This document discusses component-based entity systems for game development. It describes the disadvantages of inheritance-based models, including deep class hierarchies that are difficult to develop, maintain and extend. It then introduces an aggregation-based approach using entities composed of independent components. This approach favors composition over inheritance and improves extensibility. Finally, it describes entity system architectures where components contain data and systems contain logic, improving performance, serialization and other capabilities. Overall it advocates for entity systems as an easier way to build, maintain and extend game object models.

1. Style & Design Principles
Chapter 03: Component-Based Entity Systems
Nick Prühs

2. 5 Minute Review Session
• What’s the difference between inheritance and
aggregation?
• Explain the three types of polymorphism!
• What’s the difference between cohesion and
coupling?
• What are software design patterns?
• Explain three types of software design patterns,
and name an example for each one!
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3. Assignment Solution #2
DEMO
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4. Objectives
• To understand the disadvantages of inheritance-
based game models
• To learn how to build an aggregation-based game
models
• To understand the advantages and disadvantages of
aggregation-based game models
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5. “Say you’re an engineer…
… set out to create a new Game Object System from scratch, and you’re going to ‘do it right the first time’. You
talk to your designer and say ‘What kind of content are we going to have in this game?’
They respond with ‘Oh lots of stuff, trees, and birds, and bushes, and keys and locks and … <trailing off>’
And your eyes glaze over as you start thinking of fancy C++ ways to solve the problem.
The object oriented programming sages tell you to try to determine Is-A relationships and abstract functionality
and all that other fun stuff. You go to the book store and buy a C++ book just to be sure, and it tells you to fire
up your $5000 UML editor. [...]”
- Scott Bilas
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6. Entities
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7. Entities
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8. Entities
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9. Entities
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10. Entities
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11. Entities
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12. Entities
• Object in your game world
• Can (or cannot)…
• be visible
• move around
• attack
• explode
• be targeted
• become selected
• follow a path
• Common across all genres
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13. Entities
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14. Approach #1: Inheritance
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15. Approach #1: Inheritance
• Entity base class
• That class and its subclasses encapsulate the main
game logic
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16. Example #1: Unreal Engine 3
• Base class Actor
• Rendering
• Animation
• Sound
• Physics
• Almost everything in Unreal is an Actor
• Pawn extends by taking damage
• Projectile extends by spawning impact effects
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17. Drawbacks of
inheritance-based game models
• Diamond of Death
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18. Drawbacks of
inheritance-based game models
• Code added to the root of the inheritance tree
causes big overhead
• Code added to the leafs of the tree tends to get
copied
• Root and leaf classes tend to get very big
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19. Where is Waldo?
public override void Update(float dt)
{
this.SystemManager.Update(dt);
this.EventManager.ProcessEvents(dt);
}
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20. Where is Waldo?
public override void Update(float dt)
{
this.SystemManager.Update(dt);
this.EventManager.ProcessEvents(dt);
}
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21. Where is Waldo?
public override void Update(float dt)
{
base.Update(dt);
this.SystemManager.Update(dt);
this.EventManager.ProcessEvents(dt);
}
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22. Drawbacks of
inheritance-based game models
• Always need to understand all base classes along
the inheritance tree
• Impossible to enforce calling base class functions
• Someone will forget it. Trust me.
• And you’re gonna spend your whole evening finding that one
missing base.Update().
• Deep class hierarchies will more likely run into call
order issues
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23. Inheritance-based game
models are…
• … difficult to develop
• … difficult to maintain
• … difficult to extend
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24. “There are probably
hundreds of ways…
… you could decompose your systems and come up with a set of
classes […], and eventually, all of them are wrong. This isn’t to say
that they won’t work, but games are constantly changing,
constantly invalidating your carefully planned designs. [...]
So you hand off your new Game Object System and go work on
other things.
Then one day your designer says that they want a new type of
“alien” asteroid that acts just like a heat seeking missile, except it’s
