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A promise-aware finite state machine implementation for Ember objects

Ember FSM

A promise-aware finite state machine implementation for Ember objects

var trafficSignal = Em.FSM.Machine.create({
  events: {
    cycle: {
      transitions: [
        { initialized: 'red' },
        { red: 'green' },
        { green: 'amber' },
        { amber: 'red' }
    powerDown: {
      transition: { $all: 'off' }
// "initialized" 
// "red" 
// "green" 

A wild traffic signal demo appears!

Getting Started

The most recent builds are available in the dist directory. If you just want to drop Ember.FSM into your app and start using it, you're probably looking for: dist/globals/ember-fsm.js.

Defining a State Machine

SleepyFSM = Ember.FSM.Machine.extend({
  // Here is where you define your state machine's state-specific configuration. 
  // This section is optional. 
  states: {
    // The default initial state is "initialized" 
    initialState: 'awake'
    // If you'd like, you can choose to explicitly define the names of your 
    // states: 
    knownStates: ['sleeping', 'angry', 'awake', 'initialized', 'failed'],
    // You can define global per-state callbacks, they will fire whenever the 
    // state will be entered, was entered, will be exited, or was exited. 
    sleeping: {
      willEnter: function() { },
      didEnter: function() { },
      willExit: function() { },
      didExit: function() { }
  // Here's where you define your state machine's events, it is required. 
  events: {
    sleep: {
      // You can define global per-event callbacks. These will fire for any 
      // transition before or after this event. 
      before: function() { },
      after: function() { },
      // This is where the event's transitions are defined, it is also aliased 
      // to "transition". It can accept either a single object like one in the 
      // array below, or an array of transition definition objects: 
      transitions: [
        { awake: 'sleeping', doUnless: 'unableToSleep' },
        { awake: 'angry', doIf: 'unableToSleep' },
        { sleeping: '$same' }
    // By default this error event is injected into your state machine for you, 
    // you can override it and provide your own transitions and callbacks if 
    // you'd like. 
    error: {
      transition: { $all: 'failed' }

State Macros

For the sake of less typing (and less chances of introducing failure) the following macros can be used in transition definitions:

Macro Description
$all Expands to all known states.
$same Expands to the same state as the from state. transition: { sleeping: '$same' }
$initial Expands to the initial state.

Transition Guarding

You can specify that a transition be excluded or included in the event using doIf or doUnless. Consider SleepyFSM above, if we set unableToSleep to true then when we send in the sleep event, it will transition to the state angry because the transition { awake: 'sleeping' } will be excluded from the list.

doIf and doUnless are aliased to guard and unless respectively.

Transition Events & Callbacks

Given the SleepyFSM example above, suppose we ran the following:

var fsm = SleepyFSM.create();

Here is the series of transition events that will occurr and the corresponding callbacks that will run and where they can be defined:

Current State Is Active Event Runs callbacks
awake false beforeEvent before on events and transitions
awake true _activateTransition_ internal
awake true willExit willExit on states and transitions
awake true willEnter willEnter on states and transitions
sleeping true _setNewState_ internal
sleeping true didExit didExit on states and transitions
sleeping true didEnter didEnter on states and transitions
sleeping false _deactivateTransition_ internal
sleeping false afterEvent after on events and transitions

Some of the event names above also have aliases:

Event Aliases
beforeEvent before
afterEvent after
didEnter enter, action
didExit exit

Asynchronicity In Callbacks

If callbacks return a promise, the next callback in the chain will not fire until the promise is resolved. The return value of callbacks is stored in the transition's resolutions object. Likewise, rejections are stored in the rejections object of the transition.

Namespacing States

Ember.FSM doesn't provide true sub-state support, but you can namespace your states. For example, suppose a portion of your state workflow is related in some way; you can prefix those states with a namespace:

  • ready
  • uploading.requestingUrl
  • uploading.sendingData
  • processing.enqueuing
  • processing.working
  • finished

When you define states like this, Ember.FSM automatically generates the following boolean accessor properties for you:

  • isInReady
  • isInUploading
  • isInUploadingRequestingUrl
  • isInUploadingSendingData
  • isInProcessing
  • isInProcessingEnqueuing
  • isInProcessingWorking
  • isInFinished

Stateful Mixin

When it comes to using Ember.FSM in your application, you'll almost always want to use Ember.FSM.Stateful over sub-classing Ember.FSM.Machine. This way you can formalize a state workflow around something like file uploads where you might have to incorporate three different proceesses into on user experience.

Note states and events are renamed in the mixin to fsmStates and fsmEvents respectively, to avoid conflict with core Ember properties.

Building these sorts of workflows implicitly as-you-code-along can be a recipie for massive sadness. So why be sad? Formalize that workflow! Here's an example of how adding Ember.FSM.Stateful to a controller can remove a lot of the tedious parts of workflow managment:

App.UploadController = Em.Controller.extend(Em.FSM.Stateful, {
  needs: 'notifier',
  actions: {
    uploadFile: function(file) {
      this.set('file', file);
  fsmStates: {
    initialState: 'nofile'
  fsmEvents: {
    addFile: {
      transitions: {
        from:   ['nofile', 'failed'],
        to:     'ready',
        before: 'checkFile',
    startUpload: {
      transitions: {
        from:     'ready',
        to:       'uploading',
        before:   'getUploadURL',
        didEnter: 'performUpload',
        after:    'finishedUpload'
    finishUpload: {
      transition: { uploading: 'nofile', didEnter: 'reset' }
  reset: function() {
    this.set('file', null);
  checkFile: function() {
    var file = this.get('file');
    if (file.size > 0) {
    } else {
      this.get('controllers.notifier').warn('file must have content');
      Em.FSM.reject(); // A helper for throwing an error 
  getUploadURL: function() {
    var controller = this;
    var fileName = this.get('');
    var xhr;
    xhr = $.ajax('/api/signed_uploads', {
      type: 'put',
      data: { file: { name: fileName } }
    xhr.then(function(payload) { {
        controller.set('uploadToURL', payload.signed_upload.url);
    return xhr; // Causes transition to block until promise is settled 
  performUpload: function() {
    return $.ajax(this.get('uploadToURL'), {
      type: 'put',
      data: this.get('file')
  finishedUpload: function() {
    this.get('controllers.notifier').success('Upload complete');


cd my/fork/of/ember-fsm
rm -rf dist
broccoli build dist


Install Node.js and NPM, there are packages and binaries on the Node.js website that make it easy.

cd my/fork/of/ember-fsm
npm install -g broccoli-cli testem
npm install
bower install
broccoli serve

Then in another session:

cd my/fork/of/ember-fsm

Then do what testem tells you to do.