and notify observers      s.SubjectState = "ABC";      s.Notify();       // Wait for user      Console.ReadKey();    }  }   /// <summary>  /// The 'Subject' abstract class  /// </summary>  abstract class Subject  {    private List<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 o in _observers)      {        o.Update();      }    }  }   /// <summary>  /// The 'ConcreteSubject' class  /// </summary>  class ConcreteSubject : Subject  {    private string _subjectState;     // Gets or sets subject state    public string SubjectState    {      get { return _subjectState; }      set { _subjectState = value; }    }  }   /// <summary>  /// The 'Observer' abstract class  /// </summary>  abstract class Observer  {    public abstract void Update();  }   /// <summary>  /// The 'ConcreteObserver' class  /// </summary>  class ConcreteObserver : Observer  {    private string _name;    private string _observerState;    private ConcreteSubject _subject;     // Constructor    public ConcreteObserver(      ConcreteSubject subject, string name)    {      this._subject = subject;      this._name = name;    }     public override void Update()    {      _observerState = _subject.SubjectState;      Console.WriteLine("Observer {0}'s new state is {1}",        _name, _observerState);    }     // Gets or sets subject    public ConcreteSubject Subject    {      get { return _subject; }      set { _subject = value; }    }  }}

output

Observer X's new state is ABC
Observer Y's new state is ABC
Observer Z's new state is ABC

 

實際運用

代碼using System;using System.Collections.Generic; namespace DoFactory.GangOfFour.Observer.RealWorld{  /// <summary>  /// MainApp startup class for Real-World  /// Observer Design Pattern.  /// </summary>  class MainApp  {    /// <summary>    /// Entry point into console application.    /// </summary>    static void Main()    {      // Create IBM stock and attach investors      IBM ibm = new IBM("IBM", 120.00);      ibm.Attach(new Investor("Sorros"));      ibm.Attach(new Investor("Berkshire"));       // Fluctuating prices will notify investors      ibm.Price = 120.10;      ibm.Price = 121.00;      ibm.Price = 120.50;      ibm.Price = 120.75;       // Wait for user      Console.ReadKey();    }  }   /// <summary>  /// The 'Subject' abstract class  /// </summary>  abstract class Stock  {    private string _symbol;    private double _price;    private List<IInvestor> _investors = new List<IInvestor>();     // Constructor    public Stock(string symbol, double price)    {      this._symbol = symbol;      this._price = price;    }     public void Attach(IInvestor investor)    {      _investors.Add(investor);    }     public void Detach(IInvestor investor)    {      _investors.Remove(investor);    }     public void Notify()    {      foreach (IInvestor investor in _investors)      {        investor.Update(this);      }       Console.WriteLine("");    }     // Gets or sets the price    public double Price    {      get { return _price; }      set      {        if (_price != value)        {          _price = value;          Notify();        }      }    }     // Gets the symbol    public string Symbol    {      get { return _symbol; }    }  }   /// <summary>  /// The 'ConcreteSubject' class  /// </summary>  class IBM : Stock  {    // Constructor    public IBM(string symbol, double price)      : base(symbol, price)    {    }  }   /// <summary>  /// The 'Observer' interface  /// </summary>  interface IInvestor  {    void Update(Stock stock);  }   /// <summary>  /// The 'ConcreteObserver' class  /// </summary>  class Investor : IInvestor  {    private string _name;    private Stock _stock;     // Constructor    public Investor(string name)    {      this._name = name;    }     public void Update(Stock stock)    {      Console.WriteLine("Notified {0} of {1}'s " +        "change to {2:C}", _name, stock.Symbol, stock.Price);    }     // Gets or sets the stock    public Stock Stock    {      get { return _stock; }      set { _stock = value; }    }  }}

Output

Notified Sorros of IBM's change to $120.10
Notified Berkshire of IBM's change to $120.10
Notified Sorros of IBM's change to $121.00
Notified Berkshire of IBM's change to $121.00
Notified Sorros of IBM's change to $120.50
Notified Berkshire of IBM's change to $120.50
Notified Sorros of IBM's change to $120.75
Notified Berkshire of IBM's change to $120.75