A library for creating powerful and flexible finite state machines. Loosely inspired by Erlang/OTP's gen_fsm behavior.
Some frequent use cases for machina:
First - you need to include it in your environment (browser, node, etc.):
// If you're not using an AMD loader, machina is available on the window// Just make sure you have lodash loaded before machinavar MyFsm = machinaFsmextend /* your stuff */;// If you're using an AMD loader:require'machina'return machinaFsmextend /* your stuff */;;// node.js/CommonJS:var machina = require'machina';// FYI machina v0.3.x & earlier returned a factory// function in CommonJS environments:var lodash = require'lodash';var machina = require'machina'lodash;var MyFsm = machinaFsmextend /* your stuff */;
Great, now that we know how to pull it in, let's create an FSM to represent a vehicle traffic light at a pedestrian crosswalk:
var vehicleSignal =// the initialize method is called right after the FSM// instance is constructed, giving you a place for any// setup behavior, etc. It receives the same arguments// (options) as the constructor function.// your setup code goes here...namespace: "vehicle-signal"// `initialState` tells machina what state to start the FSM in.// The default value is "uninitialized". Not providing// this value will throw an exception in v1.0+initialState: "uninitialized"// The states object's top level properties are the// states in which the FSM can exist. Each state object// contains input handlers for the different inputs// handled while in that state.states:uninitialized:// Input handlers are usually functions. They can// take arguments, too (even though this one doesn't)// The "*" handler is special (more on that in a bit)thisdeferUntilTransition;// the `transition` method takes a target state (as a string)// and transitions to it. You should NEVER directly assign the// state property on an FSM. Also - while it's certainly OK to// call `transition` externally, you usually end up with the// cleanest approach if you endeavor to transition *internally*// and just pass input to the FSM.thistransition "green" ;green:// _onEnter is a special handler that is invoked// immediately as the FSM transitions into the new statethistimer = setTimeoutthishandle "timeout" ;bind this 30000 ;thisemit "vehicles" status: GREEN ;// If all you need to do is transition to a new state// inside an input handler, you can provide the string// name of the state in place of the input handler function.timeout: "green-interruptible"thisdeferUntilTransition "green-interruptible" ;// _onExit is a special handler that is invoked just before// the FSM leaves the current state and transitions to anotherclearTimeout thistimer ;"green-interruptible":pedestrianWaiting: "yellow"yellow:thistimer = setTimeoutthishandle "timeout" ;bind this 5000 ;// machina FSMs are event emitters. Here we're// emitting a custom event and data, etc.thisemit "vehicles" status: YELLOW ;timeout: "red"clearTimeout thistimer ;red:thistimer = setTimeoutthishandle "timeout" ;bind this 1000 ;thisemit "vehicles" status: RED ;_reset: "green"clearTimeoutthistimer;// While you can call the FSM's `handle` method externally, it doesn't// make for a terribly expressive API. As a general rule, you wrap calls// to `handle` with more semantically meaningful method calls like these:thishandle "_reset" ;thishandle "pedestrianWaiting" ;;// Now, to use it:// This call causes the FSM to transition from uninitialized -> green// & queues up pedestrianWaiting input, which replays after the timeout// causes a transition to green-interruptible....which immediately// transitions to yellow since we have a pedestrian waiting. After the// next timeout, we end up in "red".vehicleSignalpedestrianWaiting;// Once the FSM is in the "red" state, we can reset it to "green" by calling:vehicleSignalreset;
Though the code comments give you a lot of detail, let's break down what's happening in the above FSM:
optionsarg - which is an object that contains (at least) the
initialStatevalues for your FSM, as well as an optional
initializemethod (which is invoked at the end of the underlying constructor function) and any additional properties or methods you want on the FSM.
red. (Only one state can be active at a time.)
statesproperty on the FSM, and contain handlers whose names match the input types that the FSM accepts while in that state.
handledirectly and passing the input type as a string (plus any arguments), or by calling top level methods you put on your FSM's prototype that wrap the calls to
handlewith a more expressive API.
transition(stateName)to transition to a different state.
