关键代码：

在Update中调用：

void Turning ()
        {
            // Create a ray from the mouse cursor on screen in the direction of the camera.
            Ray camRay = Camera.main.ScreenPointToRay (Input.mousePosition);

            // Create a RaycastHit variable to store information about what was hit by the ray.
            RaycastHit floorHit;

            // Perform the raycast and if it hits something on the floor layer...
            if(Physics.Raycast (camRay, out floorHit, camRayLength, floorMask))
            {
                // Create a vector from the player to the point on the floor the raycast from the mouse hit.
                Vector3 playerToMouse = floorHit.point - transform.position;

                // Ensure the vector is entirely along the floor plane.
                playerToMouse.y = 0f;

                // Create a quaternion (rotation) based on looking down the vector from the player to the mouse.
                Quaternion newRotatation = Quaternion.LookRotation (playerToMouse);

                // Set the player's rotation to this new rotation.
                playerRigidbody.MoveRotation (newRotatation);
            }

涉及知识点：

1. Camera.ScreenPointToRay 屏幕位置转射线

function ScreenPointToRay (position : Vector3) : Ray

返回一条射线从摄像机通过一个屏幕点。

产生的射线是在世界空间中，从相机的近裁剪面开始并穿过屏幕position(x,y)像素坐标（position.z被忽略）。

屏幕空间以像素定义。屏幕的左下为(0,0)；右上是(pixelWidth,pixelHeight)。

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Update() {
        Ray ray = camera.ScreenPointToRay(new Vector3(200, 200, 0));
        Debug.DrawRay(ray.origin, ray.direction * 10, Color.yellow);
    }
}

2.Physics.Raycast

static function Raycast (ray : Ray, out hitInfo : RaycastHit, distance : float = Mathf.Infinity, layerMask : int = kDefaultRaycastLayers) : bool

Parameters参数

• ray

  The starting point and direction of the ray.
  射线的起点和方向

• hitInfo

  If true is returned, hitInfo will contain more information about where the collider was hit (See Also: RaycastHit).
  如果返回true，hitInfo将包含碰到器碰撞的更多信息。

• distance

  The length of the ray
  射线的长度

• layerMask

  A Layer mask that is used to selectively ignore colliders when casting a ray.
  只选定Layermask层内的碰撞器，其它层内碰撞器忽略。