Prevent closing by hardware back button in xamarin forms on android

It works with evaluating the NavigationStack (when you use NavigationPage).

In my Activity, I override the OnBackPressed

public override void OnBackPressed()
{
    if(App.Instance.DoBack)
    {
        base.OnBackPressed();
    }
}

In my xamarin forms app (App.Instance (it is a singleton)), I will evaluate the NavigationStack of the current Page like this.

public bool DoBack
{
    get
    {
        NavigationPage mainPage = MainPage as NavigationPage;
        if (mainPage != null)
        {
            return mainPage.Navigation.NavigationStack.Count > 1;
        }
        return true;                
    }
}

When there is only one page left in the NavigationStack I will not call base.OnBackPressed, so that I will not close the App.

![test]

And here's what the code could look like for a Xamarin Forms MasterDetail page scenario...

    public bool DoBack
    {
        get
        {
            MasterDetailPage mainPage = App.Current.MainPage as MasterDetailPage;

            if (mainPage != null)
            {    
                bool canDoBack = mainPage.Detail.Navigation.NavigationStack.Count > 1 || mainPage.IsPresented;

                // we are on a top level page and the Master menu is NOT showing
                if (!canDoBack)
                {
                    // don't exit the app just show the Master menu page
                    mainPage.IsPresented = true;
                    return false; 
                }
                else
                {
                    return true;
                }                    
            }
            return true;
        }
    }