private void CreateFood()
{
Point location = new Point(random.Next(arenaSize.Width) / 10 * 10, random.Next(arenaSize.Height) / 10 * 10);
Food newfood = new Food(location, this);
Foods.Add(newfood);
// 场上不止一种食物？
if (Foods.Count > 1)
{
Foods.RemoveAt(0);
}
}