machina

machina v4.0.2

What is it?

Machina.js is a JavaScript framework for highly customizable finite state machines (FSMs). Many of the ideas for machina have been loosely inspired by the Erlang/OTP FSM behaviors.

Why Would I Use It?

Finite state machines are a great conceptual model for many concerns facing developers – from conditional UI, connectivity monitoring & management to initialization and more. State machines can simplify tangled paths of asynchronous code, they're easy to test, and they inherently lend themselves to helping you avoid unexpected edge-case-state pitfalls. machina aims to give you the tools you need to model state machines in JavaScript, without being too prescriptive on the problem domain you're solving for.

Some frequent use cases for machina:

• online/offline connectivity management
• conditional UI (menus, navigation, workflow)
• initialization of node.js processes or single-page-apps
• responding to user input devices (remotes, keyboard, mouse, etc.)

Quick Example

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 machina
var MyFsm = machina.Fsm.extend({ /* your stuff */});

// If you're using an AMD loader:
require(['machina'], function(machina){
    return machina.Fsm.extend({ /* 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 = machina.Fsm.extend({ /* 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 = new machina.Fsm( {

    // 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.
    initialize: function( options ) {
        // 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)
            "*": function() {
                this.deferUntilTransition();
                // 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.
                this.transition( "green" );
            }
        },
        green: {
            // _onEnter is a special handler that is invoked
            // immediately as the FSM transitions into the new state
            _onEnter: function() {
                this.timer = setTimeout( function() {
                    this.handle( "timeout" );
                }.bind( this ), 30000 );
                this.emit( "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",
            pedestrianWaiting: function() {
                this.deferUntilTransition( "green-interruptible" );
            },
            // _onExit is a special handler that is invoked just before
            // the FSM leaves the current state and transitions to another
            _onExit: function() {
                clearTimeout( this.timer );
            }
        },
        "green-interruptible": {
            pedestrianWaiting: "yellow"
        },
        yellow: {
            _onEnter: function() {
                this.timer = setTimeout( function() {
                    this.handle( "timeout" );
                }.bind( this ), 5000 );
                // machina FSMs are event emitters. Here we're
                // emitting a custom event and data, etc.
                this.emit( "vehicles", { status: "YELLOW" } );
            },
            timeout: "red",
            _onExit: function() {
                clearTimeout( this.timer );
            }
        },
        red: {
            _onEnter: function() {
                this.timer = setTimeout( function() {
                    this.handle( "timeout" );
                }.bind( this ), 1000 );
                this.emit( "vehicles", { status: "RED" } );
            },
            _reset: "green",
            _onExit: function() {
                clearTimeout(this.timer);
            }
        }
    },

    // 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:
    reset: function() {
        this.handle( "_reset" );
    },

    pedestrianWaiting: function() {
        this.handle( "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".
vehicleSignal.pedestrianWaiting();
// Once the FSM is in the "red" state, we can reset it to "green" by calling:
vehicleSignal.reset();

Though the code comments give you a lot of detail, let's break down what's happening in the above FSM:

• When you are creating an FSM, the constructor takes one argument, the options arg - which is an object that contains (at least) the states & initialState values for your FSM, as well as an optional initialize method (which is invoked at the end of the underlying constructor function) and any additional properties or methods you want on the FSM.
• It can exist in one of five possible states: uninitialized, green, green-interruptible, yellow and red. (Only one state can be active at a time.)
• The states themselves are objects under the states property on the FSM, and contain handlers whose names match the input types that the FSM accepts while in that state.
• It starts in the uninitialized state.
• It accepts input either by calling handle directly 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 handle with a more expressive API.
• You do not assign the state value of the FSM directly, instead, you use transition(stateName) to transition to a different state.
• Special "input handlers" exist in machina: _onEnter, _onExit and *. 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 uninitialized will 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.

