/// <summary>
/// 将制定物体Pin在物理世界
/// </summary>
/// <param name="obj"></param>
/// <param name="position"></param>
/// <returns></returns>
public Handle Pin(IPin obj, Vector2D position)
{
if (!_pins.ContainsKey(obj))
{
_pins[obj] = obj.Pin(this, position);
}
else
{
throw new InvalidOperationException("Can't pin target object which was alreadly pinned.");
}
return(_pins[obj]);
}