private async void OnVisitorSelected(object sender, SelectedItemChangedEventArgs e)
        {
            if (VisitorList.SelectedItem == null)
            {
                return;
            }

            var selectedvisitor = e.SelectedItem as Visitor;

            VisitorList.SelectedItem = null;

            var page = new VisitorDetailsPage(selectedvisitor);

            page.VisitorUpdated += (source, visitor) =>
            {
                selectedvisitor.Id            = visitor.Id;
                selectedvisitor.VehicleNumber = visitor.VehicleNumber;
                selectedvisitor.Name          = visitor.Name;
                selectedvisitor.FlatNumber    = visitor.FlatNumber;
                selectedvisitor.PhoneNumber   = visitor.PhoneNumber;
                selectedvisitor.TimeIn        = visitor.TimeIn;
                selectedvisitor.Signature     = visitor.Signature;
            };

            await Navigation.PushAsync(page);
        }
        private async void OnNewVisitor(object sender, EventArgs e)
        {
            var page = new VisitorDetailsPage(new Visitor());

            page.VisitorAdded += (source, visitor) =>
            {
                _visitors.Add(visitor);
            };

            await Navigation.PushAsync(page);
        }