Kiwi is a performant and versatile entity component system focussing on fast iteration and a nice api.

To get started, read the usage guide below. A documentation site will be set up later.


The world

The world is the main object that controls the ecs.

var world = World()
defer { world.destroy() }

world.destroy() is used to clean up memory. This should always be executed when the world is not needed anymore.

Keep in mind that World is a struct, so when you pass it around be sure to use inout.


Creating components is as simple as declaring a struct:

struct Position: Component {
  var x: Int
  var y: Int

For performance reasons you might want to declare an additional property on these structs, but this is not strictly required:

struct Position: Component {
  static let id: Int = UUID().hashValue

  var x: Int
  var y: Int


An entity is spawned with a set of components:

let entityId = world.spawn(Position(x: 10, y: 10))
world.spawn(Position(x: 3, y: 5), Velocity(x: 1.5, y: 0.0))

The world.spawn(_ components: Component...) function will return the id of the newly spawned entity.



Queries can be constructed as follows:

// Immutable queries

// Query all entities having a position component
  .get() // required in every query immutable query
  .forEach { (pos: Position) in // type hint here is neccesary!

// Query all entites having a position and a velocity component and their entity ids
world.query(Position.self, Velocity.self)
  .getWithIds() // Besides querying the component, also query the entity ids
  .forEach { (id: EntityId, pos: Position, vel: Velocity) in
    // ...

// Mutable queries

// Query all entities having a position and velocity component mutably
world.queryMut(Position.self, Velocity.self)
  .mutate { (pos: inout Position, vel: inout Velocity) // type hints and inout keyword neccesary
    pos.x += vel.x
    pos.y += vel.y

// Query all entities having a position component mutably as well as their entity ids
  .mutateWithIds { (id: EntityId, pos: inout Position) in
    // ...

System Groups

system groups are currently unimplemented and are planned for a future release.


Components can’t be zero-sized. This is where a flag can be used.

Defining flags

enum Flags: FlagId {
  case Player
  case Enemy

Setting flags

let id = world.spawn()

world.setFlag(entity: id, Flags.Player)

Removing a flag

world.removeFlag(entity: id, Flags.Player)

Checking wether an entity has a flag

world.hasFlag(entity: id, Flags.Player)

Fltering queries with flags

  .filter { (id: EntityId, _: Position) in world.hasFlag(entity: id, Flags.Player) }
  .forEach { (_, pos) in

The hasFlags function is also available.

Road map

  • System groups


Contributers are welcome to open an issue requesting new features or fixes or opening a pull request for them.


The library is currenlty licensed under LGPLv3.


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