still an asteroid.”
- Scott Bilas
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25. “alien” asteroid
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26. Approach #2: Aggregation
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27. Approach #2: Aggregation
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28. Approach #2: Aggregation
• Popular since Gas Powered Games’ Dungeon Siege
• Introduced long before
• Entities are aggregations of components
• Which in turn encapsulate independent functionality
• Corresponds to recommendations by the Gang of Four
• “favor object composition over class inheritance”
• Similar approach is used by the Unity3D game engine
• Just for clarification: Unreal uses components as well, called
ActorComponent
• Got much more prominent in latest UDK release
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29. Approach #2a
• Create an Entity class
• Add references to all available components
• Has obvious disadvantages:
• Many component references will be null pointers for
most entities
• Big unnecessary memory overhead
• Entity class has to be updated each time a new
component is introduced
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30. Approach #2b
• Create an Entity class
• Introduce a common base class for components
• Entities hold a collection of Component objects
• Reduced the memory overhead
• Increased extensibility
• Already gets close to an optimal solution
• easy to build, maintain and debug
• easy to implement new design ideas without breaking
existing code
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31. However, we can do better.
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32. Approach #2c: Entity Systems
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33. Approach #2c: Entity Systems
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34. Approach #2c: Entity Systems
• Game entities are nothing more than just an id
• Thus, no data or methods on entities
• No methods on components, either: all
functionality goes into what is called a system
• PhysicsSystem
• HealthSystem
• FightSystem
• Entirely operate on their corresponding
components
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35. “All the data goes into the
Components.
All of it. Think you can take some “really common”
data, e. g. the x-/y-/z-coordinates of the in-game
object, and put it into the Entity itself? Nope. Don’t
go there. As soon as you start migrating data into the
Entity, you’ve lost. By definition the only valid place
for the data is inside the Component.”
- Adam Martin
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36. Example #2: Simple Fight
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37. Example #2: Simple Fight
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38. Example #2: Simple Fight
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39. Example #2: Simple Fight
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40. Example #2: Simple Fight
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41. Example #2: Simple Fight
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42. Example #2: Simple Fight
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43. Example #2: Simple Fight
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44. Example #2: Simple Fight
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45. Example #2: Simple Fight
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46. Inter-System Communication
Systems communicate by the means of events, only.
• No coupling between systems
• Easy to add or remove systems at any time
• Great architectural advantage for general game
features
• Need multiplayer? Just send the events over the network!
• Need AI? Just make it create events which are handled just
like player input is!
• Need replays? Just write all events with timestamps to a file!
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47. Inter-System Communication
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48. Entity Systems –
Implementation
DEMO
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49. Advantages of Entity Systems
• Update order is obvious
• Components can easily be pooled and re-used
• Independent systems can be updated by separate
threads
• Data can easily be serialized and stored in a
database
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50. Disadvantages of
Entity Systems (?)
• Lookups cause performance hit
• Resist the urge to add cross-component references – this
would make you lose all of the advantages mentioned
before
• Just don’t flood your system with unnecessary
component types – just as you would always do
• Misbelief that it takes longer to “get the job done”
• Used at the InnoGames Game Jam #3 for creating a
multi-platform multi-player real-time tactics game in just
48 hours – spending the little extra effort at the
beginning pays off
• Always.
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51. Future Prospects
• Attribute Tables
• store arbitrary key-value-pairs
• used for initializing all components of an entity
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<AttributeTable>
<Attribute keyType="System.String" valueType="System.Single">
<Key>EnemyHealthModificationComponent.EnemyHealthModifier</Key>
<Value>-3</Value>
</Attribute>
<Attribute keyType="System.String" valueType="System.String">
<Key>ActionComponent.Name</Key>
<Value>Take that!</Value>
</Attribute>
</AttributeTable>