Going Further

machina provides two constructor functions for creating an FSM: machina.Fsm and machina.BehavioralFsm:

The BehavioralFsm Constructor

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: :smile:

var vehicleSignal = new machina.BehavioralFsm( {

    initialize: function( options ) {
        // your setup code goes here...
    },

    namespace: "vehicle-signal",

    initialState: "uninitialized",

    states: {
        uninitialized: {
            "*": function( client ) {
                this.deferUntilTransition( client );
                this.transition( client, "green" );
            }
        },
        green: {
            _onEnter: function( client ) {
                client.timer = setTimeout( function() {
                    this.handle(  client, "timeout" );
                }.bind( this ), 30000 );
                this.emit( "vehicles", { client: client, status: GREEN } );
            },
            timeout: "green-interruptible",
            pedestrianWaiting: function( client ) {
                this.deferUntilTransition(  client, "green-interruptible" );
            },
            _onExit: function( client ) {
                clearTimeout( client.timer );
            }
        },
        "green-interruptible": {
            pedestrianWaiting: "yellow"
        },
        yellow: {
            _onEnter: function( client ) {
                client.timer = setTimeout( function() {
                    this.handle( client, "timeout" );
                }.bind( this ), 5000 );
                this.emit( "vehicles", { client: client, status: YELLOW } );
            },
            timeout: "red",
            _onExit: function( client ) {
                clearTimeout( client.timer );
            }
        },
        red: {
            _onEnter: function( client ) {
                client.timer = setTimeout( function() {
                    this.handle( client, "timeout" );
                }.bind( this ), 1000 );
            },
            _reset: "green",
            _onExit: function( client ) {
                clearTimeout( client.timer );
            }
        }
    },

    reset: function( client ) {
        this.handle(  client, "_reset" );
    },

    pedestrianWaiting: function( client ) {
        this.handle( 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:
vehicleSignal.pedestrianWaiting( light1 );

// Now let's signal a pedestrian waiting at light2
vehicleSignal.pedestrianWaiting( 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.)

The Fsm Constructor

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 light1 and light2 clients above (under the __machina__ property) is at the instance level on an Fsm (as it has been historically for this constructor).

Wait - What's This About Inheritance?

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 = machina.Fsm.extend({ /* your options */ });

// later/elsewhere in your code:
var trafficLight = new TrafficLightFsm();

// you can also override any of the options:
var anotherLight = new TrafficLightFsm({ initialState: "go" });

The 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!

And You Mentioned Events?

machina FSMs are event emitters, and subscribing to them is pretty easy:

// I'd like to know when the transition event occurs
trafficLight.on("transition", function (data){
    console.log("we just transitioned from " + data.fromState + " to " + data.toState);
});

// Or, maybe I want to know when ANY event occurs
trafficLight.on("*", function (eventName, data){
    console.log("this thing happened:", eventName);
});

Unsubscribing can be done a couple of ways:

//each listener gets a return value
var sub = trafficLight.on("transition", someCallback);
sub.off(); // unsubscribes the handler

// OR, we can use the FSM's prototype method -
// remove this specific subscription:
trafficLight.off("transition", someCallback);
// remove all transition subscribers
trafficLight.off("transition");
// remove ALL subscribers, period:
trafficLight.off();

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.

Things Suddenly Got Hierarchical!

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 = new machina.Fsm( {
    namespace: "pedestrian-signal",
    initialState: "uninitialized",
    reset: function() {
        this.transition( "walking" );
    },
    states: {
        uninitialized: {
            "*": function() {
                this.deferUntilTransition();
                this.transition( "walking" );
            }
        },
        walking: {
            _onEnter: function() {
                this.timer = setTimeout( function() {
                    this.handle( "timeout" );
                }.bind( this ), 30000 );
                this.emit( "pedestrians", { status: WALK } );
            },
            timeout: "flashing",
            _onExit: function() {
                clearTimeout( this.timer );
            }
        },
        flashing: {
            _onEnter: function() {
                this.timer = setTimeout( function() {
                    this.handle( "timeout" );
                }.bind( this ), 5000 );
                this.emit( "pedestrians", { status: DO_NOT_WALK, flashing: true } );
            },
            timeout: "dontwalk",
            _onExit: function() {
                clearTimeout( this.timer );
            }
        },
        dontwalk: {
            _onEnter: function() {
                this.timer = setTimeout( function() {
                    this.handle( "timeout" );
                }.bind( this ), 1000 );
            },
            _reset: "walking",
            _onExit: function() {
                clearTimeout( this.timer );
            }
        }
    }
} )

