设计模式 外观模式(Facade)

意图

　　为子系统中的一组接口提供一个一致的界面，F a c a d e 模式定义了一个高层接口，这个接口使得这一子系统更加容易使用。

适用性

　　1.当你要为一个复杂子系统提供一个简单接口时。子系统往往因为不断演化而变得越来越复杂。大多数模式使用时都会产生更多更小的类。这使得子系统更具可重用性，也更容易对子系统进行定制，但这也给那些不需要定制子系统的用户带来一些使用上的困难。F a c a d e 可以提供一个简单的缺省视图，这一视图对大多数用户来说已经足够，而那些需要更多的可定制性的用户可以越过f a c a d e 层。

　　2.客户程序与抽象类的实现部分之间存在着很大的依赖性。引入f a c a d e 将这个子系统与客户以及其他的子系统分离，可以提高子系统的独立性和可移植性。 

　　3.当你需要构建一个层次结构的子系统时，使用f a c a d e 模式定义子系统中每层的入口点。如果子系统之间是相互依赖的，你可以让它们仅通过f a c a d e 进行通讯，从而简化了它们之间的依赖关系。 

结构图

　　

Code

// Facade

// Intent: "Provide a unified interface to a set of interfaces in a subsystem.
// Facade defines a higher-level interface that makes the subsystem easier
// to use". 

// For further information, read "Design Patterns", p185, Gamma et al.,
// Addison-Wesley, ISBN:0-201-63361-2

/* Notes:
 * Many subsystems are complex to manage - and this is a dis-incentive for 
 * client code to use them. A Facasde design pattern provides a simlified 
 * high-level API to the client, thus shielding it from direct contact 
 * with the (more complex) subsystem classes.  
 * 
 * Often the code that is inside a facade would have to be inside client 
 * code without the facade. The subsystem code beneath the facade can 
 * change, without affecting the client code. 
 */
 
namespace Facade_DesignPattern
{
    using System;

    class SubSystem_class1 
    {
        public void OperationX() 
        {
            Console.WriteLine("SubSystem_class1.OperationX called");
        }
    }

    class SubSystem_class2
    {
        public void OperationY()
        {
            Console.WriteLine("SubSystem_class2.OperationY called");
        }
    }

    class SubSystem_class3 
    {
        public void OperationZ()
        {            
            Console.WriteLine("SubSystem_class3.OperationZ called");
        }    
    }

    class Facade 
    {
        private SubSystem_class1 c1 = new SubSystem_class1();
        private SubSystem_class2 c2 = new SubSystem_class2();
        private SubSystem_class3 c3 = new SubSystem_class3();

        public void OperationWrapper()
        {
            Console.WriteLine("The Facade OperationWrapper carries out complex decision-making");
            Console.WriteLine("which in turn results in calls to the subsystem classes");
            c1.OperationX();
            if (1==1 /*some really complex decision*/)
            {
                c2.OperationY();
            }
            // lots of complex code here . . .
            c3.OperationZ();
        }
        
    }

    /// <summary>
///    Summary description for Client.
/// </summary>
    public class Client
    {
          public static int Main(string[] args)
        {
            Facade facade = new Facade();
            Console.WriteLine("Client calls the Facade OperationWrapper");
            facade.OperationWrapper();      
            return 0;
        }
    }
}