@Test
public void call_open() {

    // prepare
    OpenEvent event = mock(OpenEvent.class);
    when(event.getSession()).thenReturn(session);
    // run
    caller.visit(event);
    // verify
    verify(event).getSession();
    verify(session).getService();
    verify(service).getIoHandler();
    verify(handler).sessionOpened(session);
    verifyNoMoreInteractions(session, event, handler);
}