如何在activity中显示正在加载的微调器(Xamarin.Android)？

<ProgressBar
    android:id="@+id/progressBar"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="center"
    android:visibility="gone" />

ProgressBar progressBar;

protected override void OnCreate(Bundle savedInstanceState)
{
    base.OnCreate(savedInstanceState);
    SetContentView(Resource.Layout.activity_main);

    progressBar = FindViewById<ProgressBar>(Resource.Id.progressBar);
}

progressBar.Visibility = ViewStates.Visible;

progressBar.Visibility = ViewStates.Gone;