code: function addListener(obj, eventName, target, method, once) {
    Ember.assert("You must pass at least an object and event name to Ember.addListener", !!obj && !!eventName);
  
    if (!method && 'function' === typeof target) {
      method = target;
      target = null;
    }
  
    var actions = actionsFor(obj, eventName),
        actionIndex = indexOf(actions, target, method);
  
    if (actionIndex !== -1) { return; }
  
    actions.push([target, method, once, undefined]);
  
    if ('function' === typeof obj.didAddListener) {
      obj.didAddListener(eventName, target, method);
    }
  }
docs: Add an event listener
  
  @method addListener
  @for Ember
  @param obj
  @param {String} eventName
  @param {Object|Function} targetOrMethod A target object or a function
  @param {Function|String} method A function or the name of a function to be called on `target`
  @param {Boolean} once A flag whether a function should only be called once