结构型模式篇-外观模式（Façade Pattern）-技术开发专区

结构型模式篇-外观模式（Façade Pattern）

作者：Terrylee 编辑：晓熊 2007-05-10 15:39

下面这三个类均是子系统类，示意代码：

//银行子系统
public class Bank
{
    public bool HasSufficientSavings(Customer c, int amount)
    {
        Console.WriteLine("Check bank for " + c.Name);
        return true;
    }
}

//信用子系统
public class Credit
{
    public bool HasGoodCredit(Customer c)
    {
        Console.WriteLine("Check credit for " + c.Name);
        return true;
    }
}

//贷款子系统
public class Loan
{
    public bool HasNoBadLoans(Customer c)
    {
        Console.WriteLine("Check loans for " + c.Name);
        return true;
    }
}

来看客户程序的调用：

//客户程序
public class MainApp
{
    private const int _amount = 12000;

    public static void Main()
    {
        Bank bank = new Bank();
        Loan loan = new Loan();
        Credit credit = new Credit();

        Customer customer = new Customer("Ann McKinsey");

        bool eligible = true;

        if (!bank.HasSufficientSavings(customer, _amount))
        {
            eligible = false;
        }
        else if (!loan.HasNoBadLoans(customer))
        {
            eligible = false;
        }
        else if (!credit.HasGoodCredit(customer))
        {
            eligible = false;
        }

        Console.WriteLine("\n" + customer.Name + " has been " + (eligible ? "Approved" : "Rejected"));
        Console.ReadLine();
    }
}

可以看到，在不用Façade模式的情况下，客户程序与三个子系统都发生了耦合，这种耦合使得客户程序依赖于子系统，当子系统变化时，客户程序也将面临很多变化的挑战。一个合情合理的设计就是为这些子系统创建一个统一的接口，这个接口简化了客户程序的判断操作。看一下引入Façade模式后的类结构图：

图4

门面类Mortage的实现如下：

//外观类
public class Mortgage
{
    private Bank bank = new Bank();
    private Loan loan = new Loan();
    private Credit credit = new Credit();

    public bool IsEligible(Customer cust, int amount)
    {
        Console.WriteLine("{0} applies for {1:C} loan\n",
          cust.Name, amount);

        bool eligible = true;

        if (!bank.HasSufficientSavings(cust, amount))
        {
            eligible = false;
        }
        else if (!loan.HasNoBadLoans(cust))
        {
            eligible = false;
        }
        else if (!credit.HasGoodCredit(cust))
        {
            eligible = false;
        }

        return eligible;
    }
}

顾客类和子系统类的实现仍然如下：

//银行子系统
public class Bank
{
    public bool HasSufficientSavings(Customer c, int amount)
    {
        Console.WriteLine("Check bank for " + c.Name);
        return true;
    }
}

//信用证子系统
public class Credit
{
    public bool HasGoodCredit(Customer c)
    {
        Console.WriteLine("Check credit for " + c.Name);
        return true;
    }
}

//贷款子系统
public class Loan
{
    public bool HasNoBadLoans(Customer c)
    {
        Console.WriteLine("Check loans for " + c.Name);
        return true;
    }
}

//顾客类
public class Customer
{
    private string name;

    public Customer(string name)
    {
        this.name = name;
    }

    public string Name
    {
        get { return name; }
    }
}

而此时客户程序的实现：

//客户程序类
public class MainApp
{
    public static void Main()
    {
        //外观
        Mortgage mortgage = new Mortgage();

        Customer customer = new Customer("Ann McKinsey");
        bool eligable = mortgage.IsEligible(customer, 125000);

        Console.WriteLine("\n" + customer.Name +
            " has been " + (eligable ? "Approved" : "Rejected"));
        Console.ReadLine();
    }
}

可以看到引入Façade模式后，客户程序只与Mortgage发生依赖，也就是Mortgage屏蔽了子系统之间的复杂的操作，达到了解耦内部子系统与客户程序之间的依赖。

第1页：概述第2页：Facade模式解说第3页：.NET架构中的Façade模式第4页：小结

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