Navigation: login pages

Dear all,

I followed the instructions and I can make a nice tabbed navigation. Now I'd like to move to something fancier, but I have some doubts.

The target is this: when the app starts, it must check if I have saved some credentials, and if it has saved them, switch to a profile page profile. But if it doesn't have credential, it should show a simple page with a button notloggedin. The button will pop up a new page loginpage with a Facebook login (or another oauth method). Upon cancelling or logging in, the loginpage must disappear, and either the notloggedin or profile pages will be shown.

When starting an app, I set MainPage to the tabbed page I've created. But here what will the ManPage be? And when logged in, should I change the MainPage to the profile?

I've tried to do something like this:

public class App : Application
{
    static NavigationPage _NavPage;

    public App ()
    {
        _NavPage = new NavigationPage ();

        _NavPage.PushAsync (new BaseContentPage ()); // empty page

        MainPage = _NavPage.CurrentPage;

        var account = AccountStore.Create ().FindAccountsForService (App.AppName).FirstOrDefault ();
        var p = (account != null) ? account.Properties ["token"] : null;

        Log ("credentials: " + (p == null ? "null" : p.ToString ()));

        if (p != null)
        {
            Log ("previous login is successful");

            // profile
            MainPage = new ProfilePage ();
        }
        else
        {
            Log ("No previous token present");

            // not logged in
            _NavPage.PushAsync (new notloggedPage ());
        }
    }

But something is wrong: the empty page is always there, and the notloggedin page isn't shown, but the constructor is called.

Can anyone suggest me some solutions?

Thanks!