public ValueProjectionObservable(StoreBase <TState> store, ValueProjector <TState, T> projector, Action onEmpty)
     : base(onEmpty)
 {
     _store        = store;
     _projector    = projector;
     _currentValue = projector(_store.CurrentState);
 }
 public ElementProjectionObservable(StoreBase <TState> store, ElementProjector <TState, T, K> projector, K elementKey, Action onEmpty)
     : base(onEmpty)
 {
     _store        = store;
     _projector    = projector;
     _elementKey   = elementKey;
     _currentValue = projector(_store.CurrentState, elementKey);
 }