when <these> then <those>

Honestly, it's easier to show than tell.

//we have a Backbone.Events instance at our disposal in `app.vent`
app.vent.on('rolling', function(){
	console.log('Here we go:');
});

var instruct = require('backbone.whenthen');
instruct(app.vent)
    .when('event1', 'event2', 'event3')
    .then('rolling', function(){
        console.log("We're ready to roll!!");
    });

app.vent.trigger('event1'); // nothing happens
app.vent.trigger('event2'); // nothing happens
app.vent.trigger('event3'); // this will:

// output to console: 
Here we go:
We're ready to roll!!

This is especially useful in situations where you have a lot of parallel async processes going on and you want to respond to the collective completion of these processes. For instance: complex loading and bootstrapping sequences or bespoke view transitions.

# bower
bower install backbone.whenthen

# npm
npm install backbone.whenthen

• Despite the name, backbone.whenthen does NOT have a hard dependency on Backbone, however it does expect Backbone.Events-compatible instances to control.
• underscore is required shizzle though.

backbone.whenthen can be used in node or the browser, either with:

// AMD
define([ "backbone.whenthen" ], function(instruct){
    //use `instruct` as in the examples
});

// CommonJS
var instruct = require('backbone.whenthen');
//use `instruct` as in the examples

<!-- old skool, definitely NOT recommended -->
<script src="//jashkenas.github.io/underscore/underscore-min.js"></script> <!-- must be loaded first -->
<script src="//jashkenas.github.io/backbone/backbone-min.js"></script> <!-- must be loaded first -->
<script src="/vendor/backbone.whenthen.min.js"></script>

//old skool, continued
var instruct = Backbone.WhenThen; // it's attached to Backbone
//or in case you don't load Backbone:
var instruct = WhenThen; // it's a global object... *shudders*

//use `instruct` as in the examples

The backbone.whenthen module exposes a function, which accepts a Backbone.Events(-compatible) event dispatcher and starts off an extremely simple fluent interface:

var _ = require('underscore');
var Backbone = require('backbone');
var instruct = require('backbone.whenthen');
var dispatcher = _.extend({}, Backbone.Events); // obviously any Backbone.Events dispatcher will do: Backbone.Model et cetera.
instruct(dispatcher);

You can pass as many (arrays of) event strings to when as you want, they'll be normalized:

instruct(dispatcher)
	.when('event:a', ['event:b', 'event:c'], 'event:d');

then accepts any number (and mixture) of (arrays of) strings and/or functions:

instruct(dispatcher)
	.when('event:a', ['event:b', 'event:c'], 'event:d')
	.then('event:z', ['event:y', fooFn], barFn);

Once all events registered in when have been triggered by dispatcher, all items registered with then will be either be executed (functions) or triggered (strings) in dispatcher. I.e. in the above example dispatcher will trigger event:z, event:y and call fooFn, barFn (synchronously and in order).

Example:

instruct(app.vent)
	.when('loaded:images', 'loaded:locales')
	.then('app:show:ui'); // `app.vent` triggers 'app:show:ui'

It doesn't matter in which order the when events are triggered, then will always wait until all of them have been triggered.

If you want to have a difference Backbone.Events instance triggering the completion event(s), you can use have in between:

instruct(loader)
    .when('something:ready', 'something:else:ready')
    .have(app.vent).then('app:ready');

In this case, once loader has triggered 'something:ready' and 'something:else:ready' the app.vent dispatcher will trigger 'app:ready'.

then exposes a new when. You can go on ad infinitum.

instruct(app.vent)
	.when('foo').then(fooFn)
	.when('bar').then(barFn)
// you catch the drift

have's apply strictly to the then coming right after it, e.g.:

instruct(app.vent)
	.when('foo').have(someOther).then('bar')
	.when('baz').then('qux'); // `app.vent` triggers `'qux'`

Obviously backbone.whenthen can be used as a simple event translator/relayer, for translating module events for instance. If a single event is registered with when automatically all parameters passed when triggering said event will be passed to the then events/callbacks.

instruct(app.vent)
	.when('user:selection:completed')
	.have(account.vent).then('user:selected');
//yeah, yeah I know, not the best of examples

account.vent.on('user:selected', function(user){
    console.log(user);// outputs: {name:"Camille Reynders"}
});
app.vent.trigger('user:selection:completed', {
    name: 'Camille Reynders'
});

If you need to clean up you can destroy:

var relayer = instruct(loader);
//relayer.when(...).then(...);
// later on:
relayer.destroy();

destroy unregisters all callbacks and events, and cleans up all objects. If you try to use a destroyed instance it will throw errors:

relayer.destroy();
relayer.when('foo').then('bar'); // Error: `when` instance destroyed

MIT Copyright (c) 2015 Camille Reynders