*. In fact, the very first state (
uninitialized) in this FSM is using
*. It's the "catch-all" handler which, if provided, will match any input in that state that's not explicitly matched by name. In this case, any input handled in
uninitializedwill cause the FSM to defer the input (queue it up for replay after transitioning), and immediately transfer to
green. (This is just to demonstrate how a start-up-only state can automatically transfer into active state(s) as clients begin using the FSM. )
Note - input handlers can return values. Just be aware that this is not reliable in hierarchical FSMs.
machina provides two constructor functions for creating an FSM:
BehavioralFsm is new to machina as of v1.0 (though the
Fsm constructor now inherits from it). The
BehavioralFsm constructor lets you create an FSM that defines behavior (hence the name) that you want applied to multiple, separate instances of state. A
BehavioralFsm instance does not (should not!) track state locally, on itself. For example, consider this scenario....where we get to twist our
vehicleSignal FSM beyond reason: 😄
var vehicleSignal =// your setup code goes here...namespace: "vehicle-signal"initialState: "uninitialized"states:uninitialized:thisdeferUntilTransition client ;thistransition client "green" ;green:clienttimer = setTimeoutthishandle client "timeout" ;bind this 30000 ;thisemit "vehicles" client: client status: GREEN ;timeout: "green-interruptible"thisdeferUntilTransition client "green-interruptible" ;clearTimeout clienttimer ;"green-interruptible":pedestrianWaiting: "yellow"yellow:clienttimer = setTimeoutthishandle client "timeout" ;bind this 5000 ;thisemit "vehicles" client: client status: YELLOW ;timeout: "red"clearTimeout clienttimer ;red:clienttimer = setTimeoutthishandle client "timeout" ;bind this 1000 ;_reset: "green"clearTimeout clienttimer ;thishandle client "_reset" ;thishandle client "pedestrianWaiting" ;;// Now we can have multiple 'instances' of traffic lights that all share the same FSM:var light1 = location: "Dijsktra Ave & Hunt Blvd" direction: "north-south" ;var light2 = location: "Dijsktra Ave & Hunt Blvd" direction: "east-west" ;// to use the behavioral fsm, we pass the "client" in as the first arg to API calls:vehicleSignalpedestrianWaiting light1 ;// Now let's signal a pedestrian waiting at light2vehicleSignalpedestrianWaiting light2 ;// if you were to inspect light1 and light2, you'd see they both have// a __machina__ property, which contains metadata related to this FSM.// For example, light1.__machina__.vehicleSignal.state might be "green"// and light2.__machina__.vehicleSignal.state might be "yellow" (depending// on when you check). The point is - the "clients' state" is tracked// separately from each other, and from the FSM. Here's a snapshot of// light1 right after the vehicleSignal.pedestrianWaiting( light1 ) call:"location": "Dijsktra Ave & Hunt Blvd""direction": "north-south""__machina__":"vehicle-signal":"inputQueue":"type": "transition""untilState": "green-interruptible""args":"inputType": "pedestrianWaiting""delegated": false"targetReplayState": "green""state": "green""priorState": "uninitialized""priorAction": """currentAction": """currentActionArgs":"inputType": "pedestrianWaiting""delegated": false"inExitHandler": false"timer": 11
Though we're using the same FSM for behavior, the state is tracked separately. This enables you to keep a smaller memory footprint, especially in situations where you'd otherwise have lots of individual instances of the same FSM in play. More importantly, though, it allows you to take a more functional approach to FSM behavior and state should you prefer to do so. (As a side note, it also makes it much simpler to store a client's state and re-load it later and have the FSM pick up where it left off, etc.)
If you've used machina prior to v1.0, the
Fsm constructor is what you're familiar with. It's functionally equivalent to the
BehavioralFsm (in fact, it inherits from it), except that it can only deal with one client: itself. There's no need to pass a
client argument to the API calls on an
Fsm instance, since it only acts on itself. All of the metadata that was stamped on our
light2 clients above (under the
__machina__ property) is at the instance level on an
Fsm (as it has been historically for this constructor).
machina's FSM constructor functions are simple to extend. If you don't need an instance, but just want a modified constructor function to use later to create instances, you can do something like this:
var TrafficLightFsm = machinaFsmextend /* your options */ ;// later/elsewhere in your code:var trafficLight = ;// you can also override any of the options:var anotherLight = initialState: "go" ;
extend method works similar to other frameworks (like Backbone, for example). The primary difference is this: the states object will be deep merged across the prototype chain into an instance-level
states property (so it doesn't mutate the prototype chain). This means you can add new states as well as add new (or override existing) handlers to existing states as you inherit from "parent" FSMs. This can be very useful, but – as with all things inheritance-related – use with caution!
machina FSMs are event emitters, and subscribing to them is pretty easy:
// I'd like to know when the transition event occurstrafficLighton"transition"console.log"we just transitioned from " + datafromState + " to " + datatoState;;// Or, maybe I want to know when ANY event occurstrafficLighton"*"console.log"this thing happened:" eventName;;
Unsubscribing can be done a couple of ways:
//each listener gets a return valuevar sub = trafficLighton"transition" someCallback;suboff; // unsubscribes the handler// OR, we can use the FSM's prototype method -// remove this specific subscription:trafficLightoff"transition" someCallback;// remove all transition subscriberstrafficLightoff"transition";// remove ALL subscribers, period:trafficLightoff;
You can emit your own custom events in addition to the built-in events machina emits. To read more about these events, see the wiki.
