心急的小伙伴可以直接跳到文章末尾查看最终代码，如果有问题再来看下思路。
首先我们需要确定实现思路。我想到的方案是将玩家和敌人的世界坐标转换为UI坐标，然后求玩家和敌人坐标的线段与Canvas边界的交点即为箭头坐标。
下面是求出交点的代码。
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
public class FindTest : MonoBehaviour
{
public Canvas canvas;
public Camera cam;
public Image find; //箭头UI
public GameObject player; //玩家
public GameObject target; //跟踪目标
List<Vector3> points = new List<Vector3>(); //canvas边界点
void Start()
{
//存储canvas边界点
points.Add(new Vector3(-canvas.GetComponent<RectTransform>().rect.width / 2f, -canvas.GetComponent<RectTransform>().rect.height / 2f, 0));
points.Add(new Vector3(canvas.GetComponent<RectTransform>().rect.width / 2f, -canvas.GetComponent<RectTransform>().rect.height / 2f, 0));
points.Add(new Vector3(canvas.GetComponent<RectTransform>().rect.width / 2f, canvas.GetComponent<RectTransform>().rect.height / 2f, 0));
points.Add(new Vector3(-canvas.GetComponent<RectTransform>().rect.width / 2f, canvas.GetComponent<RectTransform>().rect.height / 2f, 0));
}
void Update()
{
Vector3 pos = Vector3.zero;
Vector3 pos1 = Camera.main.WorldToScreenPoint(target.transform.position); //目标屏幕坐标
Vector3 pos2 = Camera.main.WorldToScreenPoint(player.transform.position); //玩家屏幕坐标
Vector2 worldPoint1;
Vector2 worldPoint2;
//求出目标与玩家的UI坐标
RectTransformUtility.ScreenPointToLocalPointInRectangle(canvas.transform as RectTransform, pos1, cam, out worldPoint1);
RectTransformUtility.ScreenPointToLocalPointInRectangle(canvas.transform as RectTransform, pos2, cam, out worldPoint2);
//玩家与目标UI坐标连线与Canvas边界连写交点即为箭头位置
for (int i = 0; i < points.Count; i++)
{
//这里的if是为了让最后一条边界为 ps[Count - 1]与ps[0]连线
if (i < points.Count - 1)
{
if (SegmentsInterPoint(worldPoint1, worldPoint2, points[i + 1], points[i], ref pos))
{
find.rectTransform.anchoredPosition = pos;
break;
}
}
else
{
if (SegmentsInterPoint(worldPoint1, worldPoint2, points[i], points[0], ref pos))
{
find.rectTransform.anchoredPosition = pos;
break;
}
}
}
}
public static float Cross(Vector3 a, Vector3 b)
{
return a.x * b.y - b.x * a.y;
}
//求交点
public static bool SegmentsInterPoint(Vector3 a, Vector3 b, Vector3 c, Vector3 d, ref Vector3 IntrPos)
{
//v1×v2=x1y2-y1x2
//以线段ab为准，是否c，d在同一侧
Vector3 ab = b - a;
Vector3 ac = c - a;
float abXac = Cross(ab, ac);
Vector3 ad = d - a;
float abXad = Cross(ab, ad);
if (abXac * abXad >= 0)
{
return false;
}
//以线段cd为准，是否ab在同一侧
Vector3 cd = d - c;
Vector3 ca = a - c;
Vector3 cb = b - c;
float cdXca = Cross(cd, ca);
float cdXcb = Cross(cd, cb);
if (cdXca * cdXcb >= 0)
{
return false;
}
//计算交点坐标
float t = Cross(a - c, d - c) / Cross(d - c, b - a);
float dx = t * (b.x - a.x);
float dy = t * (b.y - a.y);
IntrPos = new Vector3() { x = a.x + dx, y = a.y + dy };
return true;
}
}
效果
核心的部分写完了，但是效果差强人意。还缺少箭头朝向敌人的功能，箭头也没有完整的显示出来，而且如果敌人进入视野内箭头应该消失。
1.箭头朝向敌人
首先是箭头朝向敌人，我们可以加段转向的代码。
因为箭头默认朝上，所以第三个参数传Vector3.up。
//箭头朝向目标
UILookAt(find.transform, worldPoint1 - worldPoint2, Vector3.up);
//参数分别为：1.UI的Transform 2.朝向向量 3.起始向量
public void UILookAt(Transform transform, Vector3 dir, Vector3 lookAxis)
{
Quaternion q = Quaternion.identity;
q.SetFromToRotation(lookAxis, dir);
transform.rotation = q;
}
2.箭头完整显示
可以在代码中加一个箭头位置偏移。还有种更简单的方法就是修改UI的轴心点。如下图。
3.敌人进入视野内箭头消失
最开始打算使用OnBecameVisible和OnBecameInVisible来判断目标是否在视野内，但是这种方法不够灵活。于是考虑用另一种方法，就是通过判断玩家和目标UI坐标的线段与Canvas边界有没有交点来判断物体是否在视野内，并且给目标点设置偏移量让箭头可以提前或者延迟显隐。
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
public class FindTest : MonoBehaviour
{
public Canvas canvas;
public Camera cam;
public Image find; //箭头UI
public GameObject player; //玩家
public GameObject target; //跟踪目标
List<Vector3> points = new List<Vector3>(); //canvas边界点
bool isHaveIntersection = false; //判断有没有交点来显隐箭头
public float targetOffset; //设置目标点偏移，用来控制箭头提前或者延后显隐
void Start()
{
//存储canvas边界点
