private void Rotary_Tapped(object sender, TappedRoutedEventArgs e)
{
FrameworkElement element = sender as FrameworkElement;
Point point = e.GetPosition(element);
Point center = element.Center();
// ***
// *** The rotation angle is the opposite of the angle as
// *** calculated in the euclidean space.
// ***
double rotation = center.GetAngle(point).ToDegrees().ToRotation();
this.Rotation = rotation;
}
private void Rotary_ManipulationStarting(object sender, ManipulationStartingRoutedEventArgs e)
{
FrameworkElement element = sender as FrameworkElement;
e.Pivot = new ManipulationPivot(element.Center(), element.ActualWidth / 2.0);
}