code: function(content, idx, removedCnt, addedCnt) {
      var keys = this._keys, key, lim;
  
      lim = removedCnt>0 ? idx+removedCnt : -1;
      Ember.beginPropertyChanges(this);
  
      for(key in keys) {
        if (!keys.hasOwnProperty(key)) { continue; }
  
        if (lim>0) removeObserverForContentKey(content, key, this, idx, lim);
  
        Ember.propertyWillChange(this, key);
      }
  
      Ember.propertyWillChange(this._content, '@each');
      Ember.endPropertyChanges(this);
    }
docs: .......................................................... ARRAY CHANGES Invokes whenever the content array itself changes.