fn-machine

A tiny, functional, state machine utility

install

npm install --save fn-machine

usage

fn-machine consists of 3 functions. The first two are used to define a machine:

machine([State], 'initialState', initialContextObj, stateChangeCallback, loggerFn)

state('name', transitionsObj, enterFunction, exitFunction)

The third function is what would traditionally be called a send() function. This function is returned by calling machine(...).

Setting up a machine

// import the setup functions
import {machine, state} from 'fn-machine';

// initial context object
const initialContext = {
  loading: false,
  users: [],
};

function loadUsers() {
  // simulate a network request
  setTimeout(() => {
    // once the request completes, we can call `myMachine` (the 'send' function).
    myMachine('loaded', {users:['foo', 'bar']})
  }, 1000);
};

// initialize a machine
const myMachine = machine([
  state('initial', {
    // each method on this object represents a transition for this particular state.
    loadData: (detail, context) => {
      // a transition should return the new state, as well as the optional context.
      // here we return {state:'loadingData'} to signify we want the state to now be 'loadingData'.
      return {
        state:'loadingData',
      }
    }
  }),
  state('loadingData', {
    loaded: (detail, context) => {
      return {
        state: 'loadedData',
        context: {...context, ...detail, ...{loading: false}}
      }
    }
  }, context => {// call loadUsers when this state is entered, and return the new context.
    loadUsers();
    return {...context, ...{loading: true}};
  }),
  state('loadedData', {}) // 'loadedData' is an empty/final state. There are no transitions.
], 'initial', initialContext, newState => {
  console.log('myMachine state changed:', newState.state, newState.context);
}, console.log);// pass an optional logger function

In the loadUsers() function above, we invoke the third function provided by fn-machine, which is the send function. The send function takes a string as the first parameter, which is the name of a transition we'd like to invoke, and optionally a detail object, which contains some data we want the machine to work with, and/or update the context with.

You can also define transitions using a short-hand syntax like so:

state('myState', {
  someAction: 'newState',
});

which is equivelent to:

state('myState', {
  someAction: (detail, context) => {
    return {
      state: 'newState',
      context: {...context, ...detail},
    };
  },
});

More examples

There is an example in this repo, or you can play around with this codepen that shows a basic integration with LitElement.

mermaid

There are two utility functions to convert to and from mermaid syntax.

toMermaid([state('on', {powerOff: 'off'}, state('off', {powerOn: 'on'}))], 'off');

produces a string like that you can process with mermaidjs to visualize your machine:

stateDiagram-v2
[*] --> off
on --> off: powerOff
off --> on: powerOn

Or, you can take a mermaid string and output some stub javascript:

const mermaidStr = `
stateDiagram-v2
[*] --> off
on --> off: powerOff
off --> on: powerOn
`;
fromMermaid(mermaidStr);

which produces:

[state('on', {powerOff: 'off'}, state('off', {powerOn: 'on'}))]

These are useful for visualization and initial creation of your machines, but beware that if your machine transitions contain logic, that logic would be lost should you try to go full circle: machine -> mermaid -> machine.

Contributing

Yes! PR's are welcome. Tests are written in mocha. Run with npm run test or yarn test. Typechecking is provided by typescript via JSDoc annotations.