设计模式——装饰器模式

装饰器模式是结构型设计模式，在Python中有一个非常著名的装饰器wrapper，它的实现方法就是使用了该设计模式，装饰器可以修饰类也可以修饰函数。

从类的设计上说，他的本质是在不定义子类的情况下动态的给对象添加一些额外的功能。举个简单的例子来说，假设你有一个芭比娃娃类，你想要为它穿上不同颜色、款式的衣服，如果每一个颜色/款式都要实现一个子类，就会产生大量的类。事实上，我们可以考虑用装饰模式动态地添加和删除，而而不需要修改芭比娃娃类本身，这样使得代码更加简洁和灵活。

基本结构

组件Component：通常是抽象类或者接口，是具体组件和装饰者的父类，定义了具体组件需要实现的方法，比如说我们定义的Coffee为组件。
具体组件ConcreteComponent：实现了Component接口的具体类，是被装饰的对象。
装饰类Decorator：一个抽象类，给具体组件添加功能，但是具体的功能由子类具体装饰者完成
具体装饰类ConcreteDecorater：扩展抽象类，负责向Component对象添加新的行为，例如加牛奶的咖啡，还有加糖的咖啡也是。

代码

/*** The base Component interface defines operations that can be altered by* decorators.*/
class Component {public:virtual ~Component() {}virtual std::string Operation() const = 0;
};
/*** Concrete Components provide default implementations of the operations. There* might be several variations of these classes.*/
class ConcreteComponent : public Component {public:std::string Operation() const override {return "ConcreteComponent";}
};
/*** 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 decorators. The default implementation of the wrapping code might* include a field for storing a wrapped component and the means to initialize* it.*/
class Decorator : public Component {/*** @var Component*/protected:Component* component_;public:Decorator(Component* component) : component_(component) {}/*** The Decorator delegates all work to the wrapped component.*/std::string Operation() const override {return this->component_->Operation();}
};
/*** Concrete Decorators call the wrapped object and alter its result in some way.*/
class ConcreteDecoratorA : public Decorator {/*** Decorators may call parent implementation of the operation, instead of* calling the wrapped object directly. This approach simplifies extension of* decorator classes.*/public:ConcreteDecoratorA(Component* component) : Decorator(component) {}std::string Operation() const override {return "ConcreteDecoratorA(" + Decorator::Operation() + ")";}
};
/*** Decorators can execute their behavior either before or after the call to a* wrapped object.*/
class ConcreteDecoratorB : public Decorator {public:ConcreteDecoratorB(Component* component) : Decorator(component) {}std::string Operation() const override {return "ConcreteDecoratorB(" + Decorator::Operation() + ")";}
};
/*** The client code works with all objects using the Component interface. This* way it can stay independent of the concrete classes of components it works* with.*/
void ClientCode(Component* component) {// ...std::cout << "RESULT: " << component->Operation();// ...
}int main() {/*** This way the client code can support both simple components...*/Component* simple = new ConcreteComponent;std::cout << "Client: I've got a simple component:\n";ClientCode(simple);std::cout << "\n\n";/*** ...as well as decorated ones.** Note how decorators can wrap not only simple components but the other* decorators as well.*/Component* decorator1 = new ConcreteDecoratorA(simple);Component* decorator2 = new ConcreteDecoratorB(decorator1);std::cout << "Client: Now I've got a decorated component:\n";ClientCode(decorator2);std::cout << "\n";delete simple;delete decorator1;delete decorator2;return 0;
}

输出

Client: I've got a simple component:
RESULT: ConcreteComponentClient: Now I've got a decorated component:
RESULT: ConcreteDecoratorB(ConcreteDecoratorA(ConcreteComponent))

上述代码很好理解，就是使用了一个装饰器类继承原先的组件，然后重载组件的操作符，利用了C++中的虚函数，即动态多态实现了给对象动态添加额外功能。该模式同时间接体现单一职责原则：你可以将实现了许多不同行为的一个大类拆分为多个较小的类。

装饰模式适合应用场景

如果你希望在无需修改代码的情况下即可使用对象，且希望在运行时为对象新增额外的行为，可以使用装饰模式。装饰能将业务逻辑组织为层次结构，你可为各层创建一个装饰，在运行时将各种不同逻辑组合成对象。由于这些对象都遵循通用接口，客户端代码能以相同的方式使用这些对象。
如果用继承来扩展对象行为的方案难以实现或者根本不可行，你可以使用该模式。许多编程语言使用 final最终关键字来限制对某个类的进一步扩展。复用最终类已有行为的唯一方法是使用装饰模式：用封装器对其进行封装。