平行于视口平面的3D物体拖拽

using UnityEngine;
using System.Collections;
using UnityEngine.EventSystems;

public class Drag3DBase : MonoBehaviour,IDragHandler,IBeginDragHandler,IEndDragHandler {

    bool drag = false;
    float distance;
    Vector3 originPointPos;
    Vector3 originObjPos;

    //垂直于拖拽平面的长度
    float VerticalLineLength;
    public void OnBeginDrag(PointerEventData eventData)
    {
        if (eventData.rawPointerPress.transform == this.transform)
        {
            drag = true;
            distance = eventData.pointerPressRaycast.distance;

            VerticalLineLength = GetVerticalLine(eventData.position, distance);

            originPointPos = GetWorldPos(eventData.position, distance);
            originObjPos = this.gameObject.transform.position;
            OnBeginDragHandle(eventData);
        }
    }
    public void OnDrag(PointerEventData eventData)
    {
        if (!drag)
            return;

        Vector3 tempPos = GetWorldPos(eventData.position, GetDragDistance(eventData.position));
        Vector3 deltaPos = tempPos - originPointPos;
        this.transform.position = (originObjPos + deltaPos);
        OnDragHandle(eventData);
    }
    public void OnEndDrag(PointerEventData eventData)
    {
        if (!drag)
            return;

        drag = false;
        OnEndDragHandle(eventData);
    }
    protected Vector3 GetWorldPos(Vector2 mousePos, float distance)
    {
        Ray ray = Camera.main.ScreenPointToRay(mousePos);
        return ray.GetPoint(distance);
    }
    /// 

    /// 获取拖拽平面垂线的长度
    /// 
    /// 
    /// 
    /// 
    float GetVerticalLine(Vector2 mousePos,float distance)
    {
        Ray objRay = Camera.main.ScreenPointToRay(mousePos);
        Ray vertical = Camera.main.ScreenPointToRay(new Vector2(Screen.width/2,Screen.height/2));

        Vector3 objDir = objRay.direction;
        Vector3 verticalLineDir = vertical.direction;

        float angle = Vector3.Angle(objDir, verticalLineDir);

        return distance * Mathf.Cos(angle * Mathf.Deg2Rad);
    }
    /// 

    /// 获取拖拽物体的射线长度
    /// 
    /// 
    /// 
    float GetDragDistance(Vector2 mousePos)
    {
        Ray vertical = Camera.main.ScreenPointToRay(new Vector2(Screen.width / 2, Screen.height / 2));
        Ray objRay = Camera.main.ScreenPointToRay(mousePos);

        Vector3 objDir = objRay.direction;
        Vector3 verticalLineDir = vertical.direction;

        float angle = Vector3.Angle(objDir, verticalLineDir);

        return VerticalLineLength / Mathf.Cos(angle * Mathf.Deg2Rad);
    }
    protected virtual void OnBeginDragHandle(PointerEventData eventData)
    {

    }
    protected virtual void OnDragHandle(PointerEventData eventData)
    {

    }
    protected virtual void OnEndDragHandle(PointerEventData eventData)
    {

    }
}