Question

I have a query regarding the Delegation Design Principle in OOP. I will use the Window & Rectangle classes example from the GOF design patterns book to explain my query.

public class Window
{
    Rectangle myDelegate;

    public void Area()
    {
        return myDelegate.Area();
    }
}

public class Rectangle
{
    public int Area()
    {
        return 2*3;
    }
}

My question is: Can the Rectangle( the delegate) have a reference to the window( the parent class). i.e:

public class Window
{
    Rectangle myDelegate;
    public int myArea;

    public void Area()
    {
        myDelegate.Area();
    }
}

public class Rectangle
{
    Window myParent;

    public void Area()
    {
        myParent.myArea = 2 * 3;
    }
}

In non trivial cases the above would make it much more convenient for the delegate to update the state of the parent.
Is that logical or am I missing something here?

Thanks!

Answer 1

public interface IShape
{
  void Area();
  void SetArea(int area);
}

public class Window : IShape
{
   private IShape rectangle;
   private int myArea;

   public Window(IShape shape)
   {
      rectangle = shape;
   }

   public void SetArea(int area)
   {
      myArea = area;
   }
   public void Area()
   {
      rectangle.Area();
   }
}

public class Rectangle : IShape
{
   private IShape window;

   public Rectangle(IShape shape)
   {
       window = shape;
   }

   public void Area()
   {
      SetArea(2 * 3);
   }

   public void SetArea(int area)
   {
       window.SetArea(area);
   }
}