One of the most exciting additions in v1.0: machina now supports hierarchical state machines. Remember our earlier example of the
vehicleSignal FSM? Well, that's only part of a pedestrian crosswalk. Pedestrians need their own signal as well - typically a sign that signals "Walk" and "Do Not Walk". Let's peek at what an FSM for this might look like:
var pedestrianSignal =namespace: "pedestrian-signal"initialState: "uninitialized"thistransition "walking" ;states:uninitialized:thisdeferUntilTransition;thistransition "walking" ;walking:thistimer = setTimeoutthishandle "timeout" ;bind this 30000 ;thisemit "pedestrians" status: WALK ;timeout: "flashing"clearTimeout thistimer ;flashing:thistimer = setTimeoutthishandle "timeout" ;bind this 5000 ;thisemit "pedestrians" status: DO_NOT_WALK flashing: true ;timeout: "dontwalk"clearTimeout thistimer ;dontwalk:thistimer = setTimeoutthishandle "timeout" ;bind this 1000 ;_reset: "walking"clearTimeout thistimer ;
In many ways, our
pedestrianSignal is similar to the
uninitializedstate, and the first input causes it to transition to
walkingbefore actually processing the input.
Now - we could stand up an instance of
vehicleSignal, and subscribe them to each other's
transition events. This would make them "siblings" - where
pedestrianSignal could, for example, only transition to
vehicleSignal is in the
red state, etc. While there are scenarios where this sort of "sibling" approach is useful, what we really have is a hierarchy. There are two higher level states that each FSM represents, a "vehicles-can-cross" state and a "pedestrians-can-cross" state. With machina v1.0, we can create an FSM to model these higher states, and attach our
vehicleSignal FSMs to their parent states:
var crosswalk =namespace: "crosswalk"initialState: "vehiclesEnabled"states:vehiclesEnabled:_child: vehicleSignalthisemit "pedestrians" status: DO_NOT_WALK ;timeout: "pedestriansEnabled"pedestriansEnabled:_child: pedestrianSignalthisemit "vehicles" status: RED ;timeout: "vehiclesEnabled";
Notice how each state has a
_child property? This property can be used to assign an FSM instance to act as a child FSM for this parent state (or a factory function that produces an instance to be used, etc.). Here's how it works:
nohandlerevent, the parent FSM will take over and attempt to handle it. For example - if a
pedestrianWaitinginput is fed to the above FSM while in the
vehiclesEnabledstate, it will be passed on to the
vehicleSignalFSM to be handled there.
vehicleSignalFSMs queue up a
timeoutinput in the
redstates, respectively? However, neither of those FSMs have an input handler for
timeoutin those states. When these FSMs become part of the hierarchy above, as children of the
timeoutinput will bubble up to the parent FSM to be handled, where there are handlers for it.
_resetinput. This gives you a hook to move the child FSM to the correct state before handling any further input. For example, you'll notice our
pedestrianSignalFSM has a
_resetinput handler in the
dontwalkstate, which transitions the FSM to the
In v1.1.0, machina added the
compositeState() method to the
Fsm prototypes. This means you can get the current state of the FSM hierarchy. For example:
// calling compositeState on Fsm instancesconsole.log crosswalkcompositeState ; // vehiclesEnabled.green// calling compositeState on BehavioralFsm instances// (you have to pass the client arg)console.log crosswalkcompositeState fsmClient ; // pedestriansEnabled.walking
Caveats: This feature is very new to machina, so expect it to evolve a bit. I plan to fine-tune how events bubble in a hierarchy a bit more.
The top level
machina object has the following members:
Fsm- the constructor function used to create FSMs.
BehavioralFsm– the constructor function used to create BehavioralFSM instances.
utils- contains helper functions that can be overridden to change default behavior(s) in machina:
makeFsmNamespace- function that provides a default "channel" or "exchange" for an FSM instance. (e.g. - fsm.0, fsm.1, etc.)
on- method used to subscribe a callback to top-level machina events (currently the only event published at this level is
off- method used to unsubscribe a callback to top-level machina events.
emit- top-level method used to emit events.
eventListeners- an object literal containing the susbcribers to any top-level events.
machina.js uses gulp.js to build.
bower installto install all dependencies
npm run build- then check the lib folder for the output
npm run test
npm run coverage
npm run show-coverage
Go here to see the changelog.
Read the wiki and the source – you might find your answer and more! Check out the issue opened by @burin - a great example of how to use github issues to ask questions, provide sample code, etc. I only ask that if you open an issue, that it be focused on a specific problem or bug (not wide-open ambiguity, please).