points.Add(new Vector3(-canvas.GetComponent<RectTransform>().rect.width / 2f, -canvas.GetComponent<RectTransform>().rect.height / 2f, 0));
points.Add(new Vector3(canvas.GetComponent<RectTransform>().rect.width / 2f, -canvas.GetComponent<RectTransform>().rect.height / 2f, 0));
points.Add(new Vector3(canvas.GetComponent<RectTransform>().rect.width / 2f, canvas.GetComponent<RectTransform>().rect.height / 2f, 0));
points.Add(new Vector3(-canvas.GetComponent<RectTransform>().rect.width / 2f, canvas.GetComponent<RectTransform>().rect.height / 2f, 0));
}
void Update()
{
Vector3 pos = Vector3.zero;
Vector3 pos1 = Camera.main.WorldToScreenPoint(target.transform.position - targetOffset * (target.transform.position - player.transform.position).normalized); //目标屏幕坐标
Vector3 pos2 = Camera.main.WorldToScreenPoint(player.transform.position); //玩家屏幕坐标
Vector2 worldPoint1;
Vector2 worldPoint2;
//求出目标与玩家的UI坐标
RectTransformUtility.ScreenPointToLocalPointInRectangle(canvas.transform as RectTransform, pos1, cam, out worldPoint1);
RectTransformUtility.ScreenPointToLocalPointInRectangle(canvas.transform as RectTransform, pos2, cam, out worldPoint2);
isHaveIntersection = false; //默认没交点
//玩家与目标UI坐标连线与Canvas边界连写交点即为箭头位置
for (int i = 0; i < points.Count; i++)
{
//这里的if是为了让最后一条边界为 ps[Count - 1]与ps[0]连线
if (i < points.Count - 1)
{
if (SegmentsInterPoint(worldPoint1, worldPoint2, points[i + 1], points[i], ref pos))
{
find.rectTransform.anchoredPosition = pos;
isHaveIntersection = true;
break;
}
}
else
{
if (SegmentsInterPoint(worldPoint1, worldPoint2, points[i], points[0], ref pos))
{
find.rectTransform.anchoredPosition = pos;
isHaveIntersection = true;
break;
}
}
}
//判断有没有交点来显隐箭头
if (isHaveIntersection)
{
find.gameObject.SetActive(true);
//箭头朝向目标
UILookAt(find.transform, worldPoint1 - worldPoint2, Vector3.up);
}
else
{
find.gameObject.SetActive(false);
}
}
//参数分别为：1.UI的Transform 2.朝向向量 3.起始向量
public void UILookAt(Transform transform, Vector3 dir, Vector3 lookAxis)
{
Quaternion q = Quaternion.identity;
q.SetFromToRotation(lookAxis, dir);
transform.rotation = q;
}
public static float Cross(Vector3 a, Vector3 b)
{
return a.x * b.y - b.x * a.y;
}
//求交点
public static bool SegmentsInterPoint(Vector3 a, Vector3 b, Vector3 c, Vector3 d, ref Vector3 IntrPos)
{
//v1×v2=x1y2-y1x2
//以线段ab为准，是否c，d在同一侧
Vector3 ab = b - a;
Vector3 ac = c - a;
float abXac = Cross(ab, ac);
Vector3 ad = d - a;
float abXad = Cross(ab, ad);
if (abXac * abXad >= 0)
{
return false;
}
//以线段cd为准，是否ab在同一侧
Vector3 cd = d - c;
Vector3 ca = a - c;
Vector3 cb = b - c;
float cdXca = Cross(cd, ca);
float cdXcb = Cross(cd, cb);
if (cdXca * cdXcb >= 0)
{
return false;
}
//计算交点坐标
float t = Cross(a - c, d - c) / Cross(d - c, b - a);
float dx = t * (b.x - a.x);
float dy = t * (b.y - a.y);
IntrPos = new Vector3() { x = a.x + dx, y = a.y + dy };
return true;
}
}
PS：如果觉得目标移动的快箭头抖动可以加个过度效果，下面是我用DOTWEEN实现的，可以参考下。
for (int i = 0; i < points.Count; i++)
{
if (i < points.Count - 1)
{
if (SegmentsInterPoint(worldPoint1, worldPoint2, points[i + 1], points[i], ref pos))
{
//判断距离是为了让箭头移动距离过大的时候瞬移过去
if (Vector3.Distance(arrow.transform.localPosition, new Vector3(pos.x, pos.y, 0)) < 100)
{
//过渡效果
arrow.transform.DOLocalMove(new Vector3(pos.x, pos.y, 0),0.1f).SetEase(Ease.Linear);
}
else
{
arrow.transform.localPosition = new Vector3(pos.x, pos.y, 0);
}
isHaveIntersection = true;
break;
}
}
else
{
if (SegmentsInterPoint(worldPoint1, worldPoint2, points[i], points[0], ref pos))
{
if (Vector3.Distance(arrow.transform.localPosition, new Vector3(pos.x, pos.y, 0)) < 100)
{
arrow.transform.DOLocalMove(new Vector3(pos.x, pos.y, 0), 0.1f).SetEase(Ease.Linear);
}
else
{
arrow.transform.localPosition = new Vector3(pos.x, pos.y, 0);
}
isHaveIntersection = true;
break;
}
}
}