设计模式第七讲--Bridge 桥模式

1. “单一职责”模式

在软件组件的设计中，如果则热划分的不清晰，使用继承得到的结果往往是需求的变化，子类急剧膨胀，同时充斥着重复代码，这时候的关键是划清责任。

典型模式

Decorator

Bridge

2. 动机（Motivation）

由于某些类型的固有的实现逻辑，使得它们具有两个变化的维度，乃至多个维度的变化。

如何应对这种“多维度的变化”？如何利用面向对象技术来使得类型可以轻松的沿着两个乃至多个方向变化，而不引入额外的复杂度。

3. 代码

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#include<string>
 
using namespace std;
 
class Messager
{
public:
    virtual void Login(string username, string password) = 0;
    virtual void SendMessage(string message) = 0;
    virtual void SendPicture(Image image) = 0;
 
    virtual void PlaySound() = 0;
    virtual void DrawShape() = 0;
    virtual void WriteText() = 0;
    virtual void Connect() = 0;
 
    virtual ~Messager(){}
};
 
//平台实现
class PCMessageBase :public Messager
{
public:
    virtual void PlaySound()
    {
        //...
    }
    virtual void DrawShape()
    {
        //...
    }
    virtual void WriteText()
    {
        //...
    }
    virtual void Connect()
    {
        //...
    }
};
 
//平台实现
class MobileMessageBase :public Messager
{
public:
    virtual void PlaySound()
    {
        //...
    }
    virtual void DrawShape()
    {
        //...
    }
    virtual void WriteText()
    {
        //...
    }
    virtual void Connect()
    {
        //...
    }
};
 
//抽象业务
class PCMessageLite :public PCMessageBase
{
public:
    virtual void Login(string username, string password)
    {
        PCMessageBase::Connect();
        //...
    }
 
    virtual void SendMessage(string message)
    {
        PCMessageBase::WriteText();
        //...
    }
 
    virtual void SendPicture(Image image)
    {
        PCMessageBase::DrawShape();
        //...
    }
};
 
class PCMessagePerfect :public PCMessageBase
{
public:
    virtual void Login(string username, string password)
    {
        PCMessageBase::PlaySound();
        //...
 
        PCMessageBase::Connect();
        //...
    }
 
    virtual void SendMessage(string message)
    {
        PCMessageBase::PlaySound();
        //...
 
        PCMessageBase::WriteText();
        //...
    }
 
    virtual void SendPicture(Image image)
    {
        PCMessageBase::PlaySound();
        //...
 
        PCMessageBase::DrawShape();
        //...
    }
};
 
class MobileMessageLite :public MobileMessageBase
{
public:
    virtual void Login(string username, string password)
    {
        MobileMessageBase::Connect();
        //...
    }
 
    virtual void SendMessage(string message)
    {
        MobileMessageBase::WriteText();
        //...
    }
 
    virtual void SendPicture(Image image)
    {
        MobileMessageBase::DrawShape();
        //...
    }
};
 
class MobileMessagePerfect :public MobileMessageBase
{
public:
    virtual void Login(string username, string password)
    {
        MobileMessageBase::PlaySound();
        //...
 
        MobileMessageBase::Connect();
        //...
    }
 
    virtual void SendMessage(string message)
    {
        MobileMessageBase::PlaySound();
        //...
 
        MobileMessageBase::WriteText();
        //...
    }
 
    virtual void SendPicture(Image image)
    {
        MobileMessageBase::PlaySound();
        //...
 
        MobileMessageBase::DrawShape();
        //...
    }
};
 
void Process()
{
    //编译时装配
    Messager* m = new MobileMessagePerfect();
}　　

各类之间的继承关系如下：

使用Bridge设计模式修改之后的代码

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#include<string>
 
using namespace std;
 
class MessagerImp
{
public:
    virtual void PlaySound() = 0;
    virtual void DrawShape() = 0;
    virtual void WriteText() = 0;
    virtual void Connect() = 0;
 
    virtual ~MessagerImp() {}
};
 
class Messager
{
protected:
    MessagerImp* messagerImp;
    Messager(MessagerImp* m) :messagerImp(m) {};
public:
    virtual void Login(string username, string password) = 0;
    virtual void SendMessage(string message) = 0;
    virtual void SendPicture(Image image) = 0;
 
    virtual ~Messager() {}
};
 
//平台实现
class PCMessageImp :public MessagerImp
{
public:
    virtual void PlaySound()
    {
        //...
    }
    virtual void DrawShape()
    {
        //...
    }
    virtual void WriteText()
    {
        //...
    }
    virtual void Connect()
    {
        //...
    }
};
 
//移动设备实现
class MobileMessageImp :public MessagerImp
{
public:
    virtual void PlaySound()
    {
        //...
    }
    virtual void DrawShape()
    {
        //...
    }
    virtual void WriteText()
    {
        //...
    }
    virtual void Connect()
    {
        //...
    }
};
 
//抽象业务
class MessageLite :public Messager
{
public:
    MessageLite(MessagerImp* m) :Messager(m)
    {}
 
    virtual void Login(string username, string password)
    {
        messagerImp->Connect();
        //...
    }
 
    virtual void SendMessage(string message)
    {
        messagerImp->WriteText();
        //...
    }
 
    virtual void SendPicture(Image image)
    {
        messagerImp->DrawShape();
        //...
    }
};
 
class MessagePerfect :public  Messager
{
public:
    MessagePerfect(MessagerImp* m) :Messager(m)
    {}
 
    virtual void Login(string username, string password)
    {
        messagerImp->PlaySound();
        //...
 
        messagerImp->Connect();
        //...
    }
 
    virtual void SendMessage(string message)
    {
        messagerImp->PlaySound();
        //...
 
        messagerImp->WriteText();
        //...
    }
 
    virtual void SendPicture(Image image)
    {
        messagerImp->PlaySound();
        //...
 
        messagerImp->DrawShape();
        //...
    }
};
 
 
void Process()
{
    //运行时装配
    MessagerImp* mImp = new PCMessageImp();
 
    Messager* m = new MessagePerfect(mImp);
}