public NavigationService(
Frame frame,
IPageDefinitionRegistry pageDefinitions,
IViewModelFactory viewModelFactory,
IScheduler dispatcherScheduler,
IScheduler backgroundScheduler)
{
if (frame == null) throw new ArgumentNullException("frame");
if (pageDefinitions == null) throw new ArgumentNullException("pageDefinitions");
if (viewModelFactory == null) throw new ArgumentNullException("viewModelFactory");
if (dispatcherScheduler == null) throw new ArgumentNullException("dispatcherScheduler");
if (backgroundScheduler == null) throw new ArgumentNullException("backgroundScheduler");
_frame = frame;
_pageDefinitions = pageDefinitions;
_viewModelFactory = viewModelFactory;
_dispatcherScheduler = dispatcherScheduler;
_backgroundScheduler = backgroundScheduler;
_navigatingSubject = new Subject<INavigationRequest>();
_navigatedSubject = new Subject<INavigationRequest>();
_semaphore = new SemaphoreSlim(1);
History = _history = new NavigationHistory(RemoveHistoryEntry, ClearHistory);
_frame.Navigated += _frame_Navigated;
//_frame.BackKeyPress += _frame_BackKeyPress;
}