裝飾器模式 (Decorator Pattern)

可以動態地為物件附加額外的職責。使用裝飾器來擴展功能比使用繼承更有彈性
Head First Design Patterns, 2nd (p.91)

• 定義
  • Decorator is a structural pattern that allows adding new behaviors to objects dynamically by placing them inside special wrapper objects, called decorators
• 元素組成
  • Component
    The base Component interface defines operations that can be altered by decorators
  • ConcreteComponent
    Concrete Components provide default implementations of the operations. There might be several variations of these classes
  • Decorator
    The base Decorator class follows the same interface as the other components. The primary purpose of this class is to define the wrapping interface for all Concrete Decorator
  • Concrete Decorators
    Concrete Decorators call the wrapped object (Decorator) and alter its result
• 筆記
  • 開放 / 封閉原則: 類別應該容易擴展，但不會改變到既有舊功能
  • 遵守「開放/封閉原則」通常會引入新一層的抽象，讓程式更複雜，故應該把注意力放在最有可能改變的地方
  • 謹慎選擇要擴展的部分，到處採用此原則不但浪費，也沒必要，甚至可能寫出更複雜、難以理解的程式!
    • 缺點: 使用裝飾器，會產生大量的小類別，造成更多資源耗損
  • 在撰寫測試程式時，要注意針對抽象Component型態撰寫測試，而非具體Component，否則被包在裝飾器內部的物件可能會較難測試
  • 使用裝飾器，必須管理更多物件，更容易寫錯程式 (回傳錯層 Reference...等)。為了妥善封裝，通常會和工廠(Factory)或建造者(Builder)模式結合

範例 (與書本不同)

// DecoratorPattern.cpp : Defines the entry point for the console application.
//
 
#include "stdafx.h"
#include <iostream>
 
class IiceCream
{
public:
    virtual void Make() = 0;
    virtual ~IiceCream() { }
 
};
 
class SimpleIceCream: public IiceCream
{
public:
    virtual void Make() 
    {
        std::cout<<"\n milk + sugar +  Ice cream Powder";
    }
 
 
};
 
class IceCreamDecorator: public IiceCream
{
 
public:
    IceCreamDecorator(IiceCream& decorator):m_Decorator(decorator)
    {
 
    }
 
    virtual void Make() 
    {
        m_Decorator.Make();
    }
    private:
    IiceCream& m_Decorator;
};
 
class WithFruits : public IceCreamDecorator
{
 
public:
     WithFruits(IiceCream& decorator):IceCreamDecorator(decorator)
     {
 
     }
     virtual void Make() 
     {
         IceCreamDecorator::Make();
         std::cout<<" + Fruits";
     }
 
};
 
class WithNuts : public IceCreamDecorator
{
 
public:
    WithNuts(IiceCream& decorator):IceCreamDecorator(decorator)
    {
 
    }
 
    virtual void Make() 
    {
        IceCreamDecorator::Make();
        std::cout<<" + Nuts";
    }
 
};
 
class WithWafers : public IceCreamDecorator
{
 
public:
    WithWafers(IiceCream& decorator):IceCreamDecorator(decorator)
    {
 
    }
 
    virtual void Make() 
    {
        IceCreamDecorator::Make();
        std::cout<<" + Wafers";
    }
 
};
 
int _tmain(int argc, _TCHAR* argv[])
{
    IiceCream* pIceCreamSimple = new SimpleIceCream();
    pIceCreamSimple->Make();
 
    IiceCream* pIceCreamFruits = new WithFruits(*pIceCreamSimple);
    pIceCreamFruits->Make();
 
    IiceCream* pIceCreamNuts   = new WithNuts(*pIceCreamFruits);
    pIceCreamNuts->Make();
 
    IiceCream* pIceCreamWafers = new WithWafers(*pIceCreamNuts);
    pIceCreamWafers->Make();
 
    delete pIceCreamSimple;
    delete pIceCreamFruits;
    delete pIceCreamNuts;
    delete pIceCreamWafers;
 
    return 0;
}

Reference

[1]. https://refactoring.guru/design-patterns/decorator/cpp/example
[2]. https://cppcodetips.wordpress.com/2016/10/31/decorator-pattern-explained-with-c-sample/
[3]. https://www.bogotobogo.com/DesignPatterns/decorator.php