人生如棋、我愿为卒、行动虽缓、从未退过

意图

　　动态地给一个对象添加一些额外的职责。就增加功能来说，Decorator模式相比生成子类更为灵活。

适用性

　　1.在不影响其他对象的情况下，以动态、透明的方式给单个对象添加职责。 

　　2.处理那些可以撤消的职责。 

　　3.当不能采用生成子类的方法进行扩充时。一种情况是，可能有大量独立的扩展，为支持每一种组合将产生大量的子类，使得子类数目呈爆炸性增长。另一种情况可能是因为类定义被隐藏，或类定义不能用于生成子类。  

结构图

Code

// Decorator

/* Notes:
 * Dynamically wrap functionality with additonal functionality, so that
 * the client does not know the difference. 
 */
 
namespace Decorator_DesignPattern
{
    using System;

    abstract class Component
    {
        public abstract void Draw();         
    }

    class ConcreteComponent : Component
    {
        private string strName;
        public ConcreteComponent(string s)
        {
            strName = s;            
        }

        public override void Draw()
        {
            Console.WriteLine("ConcreteComponent - {0}", strName);            
        }        
    }

    abstract class Decorator : Component
    {
        protected Component ActualComponent;

        public void SetComponent(Component c)
        {
            ActualComponent = c;
        }
        public override void Draw()
        {
            if (ActualComponent != null)
                ActualComponent.Draw();        
        }
    }

    class ConcreteDecorator : Decorator 
    {
        private string strDecoratorName;
        public ConcreteDecorator (string str)
        {
            // how decoration occurs is localized inside this decorator
// For this demo, we simply print a decorator name
            strDecoratorName = str; 
        }
        public override void Draw()
        {
            CustomDecoration();
            base.Draw();
        }
        void CustomDecoration()
        {
            Console.WriteLine("In ConcreteDecorator: decoration goes here");
            Console.WriteLine("{0}", strDecoratorName);
        }
    }

    
    /// <summary>
///    Summary description for Client.
/// </summary>
    public class Client
    {
        Component Setup() 
        {
            ConcreteComponent c = new ConcreteComponent("This is the real component");

            ConcreteDecorator d = new ConcreteDecorator("This is a decorator for the component");

            d.SetComponent(c);

            return d;
        }
        
        public static int Main(string[] args)
        {
            Client client = new Client();
            Component c = client.Setup();    

            // The code below will work equally well with the real component, 
// or a decorator for the component

            c.Draw();
            
            return 0;
        }
    }
}