Decorator

The decorator pattern

The decorator pattern is a structural pattern. The benefit is to adorn another class to broaden its functionality. The example demonstrates this 'recursively'. It's complicated.
First an abstract VisualComponent:

abstract class VisualComponent {
    public virtual void Draw() {
        Console.WriteLine("Drawing " + this.GetType().Name);
    }
}

A nothing implementation:

class TextView : VisualComponent {
}

A quick console test of TextView:

static void Main(string[] args) {
    VisualComponent textView = new TextView();
    textView.Draw();
    Console.ReadKey();
}

Output:

Drawing TextView

Cool code complete!

New specifications land. The (production)TextView requires a border!
What to do? ...
Build a decorator! Disturbing only the composite root.
The general decorator:

class VisualComponentDecorator : VisualComponent {
    protected VisualComponent visualComponent;

    public VisualComponentDecorator(VisualComponent visualComponent) {
        this.visualComponent = visualComponent;
    }
    public override void Draw() {
        visualComponent.Draw();
    }
}

The decorator (above) inherits from abstract class VisualComponent. Also taking the abstract class as a dependency.

The new Border decorator, implementation:

class BorderVisualComponentDecorator : VisualComponentDecorator {
    private int borderWidth;

    public BorderVisualComponentDecorator(VisualComponent visualComponent, int borderWidth) : base(visualComponent) {
        this.borderWidth = borderWidth;
    }
    public override void Draw() {
        visualComponent.Draw();
        DrawBorder(borderWidth);
    }
    private void DrawBorder(int borderWidth) {
        Console.WriteLine("Drawing border with width {0}.", borderWidth);
    }
}

Adjusting the root:

static void Main(string[] args) {
    VisualComponent textView = new TextView();
    BorderVisualComponentDecorator border = new BorderVisualComponentDecorator(textView, 3);

    border.Draw();

    Console.ReadKey();
}

The output:

Drawing TextView
Drawing border with width 3.

Code complete! Time to hit the pub/gym/club...

Later, the testers complain they can't see stuff at the bottom of the page.
A scrollbar is required.
No hassle!
Build a scrollbar decorator for the border decorator:

class ScrollVisualComponentDecorator : VisualComponentDecorator {
    public ScrollVisualComponentDecorator(VisualComponent visualComponent) : base(visualComponent) { }
    public override void Draw() {
        visualComponent.Draw();
        AdornScrollbar();
    }

    private void AdornScrollbar() {
        Console.WriteLine("Adorning scrollbar.");
    }
}

Changing the corresponding root code:

static void Main(string[] args) {
    VisualComponent textView = new TextView();
    VisualComponent scrollbar = new ScrollVisualComponentDecorator(textView);
    BorderVisualComponentDecorator border = new BorderVisualComponentDecorator(scrollbar, 3);

    border.Draw();

    Console.ReadKey();
}

Now the output is:

Drawing TextView
Adorning scrollbar.
Drawing border with width 3.

Summary

The decorator pattern is complicated. Maybe feeding one instance to another can cause confusion.

References

wiki
Design Patterns [GOF]
dofactory decorator pattern
tutorialspoint decorator pattern