Caliburn.Micro Go Back called in On Activate not working in WinRT

Question

This question is specific to Windows Phone 8.1 (WinRT); it may also be applicable Windows 8.1. I am using Caliburn.Micro 2.0.1

In my ViewModel's OnActivate I check whether an item is a database, if it isn't, I want to navigate back to the previous page.

The simplist solution will be just to call GoBack in the OnActivate method (this works in Windows Phone 8.0):

INavigationService _navigationService;
protected override void OnActivate()
{
    _item = GetItemFromDB();
    if(_item == null)
    {
       _navigationService.GoBack() 
    }
}

To navigate to the view model I call:

_navigationService.NavigateToViewModel<MyViewModel>(_param);

But it does not work, it ignores the GoBack call and stays on the page which I do not want to view.

When stepping through the code you can see that the GoBack code is called inside the NavigateToViewModel method; I expect this is the reason why it does not work (something to do with a queuing issue maybe?).

I have a very "hacky" solution that involves a timer (that works), but I really despise it since it is prone to threading issues and has the possibility of being called during the NavigateToViewModel call (if it takes long to finish), which will then again not work:

protected override void OnActivate()
{
    _item = GetItemFromDB();
    if(_item == null)
    {
        DispatcherTimer navigateBackTimer = new DispatcherTimer();
        navigateBackTimer.Interval = TimeSpan.FromMilliseconds(300);
        navigateBackTimer.Tick += GoBackAfterNavigation;
        navigateBackTimer.Start();
    }
}

public void GoBackAfterNavigation(object sender, object e)
{
    _navigationService.GoBack();
    (sender as DispatcherTimer).Stop();
}

Is there a better way to navigate back? Why doesn't the GoBack work in OnActivate? Is there a way to get it to work in OnActivate?

Answer 1

You can use

Execute.OnUIThreadAsync(() => /* navigationCode */);

instead of a timer to queue the action immediately after the processing of the current stack has finished.