Lua Finite State Machine

This standalone lua module provides a finite state machine for your pleasure. Based heavily on Jake Gordon's javascript-state-machine.

Download

You can download statemachine.lua.

Alternatively:

git clone git@github.com:kyleconroy/lua-state-machine

Usage

In its simplest form, create a standalone state machine using:

local machine = require('statemachine')

local fsm = machine.create({
  initial = 'green',
  events = {
    { name = 'warn',  from = 'green',  to = 'yellow' },
    { name = 'panic', from = 'yellow', to = 'red'    },
    { name = 'calm',  from = 'red',    to = 'yellow' },
    { name = 'clear', from = 'yellow', to = 'green'  }
}})

... will create an object with a method for each event:

fsm:warn() - transition from 'green' to 'yellow'
fsm:panic() - transition from 'yellow' to 'red'
fsm:calm() - transition from 'red' to 'yellow'
fsm:clear() - transition from 'yellow' to 'green'

along with the following members:

fsm.current - contains the current state
fsm.currentTransitioningEvent - contains the current event that is in a transition.
fsm:is(s) - return true if state s is the current state
fsm:can(e) - return true if event e can be fired in the current state
fsm:cannot(e) - return true if event e cannot be fired in the current state

Multiple 'from' and 'to' states for a single event

If an event is allowed from multiple states, and always transitions to the same state, then simply provide an array of states in the from attribute of an event. However, if an event is allowed from multiple states, but should transition to a different state depending on the current state, then provide multiple event entries with the same name:

local machine = require('statemachine')

local fsm = machine.create({
  initial = 'hungry',
  events = {
    { name = 'eat',  from = 'hungry',                                to = 'satisfied' },
    { name = 'eat',  from = 'satisfied',                             to = 'full'      },
    { name = 'eat',  from = 'full',                                  to = 'sick'      },
    { name = 'rest', from = {'hungry', 'satisfied', 'full', 'sick'}, to = 'hungry'    },
}})

This example will create an object with 2 event methods:

fsm:eat()
fsm:rest()

The rest event will always transition to the hungry state, while the eat event will transition to a state that is dependent on the current state.

NOTE: The rest event could use a wildcard '*' for the 'from' state if it should be allowed from any current state.

NOTE: The rest event in the above example can also be specified as multiple events with the same name if you prefer the verbose approach.

Callbacks

4 callbacks are available if your state machine has methods using the following naming conventions:

onbeforeevent - fired before the event
onleavestate - fired when leaving the old state
onenterstate - fired when entering the new state
onafterevent - fired after the event

You can affect the event in 3 ways:

return false from an onbeforeevent handler to cancel the event.
return false from an onleavestate handler to cancel the event.
return ASYNC from an onleavestate or onenterstate handler to perform an asynchronous state transition (see next section)

For convenience, the 2 most useful callbacks can be shortened:

onevent - convenience shorthand for onafterevent
onstate - convenience shorthand for onenterstate

In addition, a generic onstatechange() callback can be used to call a single function for all state changes:

All callbacks will be passed the same arguments:

self
event name
from state
to state
(followed by any arguments you passed into the original event method)

Callbacks can be specified when the state machine is first created:

local machine = require('statemachine')

local fsm = machine.create({
  initial = 'green',
  events = {
    { name = 'warn',  from = 'green',  to = 'yellow' },
    { name = 'panic', from = 'yellow', to = 'red'    },
    { name = 'calm',  from = 'red',    to = 'yellow' },
    { name = 'clear', from = 'yellow', to = 'green'  }
  },
  callbacks = {
    onpanic =  function(self, event, from, to, msg) print('panic! ' .. msg)    end,
    onclear =  function(self, event, from, to, msg) print('thanks to ' .. msg) end,
    ongreen =  function(self, event, from, to)      print('green light')       end,
    onyellow = function(self, event, from, to)      print('yellow light')      end,
    onred =    function(self, event, from, to)      print('red light')         end,
  }
})

fsm:warn()
fsm:panic('killer bees')
fsm:calm()
fsm:clear('sedatives in the honey pots')
...

Additionally, they can be added and removed from the state machine at any time:

fsm.ongreen       = nil
fsm.onyellow      = nil
fsm.onred         = nil
fsm.onstatechange = function(self, event, from, to) print(to) end

function fsm:onstatechange(event, from, to) print(to) end

Asynchronous State Transitions

Sometimes, you need to execute some asynchronous code during a state transition and ensure the new state is not entered until your code has completed.

A good example of this is when you transition out of a menu state, perhaps you want to gradually fade the menu away, or slide it off the screen and don't want to transition to your game state until after that animation has been performed.

You can now return ASYNC from your onleavestate and/or onenterstate handlers and the state machine will be 'put on hold' until you are ready to trigger the transition using the new transition(eventName) method.

If another event is triggered during a state machine transition, the event will be triggered relative to the state the machine was transitioning to or from. Any calls to transition with the cancelled async event name will be invalidated.

During a state change, asyncState will transition from NONE to [event]WaitingOnLeave to [event]WaitingOnEnter, looping back to NONE. If the state machine is put on hold, asyncState will pause depending on which handler you returned ASYNC from.

Example of asynchronous transitions:

local machine = require('statemachine')
local manager = require('SceneManager')

local fsm = machine.create({

  initial = 'menu',

  events = {
    { name = 'play', from = 'menu', to = 'game' },
    { name = 'quit', from = 'game', to = 'menu' }
  },

  callbacks = {

    onentermenu = function() manager.switch('menu') end,
    onentergame = function() manager.switch('game') end,

    onleavemenu = function(fsm, name, from, to)
      manager.fade('fast', function()
        fsm:transition(name)
      end)
      return fsm.ASYNC -- tell machine to defer next state until we call transition (in fadeOut callback above)
    end,

    onleavegame = function(fsm, name, from, to)
      manager.slide('slow', function()
        fsm:transition(name)
      end)
      return fsm.ASYNC -- tell machine to defer next state until we call transition (in slideDown callback above)
    end,
  }
})

If you decide to cancel the async event, you can call fsm.cancelTransition(eventName)

Initialization Options

How the state machine should initialize can depend on your application requirements, so the library provides a number of simple options.

By default, if you dont specify any initial state, the state machine will be in the 'none' state and you would need to provide an event to take it out of this state:

local machine = require('statemachine')

local fsm = machine.create({
  events = {
    { name = 'startup', from = 'none',  to = 'green' },
    { name = 'panic',   from = 'green', to = 'red'   },
    { name = 'calm',    from = 'red',   to = 'green' },
}})

print(fsm.current) -- "none"
fsm:startup()
print(fsm.current) -- "green"

If you specify the name of your initial event (as in all the earlier examples), then an implicit startup event will be created for you and fired when the state machine is constructed.

local machine = require('statemachine')

local fsm = machine.create({
  inital = 'green',
  events = {
    { name = 'panic',   from = 'green', to = 'red'   },
    { name = 'calm',    from = 'red',   to = 'green' },
}})
print(fsm.current) -- "green"