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

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：

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