Bridge
動機
- 由於某些類型的固有的實現邏輯,使得它們具有兩個變化的維度,乃至多個緯度的變化。
- 如何應對這種“多維度的變化”?如何利用面向對象技術來使得類型可以輕鬆地沿着兩個乃至多個方向變化,而不引入額外的複雜度?
模式定義
將抽象部分(業務功能)與實現部分(平臺實現)分離,使它們都可以獨立地變化。 ——《設計模式》GoF
要點總結
- Bridge模式使用“對象間的組合關係”解耦了抽象和實現之間固有的綁定關係,使得抽象和實現可以沿着各自的維度來變化。所謂抽象和實現沿着各自緯度的變化,即“子類化”它們。
- Bridge模式有時候類似於多繼承方案,但是多繼承方案往往違背單一職責原則(即一個類只有一個變化的原因),複用性比較差。Bridge模式是比多繼承方案更好的解決方法。
- Bridge模式的應用一般在“兩個非常強的變化維度”,有時一個類也有多於兩個的變化維度,這時可以使用Bridge的擴展模式。
Demo
bridge1.cpp:
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(){}
};
//平臺實現
class PCMessagerBase : public Messager{
public:
virtual void PlaySound(){
//**********
}
virtual void DrawShape(){
//**********
}
virtual void WriteText(){
//**********
}
virtual void Connect(){
//**********
}
};
class MobileMessagerBase : public Messager{
public:
virtual void PlaySound(){
//==========
}
virtual void DrawShape(){
//==========
}
virtual void WriteText(){
//==========
}
virtual void Connect(){
//==========
}
};
//業務抽象
class PCMessagerLite : public PCMessagerBase {
public:
virtual void Login(string username, string password){
PCMessagerBase::Connect();
//........
}
virtual void SendMessage(string message){
PCMessagerBase::WriteText();
//........
}
virtual void SendPicture(Image image){
PCMessagerBase::DrawShape();
//........
}
};
class PCMessagerPerfect : public PCMessagerBase {
public:
virtual void Login(string username, string password){
PCMessagerBase::PlaySound();
//********
PCMessagerBase::Connect();
//........
}
virtual void SendMessage(string message){
PCMessagerBase::PlaySound();
//********
PCMessagerBase::WriteText();
//........
}
virtual void SendPicture(Image image){
PCMessagerBase::PlaySound();
//********
PCMessagerBase::DrawShape();
//........
}
};
class MobileMessagerLite : public MobileMessagerBase {
public:
virtual void Login(string username, string password){
MobileMessagerBase::Connect();
//........
}
virtual void SendMessage(string message){
MobileMessagerBase::WriteText();
//........
}
virtual void SendPicture(Image image){
MobileMessagerBase::DrawShape();
//........
}
};
class MobileMessagerPerfect : public MobileMessagerBase {
public:
virtual void Login(string username, string password){
MobileMessagerBase::PlaySound();
//********
MobileMessagerBase::Connect();
//........
}
virtual void SendMessage(string message){
MobileMessagerBase::PlaySound();
//********
MobileMessagerBase::WriteText();
//........
}
virtual void SendPicture(Image image){
MobileMessagerBase::PlaySound();
//********
MobileMessagerBase::DrawShape();
//........
}
};
void Process(){
//編譯時裝配
Messager *m =
new MobileMessagerPerfect();
}
由上可見,Messager類內部包含了兩行功能(平臺實現與業務抽象),應該職責拆分,見bridge2.cpp:
class Messager{
protected:
MessagerImp* messagerImp;//...
public:
virtual void Login(string username, string password)=0;
virtual void SendMessage(string message)=0;
virtual void SendPicture(Image image)=0;
virtual ~Messager(){}
};
// 不同維度的變化方向(業務和平臺),所以分爲兩個類
class MessagerImp{
public:
virtual void PlaySound()=0;
virtual void DrawShape()=0;
virtual void WriteText()=0;
virtual void Connect()=0;
virtual MessagerImp(){}
};
//平臺實現 n
class PCMessagerImp : public MessagerImp{
public:
virtual void PlaySound(){
//**********
}
virtual void DrawShape(){
//**********
}
virtual void WriteText(){
//**********
}
virtual void Connect(){
//**********
}
};
class MobileMessagerImp : public MessagerImp{
public:
virtual void PlaySound(){
//==========
}
virtual void DrawShape(){
//==========
}
virtual void WriteText(){
//==========
}
virtual void Connect(){
//==========
}
};
//業務抽象 m
//類的數目:1+n+m
class MessagerLite :public Messager {
//...構造函數,賦值父類messagerImp
public:
virtual void Login(string username, string password){
messagerImp->Connect();
//........
}
virtual void SendMessage(string message){
messagerImp->WriteText();
//........
}
virtual void SendPicture(Image image){
messagerImp->DrawShape();
//........
}
};
class MessagerPerfect :public Messager {
public:
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(){
//運行時裝配
MessagerImp* mImp = new PCMessagerImp();
Messager *m = new MessagerPerfect(mImp);
}
結構
兩個變化的方向獨立變化; 組合的指針作爲橋樑,連接兩邊的變化: