Q:

Prevent closing by back button in xamarin forms on android

I want to prevent closing the app by pressing the hardware back button in xamarin forms on android.
I want, that you can navigate with the hardware back button in the app (what is working), but do not want to exit, when the first page in navigation stack is reached.
I tried to use the OnSleep event in xamarin forms, but here I can not cancel the exit.
I also tried catching the back button in android:
    public override void OnBackPressed()
    {
        //base.OnBackPressed();
    }

But when using xamarin forms, I do not know which page is currently showing. So I do not know if the navigation back is allowed or not 

A:

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]

A:

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;
        }
    }

A:

Just give a blank call in the page where do you wanna prevent, like
protected override bool OnBackButtonPressed()
{
return true;
}

This will prevent the back button in XF-Droid.

