unity中Vector3自帶了一個Vector3.Angle的函式，用於計算兩個vector3向量的夾角，文件中描述的：
The angle returned is always the non reflex angle between the two vectors – ie the smaller of the two possible angles between them and never greater than 180 
參考：https://www.yxkfw.com/thread-57909-1-1.html
degrees.
這個角度返回的值是不大於180的度數，會造成逆時針120與240，返回的值其實是一樣的。

Snippet

/// <summary>
/// Determine the signed angle between two vectors, with normal 'n'
/// as the rotation axis.
/// </summary>
public static float AngleSigned(Vector3 v1, Vector3 v2, Vector3 n)
{
  return Mathf.Atan2(
      Vector3.Dot(n, Vector3.Cross(v1, v2)),
      Vector3.Dot(v1, v2)) * Mathf.Rad2Deg;
}