Creating games with entities and components

Ruby is an object-oriented language, and that is the paradigm we mainly use to write software.

Object orientation is not always the best solution for some situations. In real-time simulations, such as real-time games, techniques such as inheritance, modules, traits, etc break down.

This talk explains the entity-component-system architecture, a common pattern in modern game architecture, which explicitly breaks away from object orientation to achieve large gains in flexibility and speed.

Transcript

1. Creating games with
   entities and components
   Denis Defreyne / RUG::B / December 4, 2014
   1
   

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2. I make games*!
   2
   * Or at least I try.
   

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3. View Slide

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6. View Slide

7. Less about games.
   More about paradigms.
   7
   

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8. I am not a professional
   game developer.
   8
   DISCLAIMER
   

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9. View Slide

10. Put your data first.
    10
    

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11. If all you have is the data,

    you know what it’s about.

    If all you have is the behavior,

    you’re still in the dark.
    11
    

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12. In object orientation,
    objects encapsulate data.
    12
    

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13. Objects are defined
    by their methods,
    not by their data.
    13
    

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14. Encapsulation
    is not data-driven.
    14
    

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15. Object orientation
    is not data-driven.
    15
    

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16. You can be data-driven
    in an object-oriented language.
    16
    

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17. Objects combine
    data with behavior.
    17
    

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18. Games have data
    position, velocity, health, …
    Games have behaviors
    movement, combat, AI, …
    18
    

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19. How does the game data
    get its behavior?
    19
    

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20. class Spaceship
    end
    20
    

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21. class Spaceship
    def update
    # TODO: Write me
    end
    end
    21
    

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22. class Spaceship
    def update
    # Move
    @position_x += @velocity_x
    @position_y += @velocity_y
    # …
    end
    end
    22
    

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23. class Spaceship
    def update
    # Move
    @position_x += @velocity_x
    @position_y += @velocity_y
    # Calculate acceleration
    @acceleration = 0.0
    if Keyboard.key_down?('w')
    @acceleration = 10.0
    # TODO: Play accelerate sound
    elsif Keyboard.key_down?('s')
    @acceleration = -3.0
    end
    # …
    end
    end
    23
    

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24. class Spaceship
    def update
    # Move
    @position_x += @velocity_x
    @position_y += @velocity_y
    # Calculate acceleration
    @acceleration = 0.0
    if Keyboard.key_down?('w')
    @acceleration = 10.0
    # TODO: Play accelerate sound
    elsif Keyboard.key_down?('s')
    @acceleration = -3.0
    end
    # Rotate
    if Keyboard.key_down?('a')
    @rotation -= 5
    elsif Keyboard.key_down?('d')
    @rotation += 5
    end
    # TODO: Add rotational velocity
    # …
    end
    end
    24
    

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25. class Spaceship
    def update
    # Move
    @position_x += @velocity_x
    @position_y += @velocity_y
    # Calculate acceleration
    @acceleration = 0.0
    if Keyboard.key_down?('w')
    @acceleration = 10.0
    # TODO: Play accelerate sound
    elsif Keyboard.key_down?('s')
    @acceleration = -3.0
    end
    # Rotate
    if Keyboard.key_down?('a')
    @rotation -= 5
    elsif Keyboard.key_down?('d')
    @rotation += 5
    end
    # TODO: Add rotational velocity
    # Accelerate
    @velocity_x += Math.cos(@rotation) * @acceleration
    @velocity_y += Math.sin(@rotation) * @acceleration
    # …
    end
    end
    25
    

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26. class Spaceship
    def update
    # Move
    @position_x += @velocity_x
    @position_y += @velocity_y
    # Calculate acceleration
    @acceleration = 0.0
    if Keyboard.key_down?('w')
    @acceleration = 10.0
    # TODO: Play accelerate sound
    elsif Keyboard.key_down?('s')
    @acceleration = -3.0
    end
    # Rotate
    if Keyboard.key_down?('a')
    @rotation -= 5
    elsif Keyboard.key_down?('d')
    @rotation += 5
    end
    # TODO: Add rotational velocity
    # Accelerate
    @velocity_x += Math.cos(@rotation) * @acceleration
    @velocity_y += Math.sin(@rotation) * @acceleration
    # Cap speed
    @velocity_x = [@velocity_x, @max_velocity_x].min
    @velocity_y = [@velocity_y, @max_velocity_y].min
    # …
    end
    end
    26
    

