addObserver method

@override void addObserver(ObserverMixin observer)

Implementation

@override
void addObserver(ObserverMixin observer) {
  assert(!_isDisposed, "$this is already disposed");
  assert(() {
    if (!_observers.contains(observer)) {
      debugLog("$observer starts observing $this");
    }
    return true;
  }());
  observer.onAddedToState(this);
  _observers.add(observer);

  for (final plugin in _plugins) {
    plugin.onObserverAdded(observer);
  }
}