Design Patterns: Decorator

The Decorator Design Patterns is handy for adding behavior or state to an object during runtime.

If we are developing a game it could be used to handle item upgrades. In the following example there is a simple Sword. From this simple item we can upgrade it and add more damage to it.

public interface IWeapon
{
    int Damage { get; }
}

public class Sword : IWeapon
{
    public int Damage
    {
        get
        {
            return 10;
        }
    }
}

We now have a simple Sword. To add the decorator pattern we need to create the upgrades. The decorator class will take the weapon and increment its values.

// Decorator
public abstract class WeaponUpgrade : IWeapon
{
    protected IWeapon weapon;

    public WeaponUpgrade(IWeapon weapon)
    {
        this.weapon = weapon;
    }

    public virtual int Damage
    {
        get
        {
            return this.weapon.Damage;
        }
    }
}


public class SteelSwordDecorator : WeaponUpgrade
{
    public SteelSwordDecorator(IWeapon weapon)
        : base(weapon)
    {

    }

    // 100 damage from Steel + original weapon damage
    public override int Damage
    {
        get
        {
            return 100 + base.Damage;
        }
    }
}

To use this structure we just need to instantiate a decorator class and “put on top of” our previous class:

// sword.Damage = 10
IWeapon sword = new Sword();

// sword.Damage = 110
sword = new SteelSwordDecorator(sword);

// sword.Damage = 210
sword = new SteelSwordDecorator(sword);

We of course could have multiple types of decorators (Diamond Sword, Flame Sword and so on).
One example in Starcraft is the weapon upgrades, John Lindquist explains about it in a Pattern Craft video: PatternCraft - Decorator Pattern.