In many ways, our pedestrianSignal is similar to the vehicleSignal FSM:

• It starts in the uninitialized state, and the first input causes it to transition to walking before actually processing the input.
• It can only be in one of four states: uninitialized, walking, flashing and dontwalk.
• This FSM's input is primarily internally-executed, based on timers (setTimeout calls).

Now - we could stand up an instance of pedestrianSignal and vehicleSignal, and subscribe them to each other's transition events. This would make them "siblings" - where pedestrianSignal could, for example, only transition to walking when 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 pedestrianSignal and vehicleSignal FSMs to their parent states:

var crosswalk = new machina.Fsm( {
    namespace: "crosswalk",
    initialState: "vehiclesEnabled",
    states: {
        vehiclesEnabled: {
            _child: vehicleSignal,
            _onEnter: function() {
                this.emit( "pedestrians", { status: DO_NOT_WALK } );
            },
            timeout: "pedestriansEnabled"
        },
        pedestriansEnabled: {
            _child: pedestrianSignal,
            _onEnter: function() {
                this.emit( "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:

• When an FSM is handling input, it attempts to let the child FSM handle it first. If the child emits a nohandler event, the parent FSM will take over and attempt to handle it. For example - if a pedestrianWaiting input is fed to the above FSM while in the vehiclesEnabled state, it will be passed on to the vehicleSignal FSM to be handled there.
• Events emitted from the child FSM are bubbled up to be emitted by the top level parent (except for the nohandler event).
• If a child FSM handles input that it does not have a handler for, it will bubble the input up to the parent FSM to be handled there. Did you notice that both our pedestrianSignal and vehicleSignal FSMs queue up a timeout input in the dontwalk and red states, respectively? However, neither of those FSMs have an input handler for timeout in those states. When these FSMs become part of the hierarchy above, as children of the crosswalk FSM, the timeout input will bubble up to the parent FSM to be handled, where there are handlers for it.
• When the parent FSM transitions to a new state, any child FSM from a previous state is ignored entirely (i.e. - events emitted, or input bubbled, will not be handled in the parent). If the parent FSM transitions back to that state, it will resume listening to the child FSM, etc.
• As the parent state transitions into any of its states, it will tell the child FSM to handle a _reset input. 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 pedestrianSignal FSM has a _reset input handler in the dontwalk state, which transitions the FSM to the walking state.

In v1.1.0, machina added the compositeState() method to the BehavioralFsm and Fsm prototypes. This means you can get the current state of the FSM hierarchy. For example:

// calling compositeState on Fsm instances
console.log( crosswalk.compositeState() ); // vehiclesEnabled.green

// calling compositeState on BehavioralFsm instances
// (you have to pass the client arg)
console.log( crosswalk.compositeState( 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

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 newFsm)
• 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.

Build, Tests & Examples

machina.js uses gulp.js to build.

• Install node.js (and consider using nvm to manage your node versions)
• run npm install & bower install to install all dependencies
• To build, run npm run build - then check the lib folder for the output
• To run the examples:
  • npm start
  • navigate in your browser to http://localhost:3080/
• To run tests & examples:
  • To run node-based tests: npm run test
  • To run istanbul (code test coverage): npm run coverage
  • To see a browser-based istanbul report: npm run show-coverage

Release Notes

Go here to see the changelog.

Have More Questions?

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).

Changelog

v4.0.2

• Updated lodash to ^4.17.5