### Question:
DecisionNavigatorObserver { void onNestedElementAdded(final @Observes ExpressionEditorChanged event) { getOptionalPresenter().ifPresent(DecisionNavigatorPresenter::refresh); }  void init(final DecisionNavigatorPresenter presenter);  }### Answer:
@Test public void testOnNestedElementAdded() { observer.init(presenter); observer.onNestedElementAdded(new ExpressionEditorChanged(uuid)); verify(presenter).refresh(); }