Facade设计模式

Facade[f'sɑ:d]，建筑物的外观的意思。在程序开发中，会有很多小的类，他们之间的关系错综复杂，如果想完成一件任务，需要了解这些类的关系和调用顺序才行。将这些错综复杂的类关系用Facade类包装起来，对外提供一个窗口，就是Facade设计模式。
比如顾客（Customer）申请房屋抵押贷款，怎样根据顾客的身份和申请贷款的金额来判断能否批准，需要做非常复杂的手续：先调用银行类(Bank)，判断是不是有足够的存款，然后调用信用评定类(Credit)，判断是不是有足够的信用等级，然后调用借款类Loan，判断是不是有坏账。如果把这些复杂的评定手续包装到一个Morgate类，只对外提供一个IsEligible窗口，判断一个顾客能否批注贷款，就非常的方便。这个例子用程序实现如下：

    class MainApp
    {
        //Test
        static void Main()
        {
            // Facade
            Mortgage mortgage = new Mortgage();
            //创建一个顾客：Tom
            Customer customer = new Customer("Tom");
            //判断是否能提供抵押贷款
            bool eligible = mortgage.IsEligible(customer, 125000);

            Console.WriteLine("\n" + customer.Name +

                " has been " + (eligible ? "Approved" : "Rejected"));
            // Wait for user
            Console.ReadKey();
        }
    }
    //银行类
    class Bank
    {
        public bool HasSufficientSavings(Customer c, int amount)
        {
            Console.WriteLine("Check bank for " + c.Name);
            return true;
        }
    }
    //信用评定类
    class Credit
    {
        public bool HasGoodCredit(Customer c)
        {

            Console.WriteLine("Check credit for " + c.Name);
            return true;
        }
    }
    //以往贷款记录类
    class Loan
    {
        public bool HasNoBadLoans(Customer c)
        {
            Console.WriteLine("Check loans for " + c.Name);
            return true;
        }
    }
    //顾客类
    class Customer
    {
        private string _name;
        // Constructor
        public Customer(string name)
        {
            this._name = name;
        }
        // Gets the name
        public string Name
        {
            get { return _name; }
        }
    }
    //Facade类
    class Mortgage
    {
        private Bank _bank = new Bank();
        private Loan _loan = new Loan();
        private Credit _credit = new Credit();
        //对外提供的窗口
        public bool IsEligible(Customer cust, int amount)
        {
            Console.WriteLine("{0} applies for {1:C} loan\n",
              cust.Name, amount);
            bool eligible = true;
            // 具体评定能否贷款的手续
            if (!_bank.HasSufficientSavings(cust, amount))
            {
                eligible = false;
            }
            else if (!_loan.HasNoBadLoans(cust))
            {
                eligible = false;
            }
            else if (!_credit.HasGoodCredit(cust))
            {
                eligible = false;
            }
            return eligible;
        }
    }

执行结果：
Check bank for Tom
Check loans for Tom
Check credit for Tom

Tom has been Approved