async void OnListViewItemTapped(object sender, ItemTappedEventArgs e)
{
ChatPreview tappedItem = e.Item as ChatPreview;
var detail = new BeaconClient.ChatInterface();
detail.fromPreview = tappedItem;
await Navigation.PushAsync(detail);
}
void OnListViewItemSelected(object sender, SelectedItemChangedEventArgs e)
{
ChatPreview selectedItem = e.SelectedItem as ChatPreview;
}