State Machine

State machines are one of the basic tools for organizing your code. States allow you to switch between different behaviors, values, and setup/teardown logic based on user-defined conditions. A state machine is made up of three main components:

• States
• Triggers
• Groups

A state machine is always in exactly one state at a time, and will transition between states when certain conditions (defined by the triggers) are met. State groups are a convenient way to bundle shared logic between multiple states, but the groups are not states themselves. They provide much of the same API as a state, but distribute behavior and triggers across all of their substates.

You can add a state machine to your custom component using the stateMachine property in the component's definition. It must be a StateMachineDefiner or StateMachineDefinition. These state machines exist on each entity with the component, and will be automatically created and destroyed when the object is added to and removed from the world.

ecs.registerComponent({
  ...
  stateMachine: ({world, eid}) => {
    // Define states here
  },
})

You can also create and manage an independent state machine by calling the ecs.createStateMachine function directly:

const machineId = ecs.createMachine(world, eid, machineDef)
...
ecs.deleteStateMachine(world, machineId)

A state machine is a combination of states and the logic that transitions between those states. They are typically defined within a StateMachineDefiner function that is called when the machine is created. Here is an example of a state machine to move an object back and forth until it hits something.

const BackAndForth = ecs.registerComponent({
  name: 'back-and-forth',
  schema: {
    forthX: ecs.f32,
    forthY: ecs.f32,
    forthZ: ecs.f32,
    duration: ecs.f32,
  },
  stateMachine: ({world, eid, schemaAttribute}) => {
    const position = {...ecs.Position.get(world, eid)}
    const backX = position.x
    const backY = position.y
    const backZ = position.z
  
    const {forthX, forthY, forthZ, duration} = schemaAttribute.get(eid)
  
    const waitBeforeBack = ecs.defineState('waitBeforeBack').wait(1000, 'back')
    const back = ecs.defineState('back').initial()
      .onEvent('position-animation-complete', 'waitBeforeForth')
      .onEnter(() => {
        ecs.PositionAnimation.set(world, eid, {
          autoFrom: true
          toX: forthX,
          toY: forthY,
          toZ: forthZ,
          duration,
        })
      })
  
    const waitbeforeForth = ecs.defineState('waitBeforeForth').wait(1000, 'forth')
    const forth = ecs.defineState('forth')
      .onEvent('position-animation-complete', waitBeforeBack)
      .onEnter(() => {
        ecs.PositionAnimation.set(world, eid, {
          autoFrom: true
          toX: backX,
          toY: backY,
          toZ: backZ,
          duration,
        })
      })
    
    const stopped = ecs.defineState('stopped').onEnter(() => {
      ecs.PositionAnimation.remove(world, eid)
    })
    
    ecs.defineStateGroup([back, forth]).onEvent(ecs.Physics.COLLISION_START_EVENT, stopped)
  },
})

Here is another example of a machine of changing a character state:

const Character = ecs.registerComponent({
  name: 'character',
  schema: {
    walkingSpeed: ecs.f32,
    runningSpeed: ecs.f32,
    staminaRate: ecs.f32,
    jumpForce: ecs,f32,
  },
  stateMachine: ({world, eid, schemaAttribute}) => {
    const idle = ecs.defineState('idle').initial()
    const walking = ecs.defineState('walking')
    const running = ecs.defineState('running')
    const airborne = ecs.defineState('airborne')

    const moving = ecs.defineStateGroup([walking, running])
    const canJump = ecs.defineStateGroup([idle, walking, running])
    const recovering = ecs.defineStateGroup([idle, walking])
    const all = ecs.defineStateGroup()

    const startMoving = ecs.defineTrigger()
    const stopMoving = ecs.defineTrigger()

    idle.onTrigger(startMoving, walking)
      .onTick(() => {
        if (world.input.getAction('forward')) {
          startMoving.toggle()
        }
      })

    moving.onTrigger(stopMoving, idle)
      .onTick((currentState) => {
        const {walkingSpeed, runningSpeed} = schemaAttrbiute.get(eid)
        let speed = currentState === 'running' ? runningSpeed : walkingSpeed
        ecs.physics.applyForce(world, eid, 0, 0, speed)
        if (!world.input.getAction('forward')) {
          stopMoving.toggle()
        }
      })

    walking.onEvent('toggle-sprint', running)
    running.onEvent('toggle-sprint', walking)

    canJump.onEvent('jump', airborne)
    airborne.onEnter(() => ecs.physics.applyForce(world, eid, 0, schemaAttribute.get(eid).jumpForce, 0))
      .onEvent(ecs.Physics.COLLISION_START_EVENT, walking, {where: () => world.input.getAction('forward')})
      .onEvent(ecs.Physics.COLLISION_START_EVENT, idle)

    let stamina = 1.0
    const updateStamina = (add: boolean) => {
      const diff = schemaAttribute.get(eid).staminaRate * world.time.delta
      stamina += add ? diff : -diff
    }

    running.onTick(() => updateStamina(false))
    recovering.onTick(() => updateStamina(true))
    all.listen(eid, 'powerup-collected', () => stamina = 1.0)
  },
})

