I prefer
```
  if (_displayAvailable == isAvailable)
    return;

  _displayAvailable = isAvailable;
  if (!_listeners[DISPLAY_AVAILABLE_CHANGE_EVENT])
    return;
  
  for (var i = 0; i < _listeners[DISPLAY_AVAILABLE_CHANGE_EVENT].length; ++i) {
    _listeners[DISPLAY_AVAILABLE_CHANGE_EVENT][i].apply(null, null);
  } 
```