Game Programming 02 - Component-Based Entity Systems

Game Programming
Component-Based Entity Systems
Nick Prühs

Objectives
• To understand the disadvantages of inheritance-based game
models
• To learn how to build an aggregation-based game model
• To understand the advantages and disadvantages of aggregation-
based game models
2 / 57

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
3 / 57

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
10 / 57

Approach #1: Inheritance
12 / 57

Approach #1: Inheritance
• Entity base class
• that class and its subclasses encapsulate the main game logic
13 / 57

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
14 / 57

Drawbacks of inheritance-based game models
• Diamond of Death
15 / 57

• 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
16 / 57

Where is Waldo?
public override void TakeDamage(int damage)
{
this.Health -= damage;
}
17 / 57

Where is Waldo?
{
}
18 / 57

Where is Waldo?
{
base.TakeDamage(damage);
}
19 / 57

• always need to understand all base classes along the inheritance
tree
• impossible to enforce calling base class functions
 Someone will forget it. Trust me.
o 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
20 / 57

Inheritance-based game models are…
• … difficult to develop
• … difficult to maintain
• … difficult to extend
21 / 57

“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
22 / 57

Approach #2: Aggregation
24 / 57

25 / 57

• 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
26 / 57

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
27 / 57

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
28 / 57

However, we can do better.
29 / 57

Approach #2c: Entity Systems
30 / 57

31 / 57

• 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
32 / 57

“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
33 / 57

Example #2: Simple Fight
34 / 57

35 / 57

36 / 57

37 / 57

38 / 57

39 / 57

40 / 57

41 / 57

42 / 57

43 / 57

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!
44 / 57

Inter-System Communication
45 / 57

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
46 / 57

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
o Always.
47 / 57

Future Prospects
• Attribute Tables
 store arbitrary key-value-pairs
 used for initializing all components of an entity
48 / 57
<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>

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
49 / 57
<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>

Future Prospects
50 / 57
StarCraft II Galaxy Editor

Future Prospects
51 / 57

Future Prospects
• Hierarchical Attribute Tables
 used for overloading blueprints with specific entity
attribute values
 e.g. reduced initial health
52 / 57

Future Prospects
53 / 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;
}
54 / 57

Examples
Entitas - https://github.com/sschmid/Entitas-CSharp
55 / 12

Examples
Tome - https://github.com/npruehs/tome-editor
56 / 12

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
o scripting
o serialization
o logging
57 / 57

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.

Thank you!
http://www.npruehs.de
https://github.com/npruehs
@npruehs
nick.pruehs@daedalic.com

5 Minute Review Session
• What is an entity?
• Name a few drawbacks of inheritance-based game models!
• What is an entity component?
• What is a game system?
• How do game systems communicate with each other?
• Analyze the advantages and disadvantages of aggregation-based
game models!
• What is an entity blueprint?

Game Programming 02 - Component-Based Entity Systems

Recommended

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to Game Programming 02 - Component-Based Entity Systems (20)

More from Nick Pruehs (12)

Recently uploaded (20)

Game Programming 02 - Component-Based Entity Systems