StateMachineDefiner

A StateMachineDefiner is a function that creates a state machine and all of its parts. Inside the function you define states and groups, as well as locally scoped variables and functions to use in them. It can be passed into a component's stateMachine prop and will be called when an entity is added to the world. Alternatively you can directly pass a StateMachineDefinition, which is outlined below.

The StateMachineDefiner function accepts an object with the following properties:

Key	Type	Description
world	World	The world the machine is created in
eid	Eid	The entity owner of the machine
schemaAttribute	WorldAttribute	Access to the component's schema
dataAttribute	WorldAttribute	Access to the component's data

defineState()

Use this function to create a State from within a StateMachineDefiner

Parameter	Type	Description
name	string	The unique name of the state

Returns a StateDefiner

Example

stateMachine: ({world, eid}) => {
  const foo = ecs.defineState('foo')
  ...
}

StateDefiner

note

StateDefiner functions are 'fluent', so they all return the instance itself. This means you can chain any of the following consecutively in a single statement.

.initial()

Mark this state to be the current state of the state machine when it is created.

Example

ecs.defineState('name').initial()

.onEnter()

Set a callback to run when entering this state.

Parameter	Type	Description
cb	() => void	A function called when the state is entered

Example

ecs.defineState('name').onEnter(() => {
  // Do something
})

.onTick()

Set a callback to run every frame.

Parameter	Type	Description
cb	() => void	A function called once each frame while in this state

Example

ecs.defineState('name').onTick(() => {
  // Do something
})

.onExit()

Set a callback to run when exiting this state.

Parameter	Type	Description
cb	() => void	A function called when the state exits

Example

ecs.defineState('name').onExit(() => {
  // Do something
})

.onEvent()

Call to add an EventTrigger from this state to another that can transition when a specific event is invoked

Parameter	Type	Description
event	string	The event to listen for
nextState	StateId	The next state to transition to
args	object (see below)	Optional arguments for determining when to transition

args

Key	Type	Required	Description
target	Eid	no	The entity that is expected to receive an event (defaults to the state machine owner)
where	(QueuedEvent) => boolean	no	An optional predicate to check before transitioning; returning false will prevent the transition from occurring

Example

ecs.defineState('example').onEvent(
  ecs.input.SCREEN_TOUCH_START,
  'other',
  {
    target: world.events.globalId,
    where: (event) => event.data.position.y > 0.5
  }
)

.wait()

Call to add a TimeoutTrigger from this state to another that transitions after a set amount of time.

Parameter	Type	Description
timeout	number	The duration in milliseconds before transitioning
nextState	StateId	The next state to transition to

Example

ecs.defineState('example').wait(1000, 'other')

.onTrigger()

Call to add a CustomTrigger from this state to another that can transition at any time immediately by the user. Use ecs.defineTrigger() to create a TriggerHandle that can be manually invoked.

Parameter	Type	Description
handle	TriggerHandle	The handle that will cause a transition when manually activated
nextState	StateId	The next state to transition to

Example

const toOther = ecs.defineTrigger()
ecs.defineState('example').onTrigger(toOther, 'other')
...
toOther.trigger()

.listen()

Call to add ListenerParams to this state's set of listeners. An event listener will be automatically added when the state is entered, and removed on exit.

Parameter	Type	Description
target	Eid or () => Eid	The entity that is expected to receive an event
name	string	The event to listen for
listener	(QueuedEvent) => void	The function to call when the event is dispatched

Example

const handleCollision = (event) => { ... }
ecs.defineState('example').listen(eid, ecs.physics.COLLISION_START_EVENT, handleCollision)

defineStateGroup()

Use this function to create a StateGroup from within a StateMachineDefiner.

