float rotationAngle;
const float circle = MathHelper.Pi * 2;
const float speed = 4;
protected override void Update(GameTime gameTime)
{
float elapsed = (float)gameTime.ElapsedGameTime.TotalSeconds;
Window.Title = elapsed.ToString();
rotationAngle += elapsed * speed;
rotationAngle = rotationAngle % circle; //一周毎に値にリセットがかかる動作になる
base.Update(gameTime);
}