设计模式之八：外观模式(Facade)

外观模式：
为子系统中的一系列接口提供了一个统一的界面。外观模式定义了一个高层次的接口以使子系统更加easy使用。

Provide a unified interface to a set of interfaces in a subsystem. 
Façade defines a higher-level interface that makes the subsystem easier to use.

UML图：

主要包含：

1. Facade(MortgageApplication)：了解每一个子系统负者的功能；将client的请求分发给合适的子系统。
2. SubSystem classes（Bank,Credit,Loan）:实现子系统的功能。处理Facade指定的业务，这个类中没有Facede类的信息。

C++代码实现：

#include <iostream>


class SubSystem1
{
    public:
            void method1()
            {
                std::cout<<"SubSystem1::method1 invoke"<<std::endl;
            }
};

class SubSystem2
{
    public:
            void method2()
            {
                std::cout<<"SubSystem2::method2 invoke"<<std::endl;
            }
};

class SubSystem3
{
    public:
            void method3()
            {
                std::cout<<"SubSystem3::method3 invoke"<<std::endl;
            }
};


class Facade
{
        public:
                Facade()
                {
                    subSystem1=new SubSystem1();
                    subSystem2=new SubSystem2();
                    subSystem3=new SubSystem3();
                }
                void methodA()
                {
                    std::cout<<"Facade methodA include:"<<std::endl;
                    subSystem1->method1();
                    subSystem2->method2();
                }
                void methodB()
                {
                    std::cout<<"Facade methodB include:"<<std::endl;
                    subSystem1->method1();
                    subSystem3->method3();
                }
                ~Facade()
                {
                    if(subSystem1!=NULL)
                            delete subSystem1;
                    if(subSystem2!=NULL)
                            delete subSystem2;
                    if(subSystem3!=NULL)
                            delete subSystem3;
                }
        private:
                SubSystem1 * subSystem1;
                SubSystem2 * subSystem2;
                SubSystem3 * subSystem3;    
};

int main()
{
    std::cout<<"外观模式測试"<<std::endl;
    Facade * facade=new Facade;
    facade->methodA();
    facade->methodB();
    delete facade;
    return 0;
}

运行结果：

一个详细的样例：

Facade为Morgage，这个类用来推断是否能对客户抵押贷款(依据客户的银行账户，行用卡记录，贷款记录来推断)。

SubSystem包含Bank(即客户的银行账户是否满足要求),Credit(客户的信用卡是否符合要求),Loan(客户的是否存在不良贷款记录)。

C++代码例如以下：

#include <iostream>
#include <stdlib.h>
#include <stdio.h>
#include <string>
using namespace std;
class Customer
{
        public:
        Customer(string str=""):name(str)
        {

        }
        string getName() const
        {
                return name;
        }
        void setName(string str)
        {
            name=str;
        }

        private:
                string name;

};

class Bank
{
        public:
                bool hasSufficientSavings(Customer c,int amount)
                {
                    std::cout<<"check bank for "<<c.getName()<<std::endl;
                    return true;
                }
};

class Credit
{
        public:
                bool hasGoodCredit(Customer c)
                {
                    std::cout<<"check credit for "<<c.getName()<<std::endl;
                    return true;
                }
};

class Loan
{
        public:
                bool hasNoBadLoans(Customer c)
                {
                    std::cout<<"check loans for "<<c.getName()<<std::endl;
                    return true;
                }   
};

class Mortgage
{
        public:
                Mortgage()
                {
                    bank=new Bank;
                    credit=new Credit;
                    loan=new Loan;
                }
                bool isEligible(Customer c, int amount)
                {
                    std::cout<<c.getName()<<" applies for "<<amount<<std::endl;
                    bool eligible=true;
                    if(!bank->hasSufficientSavings(c,amount))
                    {
                        eligible=false;
                    }
                    else if(!credit->hasGoodCredit(c))
                    {
                        eligible=false;
                    }
                    else if(!loan->hasNoBadLoans(c))
                    {
                        eligible=false;
                    }

                    return eligible;

                }
                ~Mortgage()
                {
                    if(!bank)
                            delete bank;
                    if(!credit)
                            delete credit;
                    if(!loan)
                            delete loan;
                }
        private:
                Bank * bank;
                Credit *credit;
                Loan * loan;
};


int main()
{
    std::cout<<"外观模式详细样例"<<std::endl;
    Mortgage *mortgage=new Mortgage();
    Customer c(string("John"));
    bool eligible =mortgage->isEligible(c,25000);

    std::cout<<c.getName()<<" has been "<<(eligible?
"approve":"reject")<<std::endl;
    return 0;
}

运行结果：

外观模式是一个对迪米特法则非常好的应用。

迪米特法则：
假设两个类不必直接通信，那么这两个类就不应当发生直接的相互作用。假设当中一个类须要调用还有一个类的某一个方法的话。能够通过第三者转发这个调用。
这也是减少耦合性的思想。