设计模式学习之路——Decorator装饰模式(结构模式)

子类复子类，子类何其多

假如我们需要为游戏中开发一种坦克，除了各种不同型号的坦克外，我们还希望在不同场合中为其增加以下一种或多种功能：比如红外线夜视功能，比如水陆两栖功能，比如卫星定位功能等等。

动机（Motivation）

上述描述的问题根源在于我们“过度地使用了继承来扩展对象的功能”，由于继承为类型引入的静态特质，使得这种扩展方式缺乏灵活性；并且随着子类的增多（扩展功能的增多），各种子类的组合（扩展功能的组合）会导致更多子类的膨胀（多继承）。如何使“对象功能的扩展”能够根据需要来动态地实现？同时避免“扩展功能的增多”带来的子类膨胀问题？从而使得任何“功能扩展变化”所导致的影响将为最低？

意图（Intent）

动态地给一个对象增加一些额外的职责。就增加功能而言，Decorator模式比生成子类更为灵活。

——《设计模式》GoF

结构（Structure）

 

代码结构

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using System;   namespace Decorator.Structural {     /// <summary>     /// MainApp startup class for Structural     /// Decorator Design Pattern.     /// </summary>     class MainApp     {         static void Main()         {             // Create ConcreteComponent and two Decorators             ConcreteComponent c = new ConcreteComponent();             ConcreteDecoratorA d1 = new ConcreteDecoratorA();             ConcreteDecoratorB d2 = new ConcreteDecoratorB();               // Link decorators             d1.SetComponent(c);             d2.SetComponent(d1);               d2.Operation();             // Wait for user             Console.ReadKey();         }     }       /// <summary>     /// The 'Component' abstract class     /// </summary>     abstract class Component     {         public abstract void Operation();     }       /// <summary>     /// The 'ConcreteComponent' class     /// </summary>     class ConcreteComponent : Component     {         public override void Operation()         {             Console.WriteLine("ConcreteComponent.Operation()");         }     }       /// <summary>     /// The 'Decorator' abstract class     /// </summary>     abstract class Decorator : Component     {         protected Component component;         public void SetComponent(Component component)         {             this.component = component;         }         public override void Operation()         {             if (component != null)             {                 component.Operation();             }         }     }       /// <summary>     /// The 'ConcreteDecoratorA' class     /// </summary>     class ConcreteDecoratorA : Decorator     {         public override void Operation()         {             base.Operation();             Console.WriteLine("ConcreteDecoratorA.Operation()");         }     }       /// <summary>     /// The 'ConcreteDecoratorB' class     /// </summary>     class ConcreteDecoratorB : Decorator     {         public override void Operation()         {             base.Operation();             AddedBehavior();             Console.WriteLine("ConcreteDecoratorB.Operation()");         }         void AddedBehavior()         {         }     } }

输出：

 

代码示例：

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class Program     {         static void Main(string[] args)         {             // Create book             Book book = new Book("Worley", "Inside ASP.NET", 10);             book.Display();               // Create video             Video video = new Video("Spielberg", "Jaws", 23, 92);             video.Display();               // Make video borrowable, then borrow and display             Console.WriteLine("\nMaking video borrowable:");               Borrowable borrowvideo = new Borrowable(video);             borrowvideo.BorrowItem("Customer #1");             borrowvideo.BorrowItem("Customer #2");               borrowvideo.Display();               // Wait for user             Console.ReadKey();           }     }       /// <summary>     /// The 'Component' abstract class     /// </summary>     abstract class LibraryItem     {         private int _numCopies;           // Property         public int NumCopies         {             get { return _numCopies; }             set { _numCopies = value; }         }           public abstract void Display();     }       /// <summary>     /// The 'ConcreteComponent' class     /// </summary>     class Book : LibraryItem     {         private string _author;         private string _title;           // Constructor         public Book(string author, string title, int numCopies)         {             this._author = author;             this._title = title;             this.NumCopies = numCopies;         }           public override void Display()         {             Console.WriteLine("\nBook ------ ");             Console.WriteLine(" Author: {0}", _author);             Console.WriteLine(" Title: {0}", _title);             Console.WriteLine(" # Copies: {0}", NumCopies);         }     }       /// <summary>     /// The 'ConcreteComponent' class     /// </summary>     class Video : LibraryItem     {         private string _director;         private string _title;         private int _playTime;           // Constructor         public Video(string director, string title,           int numCopies, int playTime)         {             this._director = director;             this._title = title;             this.NumCopies = numCopies;             this._playTime = playTime;         }           public override void Display()         {             Console.WriteLine("\nVideo ----- ");             Console.WriteLine(" Director: {0}", _director);             Console.WriteLine(" Title: {0}", _title);             Console.WriteLine(" # Copies: {0}", NumCopies);             Console.WriteLine(" Playtime: {0}\n", _playTime);         }     }       /// <summary>     /// The 'Decorator' abstract class     /// </summary>     abstract class Decorator : LibraryItem     {         protected LibraryItem libraryItem;           // Constructor         public Decorator(LibraryItem libraryItem)         {             this.libraryItem = libraryItem;         }           public override void Display()         {             libraryItem.Display();         }     }       /// <summary>     /// The 'ConcreteDecorator' class     /// </summary>     class Borrowable : Decorator     {         protected List<string> borrowers = new List<string>();           // Constructor         public Borrowable(LibraryItem libraryItem)             : base(libraryItem)         {         }           public void BorrowItem(string name)         {             borrowers.Add(name);             libraryItem.NumCopies--;         }           public void ReturnItem(string name)         {             borrowers.Remove(name);             libraryItem.NumCopies++;         }           public override void Display()         {             base.Display();               foreach (string borrower in borrowers)             {                 Console.WriteLine(" borrower: " + borrower);             }         }     }

输出：

 

Decorator模式的几个要点

• 通过采用组合、而非继承的手法， Decorator模式实现了在运行时动态地扩展对象功能的能力，而且可以根据需要扩展多个功能。避免了单独使用继承带来的“灵活性差”和“多子类衍生问题”。
• Component类在Decorator模式中充当抽象接口的角色，不应该去实现具体的行为。而且Decorator类对于Component类应该透明——换言之Component类无需知道Decorator类，Decorator类是从外部来扩展Component类的功能。
• Decorator类在接口上表现为is-a Component的继承关系，即Decorator类继承了Component类所具有的接口。但在实现上又表现为has-a Component的组合关系，即Decorator类又使用了另外一个Component类。我们可以使用一个或者多个Decorator对象来“装饰”一个Component对象，且装饰后的对象仍然是一个Component对象。
• Decorator模式并非解决“多子类衍生的多继承”问题，Decorator模式应用的要点在于解决“主体类在多个方向上的扩展功能”——是为“装饰”的含义。

.NET框架中的Decorator应用