Event-driven Finite State Machine

This is a helper module for implementing finite state machines that react to events on an event bus.

The event bus is provided externally and must have implemented the following methods:

• emit(event, arg)
• on(event, handler) with handle: (arg) => {...}
• removeListener(event, handler)

For further details have a look into the documentation of native events provided by NodeJS.

Example

A little guessing game:

// Input event bus
const rl = require('readline');
const input = rl.createInterface({ input: process.stdin });

// Output event bus
const EventEmitter = require('events');
const output = new EventEmitter();
output.on('log', (line) => console.log(line));
output.on('err', (line) => console.error(line));

// FSM
const EDFSM = require('edfsm');
EDFSM({
	fsmName: 'guess',
	input: input,
	output: output,
	firstState: 'getName'
}).state('getName', (ctx, i, o, next) => {
	// Get the user's name
	o('log', 'Hi! What\'s your name?');
	i('line', (name) => {
		ctx.name = name;
		if (name === 'Chuck Norris') {
			next('win');
		} else {
			next('guess');
		}
	});
}).state('guess', (ctx, i, o, next) => {
	// Generate an random number
	const no = Math.floor(Math.random() * 16);

	// Let the user guess a number
	o('log', 'Guess a number between 0 and 15');
	i('line', (number) => {
		number = parseInt(number);
		if (number === no) {
			next('win');
		} else {
			next('loose');
		}
	});

	// The user has to answer within 3s
	next.timeout(3000, 'timeout');
}).state('win', (ctx, i, o, next) => {
	o('log', `You won, ${ctx.name}`);
	next('getName');
}).state('loose', (ctx, i, o, next) => {
	o('log', `You lost, ${ctx.name}`);
	next('getName');
}).state('timeout', (ctx, i, o, next) => {
	o('error', 'You are too slow!');
	next('getName');
}).run({});

API

Creating a new FSM factory:

const EDFSM = require('edfsm');
const edfsmFactory = EDFSM(opts);

Creates a new FSM factory with given opts:

• fsmName: Human-readable name describing the FSM. Used for logging.
• inputs: Object containing multiple input busses. Their on() methods are accessible by calling i[key](event, () => {...}) inside the states.
• outputs: Object containing multiple output busses. Events are emitted by calling o[key](event, () => {...}) inside the states.
• input: Instance of the main input event bus. Can be used instead of the inputs option. Event listeners are installed by calling i(event, () => {...}) inside the state.
• output: Instance of the output event bus. Can be used instead of the outputs option. Can be the same instance as input. Events can be emitted by calling o(event, arg) inside the states.
• firstState: The name of the entry state. If not given, the first state will become the entry state.
• log: Object containing logging callbacks. All callbacks have this interface: (msg, obj) => {}, whereas msg is the log message and obj contains further machine-readable details.
  • debug: Message about FMS construction, destruction and state changes
  • warn: Warnings about unconsumed output messages
  • error: Error log.

States are defined and attached like this:

edfsmFactory.state(name, (ctx, i, o, next) => {...});

• name: The state's name for addressing it.
• ctx: Context object of the current instance of the FSM. You can store any kind of information in there.
• i: This interface is used if the FSM option input is set. Helper method for setting up listeners to input events: i(event, (arg) => {...}). The listener will be removed automatically when the state is left. You do not have to take care about that.
• i[key]: This interface is used if the FSM option inputs is set. Helper method for setting up listeners to input events: i[key](event, (arg) => {...}). key is the respective input item. The listener will be removed automatically when the state is left. You do not have to take care about that.
• o: This interface is used if the FSM option output is set. Helper method for emitting events on the output event bus: o(event, arg).
• o[key]: This interface is used if the FSM option outputs is set. Helper method for emitting events on the output event bus: o[key](event, arg). key is the respective output item.
• next: Method to be called if the current state shall be left: next(state). For transitioning to another state, put the state's name in the argument. If you want to destroy the current FSM instance, state null or an instance of Error as the first argument.
• next.timeout: Like next but delayed by timeout milliseconds: next.timeout(timeout, state). If the current state is left before the timeout elapsed, it will be cleared automatically. The timeout can be retriggered by calling this method again.

You can define a special state that will be entered before the FSM instance gets destroyed:

edfsmFactory.final((ctx, i, o, next, err, lastState) => {...});

In addition to the handler's arguments as they are described above, these arugments are handed over to the handler:

• err: Will contain the error if the next call in the previous state contained an error. Otherwise this is undefined.
• lastState: The name of the previous state.

Instances can be created by calling:

edfsmFactory.run(ctx[, onEnd]);

• ctx: Object containing the context that the state handlers will access and use for storing instance-related information.
• onEnd: Callback that will be called once the FSM instance has been destroyed.