How to intercept Navigation Bar Back Button Clicked in Xamarin Forms?

Question

I have a xamarin form page where a user can update some data in a form. I need to intercept the Navigation Bar Back Button Clicked to warn the user if some data have not bee

Answer 1

As has already been said - you cannot do this cross-platform. However, you can handle it natively with arguably not so much effort: https://theconfuzedsourcecode.wordpress.com/2017/03/12/lets-override-navigation-bar-back-button-click-in-xamarin-forms/

The article covers iOS and Android. If you have a UWP project you'll have to hammer your own solution for it.

Edit: Here is the UWP solution! It actually turned out to be pretty easy – there is just one back button and it’s supported by Forms so you just have to override ContentPage’s OnBackButtonPressed:

    protected override bool OnBackButtonPressed()
    {
        if (Device.RuntimePlatform.Equals(Device.UWP))
        {
            OnClosePageRequested();
            return true;
        }
        else
        {
            base.OnBackButtonPressed();
            return false;
        }
    }

    async void OnClosePageRequested()
    {
        var tdvm = (TaskDetailsViewModel)BindingContext;
        if (tdvm.CanSaveTask())
        {
            var result = await DisplayAlert("Wait", "You have unsaved changes! Are you sure you want to go back?", "Discard changes", "Cancel");

            if (result)
            {
                tdvm.DiscardChanges();
                await Navigation.PopAsync(true);
            }
        }
        else
        {
            await Navigation.PopAsync(true);
        }           
    }