public Animation(Action <double> callback, double start = 0.0f, double end = 1.0f, Easing easing = null, Action finished = null)
{
_children = new List <Animation>();
_easing = easing ?? Easing.Linear;
_finished = finished;
Func <double, double> transform = AnimationExtensions.Interpolate(start, end);
_step = f => callback(transform(f));
}