Parameter	Type	Required	Description
substates	StateId[]	no	The list of states that make up this group; excluding this parameter is equivalent to listing all states

Returns a StateGroupDefiner

Example

const foo = ecs.defineState('foo')
const fizz = ecs.defineState('fizz')
const buzz = ecs.defineState('buzz')
const fizzBuzz = ecs.defineStateGroup([fizz, 'buzz'])

StateGroupDefiner

note

StateGroupDefiner functions are 'fluent', so they all return the instance itself. This means you can chain any of the following consecutively in a single statement.

.onEnter()

Set a callback to run when entering this group from outside it.

Parameter	Type	Description
cb	(string) => void	A function called when a substate is entered from a non-substate, optionally providing the specific state that was entered

Example

ecs.defineStateGroup(['a', 'b']).onEnter(() => {
  // Do something
})

.onTick()

Set a callback to run every frame.

Parameter	Type	Description
cb	(string) => void	A function called once each frame while in any substate, optionally providing the current state

Example

ecs.defineStateGroup(['a', 'b']).onTick(() => {
  // Do something
})

.onExit()

Set a callback to run when exiting this state.

Parameter	Type	Description
cb	(string) => void	A function called when a substate exits to a non-substate, optionally providing the specific state that was exited

Example

ecs.defineStateGroup(['a', 'b']).onExit(() => {
  // Do something
})

.onEvent()

Call to add an EventTrigger from any state in this group to some other state that can transition when a specific event is invoked

Parameter	Type	Description
event	string	The event to listen for
nextState	StateId	The state to transition to
args	object (see below)	optional arguments for determining when to transition

args

Key	Type	Required	Description
target	Eid	no	The entity that is expected to receive an event
where	(QueuedEvent) => boolean	no	An optional predicate to check before transitioning; returning false will prevent the transition from occurring

Example

ecs.defineStateGroup(['a', 'b']).onEvent('click', 'c')

.wait()

Call to add a TimeoutTrigger from any state in this group to some other state that transitions after a set amount of time.

Parameter	Type	Description
timeout	number	The duration in milliseconds before transitioning
nextState	StateId	The state to transition to

Example

ecs.defineStateGroup(['a', 'b']).wait(1000, 'c')

.onTrigger()

Call to add a CustomTrigger from any state in this group to some other state that can transition at any time immediately by the user. Use ecs.defineTrigger() to create a TriggerHandle that can be manually invoked.

Parameter	Type	Description
handle	TriggerHandle	The handle that will cause a transition when manually activated
nextState	StateId	The state to transition to

Example

const toC = ecs.defineTrigger()
ecs.defineStateGroup(['a', 'b']).onTrigger(toC, 'c')
...
toC.trigger()

.listen()

Call to add ListenerParams to this group's set of listeners. An event listener will be automatically added when the group is entered, and removed on exit.

Parameter	Type	Description
target	Eid or () => Eid	The entity that is expected to receive an event
name	string	The event to listen for
listener	(QueuedEvent) => void	The function to call when the event is dispatched

Example

const handleCollision = (event) => { ... }
ecs.defineStateGroup(['a', 'b']).listen(eid, ecs.physics.COLLISION_START_EVENT, handleCollision)

StateMachineDefinition

Instead of using the StateMachineDefiner to define states and transitions, you can pass a StateMachineDefinition, which is an object containing all of the states and their transitions.

Key	Type	Required	Description
initialState	string	yes	Name of the starting state of the state machine
states	Record<string, State>	yes	A map that stores state names and their definition
groups	StateGroup[]	no	An optional list of state groups

Example

const stateMachine = {
  initialState: 'a'
  states: {
    'a': {
      onExit: () => console.log('exit a'),
      triggers: {
        'b': [{ type: 'timeout', timeout: 1000 }],
      },
    },
    'b': { onEnter: () => console.log('enter b') },
  },
  groups: [{
      substates: ['a', 'b'],
      listeners: [{
          target: world.events.globalId,
          name: ecs.input.SCREEN_TOUCH_START,
          listener: (event) => console.log('touch'),
        }]
  }],
}

State

A state is the fundamental atomic unit of a state machine. They can be defined directly or with the fluent StateMachineDefiner API outlined above. A state machine is always in exactly one state at a time, and will transition according to user-defined triggers associated with the current state.

