How to listen to an event on multiple objects using Backbone.js

whenAll.js

function whenAll (objects, event, callback) {
  var callbackWrapper = _.after(objects.length, callback);
  _.each(objects, function (obj) {
    obj.once(event, callbackWrapper, this);
  }, this);
}

Inspired by How to listen to an event on multiple objects using Backbone.js.

I use it like this:

preload: function (callback) {
  var collections = [users, messages];
  whenAll(collections, 'sync', callback);
  for (var i = collections.length - 1; i >= 0; i--) {
    collections[i].fetch();
  }
}