src/core/mixins/events.js

let each = require("lodash/collection/each"),
    once = require("lodash/function/once"),
    keys = require("lodash/object/keys"),
    isEmpty = require("lodash/lang/isEmpty"),
    uniqueId = require("lodash/utility/uniqueId"),
    slice = Array.prototype.slice;

// Regular expression used to split event strings.
let eventSplitter = /\s+/;

// Implement fancy features of the Events API such as multiple event
// names `"change blur"` and jQuery-style event maps `{change: action}`
// in terms of the existing API.
function eventsApi(obj, action, name, rest) {
  if (!name) return true;

  // Handle event maps.
  if (typeof name === "object") {
    for (let key in name) {
      obj[action].apply(obj, [key, name[key]].concat(rest));
    }
    return false;
  }

  // Handle space separated event names.
  if (eventSplitter.test(name)) {
    let names = name.split(eventSplitter);
    for (let i = 0, l = names.length; i < l; i++) {
      obj[action].apply(obj, [names[i]].concat(rest));
    }
    return false;
  }

  return true;
}

// A difficult-to-believe, but optimized internal dispatch function for
// triggering events. Tries to keep the usual cases speedy (most internal
// Backbone events have 3 arguments).
function triggerEvents(events, args) {
  let ev, i = -1, l = events.length, a1 = args[0], a2 = args[1], a3 = args[2];
  switch (args.length) {
    case 0: while (++i < l) (ev = events[i]).callback.call(ev.ctx); return;
    case 1: while (++i < l) (ev = events[i]).callback.call(ev.ctx, a1); return;
    case 2: while (++i < l) (ev = events[i]).callback.call(ev.ctx, a1, a2); return;
    case 3: while (++i < l) (ev = events[i]).callback.call(ev.ctx, a1, a2, a3); return;
    default: while (++i < l) (ev = events[i]).callback.apply(ev.ctx, args); return;
  }
}

let Events = {
  // Bind an event to a `callback` function. Passing `"all"` will bind
  // the callback to all events fired.
  on: function(name, callback, context) {
    if (!eventsApi(this, "on", name, [callback, context]) || !callback) return this;
    this._events || (this._events = {});
    let events = this._events[name] || (this._events[name] = []);
    events.push({callback: callback, context: context, ctx: context || this});
    return this;
  },

  // Bind an event to only be triggered a single time. After the first time
  // the callback is invoked, it will be removed.
  once: function(name, callback, context) {
    if (!eventsApi(this, "once", name, [callback, context]) || !callback) return this;
    let self = this;
    let onced = once(function() {
      self.off(name, once);
      callback.apply(this, arguments);
    });
    onced._callback = callback;
    return this.on(name, onced, context);
  },

  // Remove one or many callbacks. If `context` is null, removes all
  // callbacks with that function. If `callback` is null, removes all
  // callbacks for the event. If `name` is null, removes all bound
  // callbacks for all events.
  off: function(name, callback, context) {
    let retain, ev, events, names, i, l, j, k;
    if (!this._events || !eventsApi(this, "off", name, [callback, context])) return this;
    if (!name && !callback && !context) {
      this._events = void 0;
      return this;
    }
    names = name ? [name] : keys(this._events);
    for (i = 0, l = names.length; i < l; i++) {
      name = names[i];
      if (events = this._events[name]) {
        this._events[name] = retain = [];
        if (callback || context) {
          for (j = 0, k = events.length; j < k; j++) {
            ev = events[j];
            if ((callback && callback !== ev.callback && callback !== ev.callback._callback) ||
                (context && context !== ev.context)) {
              retain.push(ev);
            }
          }
        }
        if (!retain.length) delete this._events[name];
      }
    }

    return this;
  },

  // Trigger one or many events, firing all bound callbacks. Callbacks are
  // passed the same arguments as `trigger` is, apart from the event name
  // (unless you're listening on `"all"`, which will cause your callback to
  // receive the true name of the event as the first argument).
  trigger: function(name) {
    if (!this._events) return this;
    let args = slice.call(arguments, 1);
    if (!eventsApi(this, "trigger", name, args)) return this;
    let events = this._events[name];
    let allEvents = this._events.all;
    if (events) triggerEvents(events, args);
    if (allEvents) triggerEvents(allEvents, arguments);
    return this;
  },

  // Tell this object to stop listening to either specific events ... or
  // to every object it's currently listening to.
  stopListening: function(obj, name, callback) {
    let listeningTo = this._listeningTo;
    if (!listeningTo) return this;
    let remove = !name && !callback;
    if (!callback && typeof name === "object") callback = this;
    if (obj) (listeningTo = {})[obj._listenId] = obj;
    for (let id in listeningTo) {
      obj = listeningTo[id];
      obj.off(name, callback, this);
      if (remove || isEmpty(obj._events)) delete this._listeningTo[id];
    }
    return this;
  }
};

let listenMethods = {listenTo: "on", listenToOnce: "once"};

// Inversion-of-control versions of `on` and `once`. Tell *this* object to
// listen to an event in another object ... keeping track of what it's
// listening to.
each(listenMethods, function(implementation, method) {
  Events[method] = function(obj, name, callback) {
    let listeningTo = this._listeningTo || (this._listeningTo = {});
    let id = obj._listenId || (obj._listenId = uniqueId("l"));
    listeningTo[id] = obj;
    if (!callback && typeof name === "object") callback = this;
    obj[implementation](name, callback, this);
    return this;
  };
});

// Aliases for backwards compatibility.
Events.bind   = Events.on;
Events.unbind = Events.off;

export default Events;