Key	Type	Required	Description
triggers	Record<string, Trigger[]>	yes	All outgoing transitions indexed by their destination state
onEnter	() => void	no	A function called when the state is entered
onTick	() => void	no	A function called once each frame while in this state
onExit	() => void	no	A function called when the state exits
listeners	ListenerParams[]	no	Parameters for event listeners that will be automatically added and removed when entering and exiting the state

Example:

const forth = {
  triggers: {
    'waitBeforeBack': {
      type: 'event',
      event: 'animation-complete',
    },
  },
  onEnter: () => {
    ecs.PositionAnimation.set(world, eid, {
      autoFrom: true
      toX: backX,
      toY: backY,
      toZ: backZ,
      duration,
    })
  },
},

StateGroup

A state group is an abstract wrapper around multiple states of a state machine. It is a useful tool for organizing code, preventing repetitive statements, and simplifying logic. It is not a state itself and thus cannot be transitioned to. It can however have outgoing transitions, which are essentially shorthand for defining those same outgoing transitions individually on all substates. Its onEnter and onExit callbacks will not be invoked when transitioning between its substates.

Key	Type	Required	Description
substates	StateId[]	no	The list of states that make up this group; excluding this parameter is equivalent to listing all states
triggers	Record<string, Trigger[]>	yes	All outgoing transitions indexed by their destination state
onEnter	() => void	no	A function called when a substate is entered from a non-substate
onTick	() => void	no	A function called once each frame while in any substate
onExit	() => void	no	A function called when a substate exits to a non-substate
listeners	ListenerParams[]	no	Parameters for event listeners that will be automatically added and removed when entering and exiting the group

Example:

const forth = {
  triggers: {
    'waitBeforeBack': {
      type: 'event',
      event: 'animation-complete',
    },
  },
  onEnter: () => {
    ecs.PositionAnimation.set(world, eid, {
      autoFrom: true
      toX: backX,
      toY: backY,
      toZ: backZ,
      duration,
    })
  },
},

Trigger

There are multiple types of triggers for transitioning under various circumstances

EventTrigger

EventTriggers are used to optionally transition when a specified event is invoked. The event data can be used to make a runtime decision whether to transition or not.

Key	Type	Required	Description
type	'event'	yes	A constant to indicate the type of the trigger
event	string	yes	The name of the event to listen for
target	Eid	no	The entity that is expected to receive an event
where	(QueuedEvent) => boolean	no	An optional predicate to check before transitioning; returning false will prevent the transition from occurring

Example:

const example = {
  triggers:
    'other': [
      {
        type: 'event',
        event: ecs.input.SCREEN_TOUCH_START,
        target: world.events.globalId
        where: (event) => event.data.position.y > 0.5
      },
    ]
}

TimeoutTrigger

TimeoutTriggers are used to cause a transition to occur after a fixed amount of time from entering a state or group.

Key	Type	Required	Description
type	'timeout'	yes	A constant to indicate the type of the trigger
timeout	number	yes	The number of milliseconds to wait before transitioning

Example:

const example = {
  triggers:
    'other': [
      {
        type: 'timeout',
        timeout: 1000,
      },
    ]
}

CustomTrigger

CustomTriggers are transitions that can be triggered at any time, causing an immediate transition. Use ecs.defineTrigger() to create a TriggerHandle that can be manually invoked.

Key	Type	Required	Description
type	'custom'	yes	A constant to indicate the type of the trigger
handle	TriggerHandle	yes	The handle that will cause a transition when manually activated

Example:

const toOther = ecs.defineTrigger()
const example = {
  triggers:
    'other': [
      {
        type: 'custom',
        trigger: toOther,
      },
    ]
}
...
toOther.trigger()

StateId

StateIds are used for specifying transition destinations. Can be either a StateDefiner or the state name itself as a string.

const a = ecs.definestate('a').wait(1000, 'b')
const b = ecs.defineState('b').wait(1000, a)

ListenerParams

A ListenerParams object is a type that is used for storing the parameters needed to supply an event listener. They are passed into a state via the listeners property of a State or StateGroup object, or created automatically when calling listen() on a StateDefiner or StateGroupDefiner.

Key	Type	Required	Description
target	Eid or () => Eid	yes	The entity that is expected to receive an event
name	string	yes	The event to listen for
listener	(QueuedEvent) => void	yes	The function to call when the event is dispatched

TriggerHandle

An object that is used to define an arbitrary transition between states. It must be created via ecs.defineTrigger, and is used by onTrigger or CustomTrigger.

Key	Type	Description
trigger()	() => void	Call this function to cause any active CustomTrigger transitions to occur

Example

const go = ecs.defineTrigger()
const stopped = ecs.defineState('stoppe').onTick(() => {
  if (world.input.getAction('start-going')) {
    go.trigger()
  }
}).onTrigger(go, 'going')
const going = ecs.defineState('going')