데코레이터 패턴

데코레이터 패턴(Decorator pattern)이란 주어진 상황 및 용도에 따라 어떤 객체에 책임을 덧붙이는 패턴으로, 기능 확장이 필요할 때 서브클래싱 대신 쓸 수 있는 유연한 대안이 될 수 있다.

예

자바

// the Window interface
interface Window {
    public void draw(); // draws the Window
    public String getDescription(); // returns a description of the Window
}

// implementation of a simple Window without any scrollbars
class SimpleWindow implements Window {
    public void draw() {
        // draw window
    }

    public String getDescription() {
        return "simple window";
    }
}

아래의 클래스들은 모든 Window 클래스들의 데코레이터를 포함하고 있다.

// abstract decorator class - note that it implements Window
abstract class WindowDecorator implements Window {
    protected Window decoratedWindow; // the Window being decorated

    public WindowDecorator (Window decoratedWindow) {
        this.decoratedWindow = decoratedWindow;
    }
}

// the first concrete decorator which adds vertical scrollbar functionality
class VerticalScrollBarDecorator extends WindowDecorator {
    public VerticalScrollBarDecorator (Window decoratedWindow) {
        super(decoratedWindow);
    }

    public void draw() {
        drawVerticalScrollBar();
        decoratedWindow.draw();
    }

    private void drawVerticalScrollBar() {
        // draw the vertical scrollbar
    }

    public String getDescription() {
        return decoratedWindow.getDescription() + ", including vertical scrollbars";
    }
}

// the second concrete decorator which adds horizontal scrollbar functionality
class HorizontalScrollBarDecorator extends WindowDecorator {
    public HorizontalScrollBarDecorator (Window decoratedWindow) {
        super(decoratedWindow);
    }

    public void draw() {
        drawHorizontalScrollBar();
        decoratedWindow.draw();
    }

    private void drawHorizontalScrollBar() {
        // draw the horizontal scrollbar
    }

    public String getDescription() {
        return decoratedWindow.getDescription() + ", including horizontal scrollbars";
    }
}

Window 인스턴스를 만드는 테스트 프로그램은 아래와 같다.

public class DecoratedWindowTest {
    public static void main(String[] args) {
        // create a decorated Window with horizontal and vertical scrollbars
        Window decoratedWindow = new HorizontalScrollBarDecorator (
                new VerticalScrollBarDecorator(new SimpleWindow()));

        // print the Window's description
        System.out.println(decoratedWindow.getDescription());
    }
}

C++

#include <iostream>

using namespace std;

/* Component (interface) */
class Widget {

public:
  virtual void draw() = 0;
  virtual ~Widget() {}
};

/* ConcreteComponent */
class TextField : public Widget {

private:
   int width, height;

public:
   TextField( int w, int h ){
      width  = w;
      height = h;
   }

   void draw() {
      cout << "TextField: " << width << ", " << height << '\n';
   }
};

/* Decorator (interface) */
class Decorator : public Widget {

private:
   Widget* wid;       // reference to Widget

public:
   Decorator( Widget* w )  {
     wid = w;
   }

   void draw() {
     wid->draw();
   }

   ~Decorator() {
     delete wid;
   }
};

/* ConcreteDecoratorA */
class BorderDecorator : public Decorator {

public:
   BorderDecorator( Widget* w ) : Decorator( w ) { }
   void draw() {
      Decorator::draw();
      cout << "   BorderDecorator" << '\n';
   }
};

/* ConcreteDecoratorB */
class ScrollDecorator : public Decorator {
public:
   ScrollDecorator( Widget* w ) : Decorator( w ) { }
   void draw() {
      Decorator::draw();
      cout << "   ScrollDecorator" << '\n';
   }
};

int main( void ) {

   Widget* aWidget = new BorderDecorator(
                     new ScrollDecorator(
                     new TextField( 80, 24 )));
   aWidget->draw();
   delete aWidget;
}