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27. class Spaceship
    def update
    # Move
    @position_x += @velocity_x
    @position_y += @velocity_y
    # Calculate acceleration
    @acceleration = 0.0
    if Keyboard.key_down?('w')
    @acceleration = 10.0
    # TODO: Play accelerate sound
    elsif Keyboard.key_down?('s')
    @acceleration = -3.0
    end
    # Rotate
    if Keyboard.key_down?('a')
    @rotation -= 5
    elsif Keyboard.key_down?('d')
    @rotation += 5
    end
    # TODO: Add rotational velocity
    # Accelerate
    @velocity_x += Math.cos(@rotation) * @acceleration
    @velocity_y += Math.sin(@rotation) * @acceleration
    # Cap speed
    @velocity_x = [@velocity_x, @max_velocity_x].min
    @velocity_y = [@velocity_y, @max_velocity_y].min
    # Render
    Graphics.translate(@position_x, @position_y) do
    Graphics.rotate(@rotation) do
    @sprite.draw
    end
    end
    end
    end
    27
    

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28. class Spaceship
    def update
    # Move
    @position_x += @velocity_x
    @position_y += @velocity_y
    # Calculate acceleration
    @acceleration = 0.0
    if Keyboard.key_down?('w')
    @acceleration = 10.0
    # TODO: Play accelerate sound
    elsif Keyboard.key_down?('s')
    @acceleration = -3.0
    end
    # Rotate
    if Keyboard.key_down?('a')
    @rotation -= 5
    elsif Keyboard.key_down?('d')
    @rotation += 5
    end
    # TODO: Add rotational velocity
    # Accelerate
    @velocity_x += Math.cos(@rotation) * @acceleration
    @velocity_y += Math.sin(@rotation) * @acceleration
    # Cap speed
    @velocity_x = [@velocity_x, @max_velocity_x].min
    @velocity_y = [@velocity_y, @max_velocity_y].min
    # Render
    Graphics.translate(@position_x, @position_y) do
    Graphics.rotate(@rotation) do
    if @acceleration > 0.0
    @flame_animation.step
    @flame_sprite = @flame_animation.sprite
    @flame_sprite.draw
    end
    @sprite.draw
    end
    end
    end
    end
    28
    

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29. class Spaceship < Movable
    …
    end
    29
    ?
    

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30. class Spaceship
    include Movable
    include Decoration
    …
    end
    30
    ?
    

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31. View Slide

32. How can you be data-driven
    in an object oriented language?
    32
    

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33. A better approach:
    entities, components, systems.
    33
    

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34. Objects can have logic.
    That doesn’t mean they should.
    34
    

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35. class Spaceship
    attr_reader :position_x, :position_y
    attr_reader :velocity_x, :velocity_y
    attr_reader :acceleration_x, :acceleration_y
    attr_reader :rotation
    attr_reader :shield_cur, :shield_max, :shield_rate
    attr_reader :armor_cur, :armor_max
    def initialize(params = {})
    …
    end
    end
    35
    

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36. Entities are stupid objects;
    they’re just structs.
    36
    

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37. How do these entities
    get their behaviors?
    37
    

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38. Behaviors are handled
    by systems.
    38
    

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39. A system is essentially
    a procedure* that is called
    thirty times per second.
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    * A function with only side effects
    

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40. If you mutate state,
    you might as well
    be explicit about it.
    40
    

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41. spaceship = Spaceship.new(

    position_x: 200,

    position_y: 150,

    velocity_x: 10,

    velocity_y: -5,

    armor_cur: 100,

    armor_max: 100)

    movement_system = MovementSystem.new
    movement_system.update(spaceship)

    p [spaceship.position_x, spaceship.position_y]
    # => [210, 145]
    41
    

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42. class MovementSystem

    def update(entity)

    entity.position_x += entity.velocity_x
    entity.position_y += entity.velocity_y
    end
    end
    42
    

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43. class MovementSystem
    class HealthSystem
    class InputSystem
    class AISteeringSystem
    class RenderingSystem
    class …
    43
    

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44. Systems are decoupled.
    44
    

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45. Data is stored in components.
    45
    

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46. class Spaceship
    attr_reader :position_x, :position_y
    attr_reader :velocity_x, :velocity_y
    attr_reader :acceleration_x, :acceleration_y
    attr_reader :rotation
    attr_reader :shield_cur, :shield_max, :shield_rate
    attr_reader :armor_cur, :armor_max
    def initialize(params = {})
    …
    end
    end
    46
    