52. Future Prospects
• Blueprints
• consist of a list of components and an attribute table
• created with some kind of editor tool by designers
• used for creating entites at run-time
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<Blueprint>
<ComponentTypes>
<ComponentType>FreudBot.Logic.Components.ActionComponent</ComponentType>
<ComponentType>FreudBot.Logic.Components.EnemyHealthModificationComponent</ComponentType>
</ComponentTypes>
<AttributeTable>
<Attribute keyType="System.String" valueType="System.Single">
<Key>EnemyHealthModificationComponent.EnemyHealthModifier</Key>
<Value>-3</Value>
</Attribute>
<Attribute keyType="System.String" valueType="System.String">
<Key>ActionComponent.Name</Key>
<Value>Take that!</Value>
</Attribute>
</AttributeTable>
</Blueprint>

53. Future Prospects
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StarCraft II Galaxy Editor

54. Future Prospects
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StarCraft II Galaxy Editor

55. Future Prospects
• Hierarchical Attribute Tables
• used for overloading blueprints with specific entity attribute
values
• e.g. reduced initial health
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56. Future Prospects
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StarCraft II Galaxy Editor

57. Future Prospects
public int CreateEntity(Blueprint blueprint, IAttributeTable configuration)
{
int entityId = this.CreateEntity();
// Setup attribute table.
HierarchicalAttributeTable attributeTable = new HierarchicalAttributeTable();
attributeTable.AddParent(blueprint.GetAttributeTable());
attributeTable.AddParent(configuration);
// Add components.
foreach (Type componentType in blueprint.GetAllComponentTypes())
{
// Create component.
IEntityComponent component = (IEntityComponent)Activator.CreateInstance(componentType);
// Add component to entity.
this.AddComponent(entityId, component);
// Initialize component with the attribute table data.
component.InitComponent(attributeTable);
}
// Raise event.
this.game.EventManager.QueueEvent(FrameworkEventType.EntityInitialized, entityId);
return entityId;
}
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58. Takeaway
• inheritance-based game models show a lot of
disadvantages
• entity systems are easy to maintain and debug
• provide great extensibility without the necessity of
modifying existing code
• show better performance characteristics for both
memory and CPU load
• easy to implement commonly used features
• scripting
• serialization
• logging
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59. References
• Mick West. Evolve Your Hierarchy. http://cowboyprogramming.com/2007/01/05/evolve-your-
heirachy/, January 5, 2007.
• Levi Baker. Entity Systems Part 1: Entity and EntityManager.
http://blog.chronoclast.com/2010/12/entity-systems-part-1-entity-and.html, December 24,
2010.
• Kyle Wilson. Game Object Structure: Inheritance vs. Aggregation.
http://gamearchitect.net/Articles/GameObjects1.html, July 3, 2002.
• Adam Martin. Entity Systems are the future of MMOG development – Part 1. http://t-
machine.org/index.php/2007/09/03/entity-systems-are-the-future-of-mmog-development-part-
1/, September 3, 2007.
• Adam Martin. Entity Systems: what makes good Components? good Entities? http://t-
machine.org/index.php/2012/03/16/entity-systems-what-makes-good-components-good-
entities/, March 16, 2012.
• Scott Bilas. A Data-Driven Game Object System.
http://scottbilas.com/files/2002/gdc_san_jose/game_objects_slides_with_notes.pdf, Slides,
GDC 2002.
• Scott Bilas. A Data-Driven Game Object System.
http://scottbilas.com/files/2002/gdc_san_jose/game_objects_paper.pdf, Paper, GDC 2002.
• Insomniac Games. A Dynamic Component Architecture for High Performance Gameplay.
http://www.insomniacgames.com/a-dynamic-component-architecture-for-high-performance-
gameplay/, June 1, 2010.
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60. Thank you for your attention!
Contact
Mail
dev@npruehs.de
Blog
http://www.npruehs.de
Twitter
@npruehs
Github
https://github.com/npruehs
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