Entity and Components (old)

To create and add entities to a world use the entity function of the world. Let's create an entity with a PositionComponent and SpriteComponent:

data class Position(var x: Float = 0f, var y: Float = 0f)
data class Sprite(var texturePath: String = "")

fun main() {
    val world = world {}

    val entity: Entity = world.entity {
        add<Position> { x = 5f }
        add<Sprite>()
    }
    
    // if needed, you can already access the new entity within the entity{} block
    val entity2 = world.entity { e -> 
        // e is the same as entity2
        // this can be useful in some cases where you want to set the entity as
        // custom 'userData' on certain third-party library objects directly
    }
}

There might be situations where you need to execute a specific code when a component gets added or removed from an entity. This can be done via ComponentListener in Fleks. They are created in a similar way like systems meaning that they are created by Fleks using dependency injection. The world of a ComponentListener is automatically available as a dependency like any ComponentMapper.

Here is an example of a listener that reacts on add/remove of a Box2dComponent and destroys the body when the component gets removed from an entity:

JVM snippet

data class Box2dComponent{
    lateinit var body: Body
}

class Box2dComponentListener(
    private val b2dWorld : com.badlogic.gdx.physics.box2d.World
) : ComponentListener<Box2dComponent> {
    override fun onComponentAdded(entity: Entity, component: Box2dComponent) {
        component.body = b2dWorld.createBody( /* body creation code omitted */ )
        component.body.userData = entity
    }
    
    override fun onComponentRemoved(entity: Entity, component: Box2dComponent) {
        component.body.world.destroyBody(body)
        component.body.userData = null
    }
}

fun main() {
    val b2dWorld = com.badlogic.gdx.physics.box2d.World()
    val world = world {
        injectables {
            add(b2dWorld)
        }

        components {
            // register listener to the world
            add<Box2dComponentListener>()
        }
    }
}

KMP snippet

data class Box2dComponent{
    lateinit var body: Body
}

class Box2dComponentListener : ComponentListener<Box2dComponent> {
    private val b2dWorld : com.badlogic.gdx.physics.box2d.World = Inject.dependency()

    override fun onComponentAdded(entity: Entity, component: Box2dComponent) {
        component.body = b2dWorld.createBody( /* body creation code omitted */ )
        component.body.userData = entity
    }
    
    override fun onComponentRemoved(entity: Entity, component: Box2dComponent) {
        component.body.world.destroyBody(body)
        component.body.userData = null
    }
}

fun main() {
    val b2dWorld = com.badlogic.gdx.physics.box2d.World()
    val world = world {
        injectables {
            add(b2dWorld)
        }

        components {
            // register component together with its listener to the world
            add(::Box2dComponent, ::Box2dComponentListener)
        }
    }
}