public override void Update(double x, KeyFrameAnimationState <TValue> state)
 {
     state.Target.SetValue(state.TargetProperty, GetInterpolatedValue(x, state.PreviousKeyFrameValue));
 }
 public override void Update(double x, KeyFrameAnimationState <TValue> state)
 {
     state.Target.SetValue(state.TargetProperty, Value);
 }