### Question:
DecisionNavigatorObserver { void onDMNDiagramSelected(final @Observes DMNDiagramSelected event) { getOptionalPresenter().ifPresent(DecisionNavigatorPresenter::refresh); }  void init(final DecisionNavigatorPresenter presenter);  }### Answer:
@Test public void testOnDMNDiagramSelected() { observer.init(presenter); observer.onDMNDiagramSelected(new DMNDiagramSelected(null)); verify(presenter).refresh(); }