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.

  1. public interface IWeapon
  2. {
  3. int Damage { get; }
  4. }
  6. public class Sword : IWeapon
  7. {
  8. public int Damage
  9. {
  10. get
  11. {
  12. return 10;
  13. }
  14. }
  15. }

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.

  1. // Decorator
  2. public abstract class WeaponUpgrade : IWeapon
  3. {
  4. protected IWeapon weapon;
  6. public WeaponUpgrade(IWeapon weapon)
  7. {
  8. this.weapon = weapon;
  9. }
  11. public virtual int Damage
  12. {
  13. get
  14. {
  15. return this.weapon.Damage;
  16. }
  17. }
  18. }
  21. public class SteelSwordDecorator : WeaponUpgrade
  22. {
  23. public SteelSwordDecorator(IWeapon weapon)
  24. : base(weapon)
  25. {
  27. }
  29. // 100 damage from Steel + original weapon damage
  30. public override int Damage
  31. {
  32. get
  33. {
  34. return 100 + base.Damage;
  35. }
  36. }
  37. }

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

  1. // sword.Damage = 10
  2. IWeapon sword = new Sword();
  4. // sword.Damage = 110
  5. sword = new SteelSwordDecorator(sword);
  7. // sword.Damage = 210
  8. 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.