Making features independent with internal events

2 Feb 2015 · by David DeSandro

Flickity is not lightweight. flickity.pkgd.js is currently 5000 lines of code weighing 132 kb. That's about half the size of jQuery. Having a larger file size is not necessarily a bad thing. Using the compressed flickity.pkgd.min.js, the file size is equivalent to a small image — nothing to worry about. What did concern me was how Flickity had grown to become a monolith.

Although I was breaking up code into logical sections and features, these features were hard-coded together. For example, activating page dots and previous/next buttons was handled within Flickity.prototype.activate().

Flickity.prototype.activate = function() {
  // ...
  // activate prev/next buttons, page dots
  if ( this.prevButton ) {
    this.prevButton.activate();
  }
  if ( this.nextButton ) {
    this.nextButton.activate();
  }
  if ( this.pageDots ) {
    this.pageDots.activate();
  }
}

Flickity.prototype.pointerDown() made a direct call to this.player.

Flickity.prototype.pointerDown = function() {
  // ...
  // stop auto play
  this.player.stop();
};

Everything was interconnected. If you wanted to use Flickity, you would need to use all its parts.

Flickity dependency chart

Ideally, Flickity could be structured in a way that features could be completely optional. If you didn't want to use page dots or autoPlay, you wouldn't have to include its .js source. While these features are related, they are not dependent on one another. They could be separated and made independent. To maintain functionality between features, Flickity could use its own events.

EventEmitter

I use Wolfy87/EventEmitter for event handling in all my libraries (Flickity, Isotope, Draggabilly, imagesLoaded). EventEmitter was developed as a browser-port of node's EventEmitter class. It emits evens with .emit() or .emitEvent(), and binds events with .on(), .off(), and .once().

function Library() {}
// inherit EventEmitter methods
Library.prototype = new EventEmitter();

var lib = new Library();
lib.on( 'tacoTuesday', function( message, hours ) {
  console.log( 'REJOICE! TT ' + message + ' for ' + hours + ' hours' );
});
lib.emit( 'tacoTuesday', 'gonna be chill', 4 );
// -> logs 'REJOICE! TT gonna be chill for 4 hours'

With EventEmitter, I have a proper API for pub-sub.

Internal events

You might typically think of events as hooks that are used by developers and third-party libraries to build on top of a library. For example, Bootstrap's carousel triggers slide.

$('#myCarousel').on( 'slide', function () {
  console.log('carousel slide happening')
})

Events can also be used internally within the library. Within Flickity, I was already using events across features. Previous/next buttons and page dots were being updated by listening to the select event.

Flickity.prototype.select = function( index ) {
  // ...
  this.selectedIndex = index;
  // emit select event
  this.emit('select');
};

PrevNextButton.prototype._create = function() {
  // ...
  // update on select event
  var _this = this;
  this.parent.on( 'select', function() {
    _this.update();
  });
};

I used this same concept to replace all hard-coded features with events. For example, in Flickity.prototype.activate(), I removed checking for features like this.prevButton, and replaced it with emitting the activate event.

Flickity.prototype.activate = function() {
  // ...
  this.emit('activate');
};

I moved the previous/next button activation logic into prev-next-button.js. This file adds necessary methods to Flickity.prototype that are specific to the previous/next buttons.

Flickity.createMethods.push('_createPrevNextButtons');

Flickity.prototype._createPrevNextButtons = function() {
  if ( !this.options.prevNextButtons ) {
    return;
  }
  // create buttons
  this.prevButton = new PrevNextButton( -1, this );
  this.nextButton = new PrevNextButton( 1, this );
  // listen to activate
  this.on( 'activate', this.activatePrevNextButtons );
};

Flickity.prototype.activatePrevNextButtons = function() {
  this.prevButton.activate();
  this.nextButton.activate();
};

Instead of Flickity.prototype.pointerDown() calling this.player.stop(), the Player class listens to the pointerDown event.

Flickity.createMethods.push('_createPlayer');

Flickity.prototype._createPlayer = function() {
  this.player = new Player( this );
  // ...
  this.on( 'pointerDown', this.stopPlayer );
};

Flickity.prototype.stopPlayer = function() {
  this.player.stop();
};

There's some extra glue required to put this together. Feature-specific Flickity.prototype methods need to be triggered on creation. Events need to be bound before they are emitted. To handle this, within Flickity.prototype.create(), each method in Flickity.createMethods gets triggered.

// flickity.js
Flickity.prototype._create = function() {
  // ...
  // trigger each method within createMethods
  for ( var i=0, len = Flickity.createMethods.length; i < len; i++ ) {
    var method = Flickity.createMethods[i];
    this[ method ]();
  }
};

Features can then register their create methods:

// feature.js
Flickity.createMethods.push('_createFeature');

Flickity.prototype._createFeature = function() {
  // do feature-specific creation and event binding
};

A feature's code exists only in that feature's .js file, and not sprinkled through out the project.

The result is that the dependencies are reversed. Instead of Flickity requiring each feature to be in place, features are dependent on the core of Flickity. Features can be added or removed independently of one another.

Flickity dependency chart

You could even build a Flickity gallery with no dragging or no UI, with just the core API as its interface.

See the Pen Flickity - no drag, no UI, API only by David DeSandro (@desandro) on CodePen.

Even though it increases Flickity's total file size, this refactor makes me feel at ease. It provides file-size-snobs a way to build streamlined Flickity packages without any cruft. Flickity now has a extendable API that allows anyone to build add-on features that can be used on demand. This will be useful as more features get requested. Flickity's feature-set can grow without having to build into the core project.

Previously

If you're just joining us, I'm making a new gallery library! The story thus far...

Flickity beta is out. Give 'er a flick!

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