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47. class Spaceship
    attr_reader :position
    attr_reader :velocity
    attr_reader :acceleration
    attr_reader :rotation
    attr_reader :shield
    attr_reader :armor
    def initialize(params = {})
    …
    end
    end
    47
    

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48. Position = Struct.new(:x, :y)
    Velocity = Struct.new(:x, :y)
    Acceleration = Struct.new(:x, :y)
    Rotation = Struct.new(:rad)
    Shield = Struct.new(:cur, :max, :rate)
    Armor = Struct.new(:cur, :max)
    48
    

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49. Components have no behavior.
    49
    

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50. Entities are essentially just
    collections of components.
    50
    

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51. class Spaceship
    attr_reader :position
    attr_reader :velocity
    attr_reader :acceleration
    attr_reader :rotation
    attr_reader :shield
    attr_reader :armor
    def initialize(params = {})
    …
    end
    end
    51
    

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52. spaceship = Entity.new
    52
    

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53. spaceship = Entity.new
    spaceship.add(Position.new(400, 200))
    spaceship.add(Velocity.new(0, 0))
    spaceship.add(Acceleration.new(0, 0))
    spaceship.add(Rotation.new(Math::PI / 4))
    spaceship.add(Armor.new(100, 100))
    spaceship.add(Shield.new(100, 100, 10))
    53
    

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54. class MovementSystem
    def update(entity)
    entity[Position] += entity[Velocity]
    end
    end
    54
    

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55. Entities group components.
    Components contain data.
    Systems update components.
    55
    

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56. What are the advantages
    being data-driven?
    56
    

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57. It gives us the power to
    change behavior at runtime!
    57
    WIN #1
    

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58. # Become invisible
    spaceship.remove(Sprite)
    # Become mortal
    angel.add(Health.new(100))
    # Become a ghost
    asteroid.remove(CollisionShape)
    # Mind control enemy
    enemy.remove(AISteering)
    enemy.add(PlayerSteering)
    58
    

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59. It gives us data locality*!
    59
    * Not so much in Ruby.
    WIN #2
    

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60. 60
    HD
    RAM
    Cache
    ~ 1 000 000 ns
    ~ 1-10 ns
    ~ 500 ns
    

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61. View Slide

62. View Slide

63. To avoid cache misses,
    keep similar data
    together in memory.
    63
    

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64. 64
    position velocity rotation armor shield
    

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65. 65
    position velocity
    14 used bytes / 32 total bytes = 44% efficiency
    (for movement system)
    rotation armor shield
    

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66. View Slide

67. View Slide

68. Store components
    in contiguous arrays.
    68
    

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69. positions[47] = Position.new(400, 200)
    velocities[47] = Velocity.new(0, 0)
    shields[47] = Shield.new(cur: 100, max: 100, rate: 2)
    armors[47] = Armor.new(cur: 50, max: 50)
    69
    

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70. Smart use of the CPU cache
    can lead to a 50x speedup!
    70
    * Source: Game Programming Patterns by Robert Nystrom
    

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71. Saving and loading
    becomes trivial!
    71
    WIN #3
    

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72. # Save
    File.write('save.json', JSON.dump(@world))
    # Load
    @world = JSON.parse(File.read('save.json'))
    72
    

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73. Level data becomes plaintext!
    73
    WIN #4
    

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74. {
    "position": [400, 200],
    "velocity": [0, 0],
    "shield": {"cur": 100, "max": 100, "rate": 2},
    "armor": {"cur": 50, "max": 50}
    }
    74
    

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75. In-game editors
    become easy to write!
    75
    WIN #5
    

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76. View Slide

77. Entities group components.
    Components contain data.
    Systems update components.
    77
    

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78. DEMO
    78
    

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79. 79
    @ddfreyne
    denis@stoneship.org
    Denis Defreyne,
    expert game dev newbie.

    

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80. 80
    This talk would not have been the same without some great assets that I
    could use, either for free or for a well-deserved donation.
    The fonts in this presentation are Clear Sans by Intel (01.org/clear-sans)
    and Ubuntu Mono by Canonical Ltd (font.ubuntu.com).
    Most of the sprites are by Kenney Vleugels (kenney.nl) and are part of the
    Kenney Donation Pack (kenney.itch.io/kenney-donation). The planet
    sprite in the 2D space shooter example is by Justin Nichol. Assets in the
    initial screenshot are by Sven Ahlgrimm.
    

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