首先创建一个边界类

[Serializable]
public class Boundary
{
    public float MaxX;
    public float MinX;
    public float MaxY;
    public float MinY;
}

解释
Mathf.Clamp 限制
static function Clamp (value : float, min : float, max : float) : float
Description描述
Clamps a value between a minimum float and maximum float value.
限制value的值在min和max之间， 如果value小于min，返回min。 如果value大于max，返回max，否则返回value

        transform.position = new Vector3(Mathf.Clamp(transform.position.x, boundary.MinX, boundary.MaxX),
                                          Mathf.Clamp(transform.position.y, boundary.MinY, boundary.MaxY),
                                        0);