思路-绘制空心矩形
拓展UGUI的Graphic类,实现拖拽接口。
开始拖拽时记录鼠标位置，
使用拖拽中的鼠标位置和记录的位置，计算矩形顶点，绘制矩形。
两个三角形合并为一个矩形，作为空心矩形的一条边，四个边合并为空心矩形。
示例-绘制空心矩形
using UnityEngine;
using UnityEngine.EventSystems;
using UnityEngine.UI;public class TestDraw : Graphic, IBeginDragHandler, IDragHandler, IEndDragHandler
{UIVertex[] rectangle1 = new UIVertex[4];UIVertex[] rectangle2 = new UIVertex[4];UIVertex[] rectangle3 = new UIVertex[4];UIVertex[] rectangle4 = new UIVertex[4];[SerializeField] float width = 5f;Vector3 lastPoint;protected override void Awake(){Init(rectangle1);Init(rectangle2);Init(rectangle3);Init(rectangle4);void Init(UIVertex[] uIVertices){var length = uIVertices.Length;for (int i = 0; i < length; i++)uIVertices[i] = new UIVertex();}}protected override void OnPopulateMesh(VertexHelper vh){vh.Clear();vh.AddUIVertexQuad(rectangle1);vh.AddUIVertexQuad(rectangle2);vh.AddUIVertexQuad(rectangle3);vh.AddUIVertexQuad(rectangle4);}public void ClearDraw(){Clear(rectangle1);Clear(rectangle2);Clear(rectangle3);Clear(rectangle4);void Clear(UIVertex[] uIVertices){var length = uIVertices.Length;for (int i = 0; i < length; i++)uIVertices[i].position = Vector3.zero;}SetVerticesDirty();}public void OnBeginDrag(PointerEventData eventData){lastPoint = ScreenPointToLocalPoint(rectTransform, eventData.position);}public void OnDrag(PointerEventData eventData){Vector3 point = ScreenPointToLocalPoint(rectTransform, eventData.position);if (lastPoint.x < point.x && lastPoint.y < point.y)//起点在左下角{//水平方向投影向量 var horizontalNormal = Vector3.Project(point - lastPoint, Vector3.right);SetRectangleVertex(point - horizontalNormal, point, lastPoint + horizontalNormal, lastPoint);SetRectangleColor(color);SetVerticesDirty();}else if (lastPoint.x > point.x && lastPoint.y > point.y)//起点在右上角{var horizontalNormal = Vector3.Project(point - lastPoint, Vector3.right);SetRectangleVertex(lastPoint + horizontalNormal, lastPoint, point - horizontalNormal, point);SetRectangleColor(color);SetVerticesDirty();}else if (lastPoint.x > point.x && lastPoint.y < point.y)//起点在右下角{var horizontalNormal = Vector3.Project(point - lastPoint, Vector3.right);SetRectangleVertex(point, point - horizontalNormal, lastPoint, lastPoint + horizontalNormal);SetRectangleColor(color);SetVerticesDirty();}else if (lastPoint.x < point.x && lastPoint.y > point.y)//起点在左上角{var horizontalNormal = Vector3.Project(point - lastPoint, Vector3.right);SetRectangleVertex(lastPoint, lastPoint + horizontalNormal, point, point - horizontalNormal);SetRectangleColor(color);SetVerticesDirty();}}public void OnEndDrag(PointerEventData eventData){lastPoint = Vector3.zero;}void SetRectangleVertex(Vector3 topLeftUp, Vector3 topRigthUp, Vector3 bottomRightDown, Vector3 bottomLeftDown){//top矩形rectangle1[0].position = topLeftUp;rectangle1[1].position = topRigthUp;rectangle1[2].position = topRigthUp - Vector3.up * width;rectangle1[3].position = topLeftUp - Vector3.up * width;//bottom矩形rectangle2[0].position = bottomRightDown;rectangle2[1].position = bottomLeftDown;rectangle2[2].position = bottomLeftDown + Vector3.up * width;rectangle2[3].position = bottomRightDown + Vector3.up * width;//left矩形rectangle3[0].position = topLeftUp - Vector3.up * width;rectangle3[1].position = topLeftUp - Vector3.up * width + Vector3.right * width;rectangle3[2].position = bottomLeftDown + Vector3.up * width + Vector3.right * width;rectangle3[3].position = bottomLeftDown + Vector3.up * width;//right矩形rectangle4[0].position = topRigthUp - Vector3.up * width;rectangle4[1].position = bottomRightDown + Vector3.up * width;rectangle4[2].position = bottomRightDown + Vector3.up * width - Vector3.right * width;rectangle4[3].position = topRigthUp - Vector3.up * width - Vector3.right * width;}void SetRectangleColor(Color color){var length = rectangle1.Length;for (int i = 0; i < length; i++)rectangle1[i].color = color;length = rectangle2.Length;for (int i = 0; i < length; i++)rectangle2[i].color = color;length = rectangle3.Length;for (int i = 0; i < length; i++)rectangle3[i].color = color;length = rectangle4.Length;for (int i = 0; i < length; i++)rectangle4[i].color = color;}Vector2 ScreenPointToLocalPoint(RectTransform rect, Vector2 mousePoint){Vector2 result = Vector2.zero;switch (canvas.renderMode){case RenderMode.ScreenSpaceOverlay:RectTransformUtility.ScreenPointToLocalPointInRectangle(rect, mousePoint, null, out result);break;case RenderMode.ScreenSpaceCamera:RectTransformUtility.ScreenPointToLocalPointInRectangle(rect, mousePoint, canvas.worldCamera, out result);break;case RenderMode.WorldSpace:RectTransformUtility.ScreenPointToLocalPointInRectangle(rect, mousePoint, canvas.worldCamera, out result);break;}return result;}void Update(){if (Input.GetKeyDown(KeyCode.Space))ClearDraw();}
}
场景结构
TestDraw对象挂载TestDraw脚本，该对象需要CanvsRenderer脚本，大小为屏幕大小
运行结果
上方为矩形网格
运行，拖拽鼠标显示矩形。
按下空格键，清除矩形。
知识点
三角形的绘制需要三个顶点，三角形使用顺时针的顺序进行绘制。
其他几何图形的绘制思路
线：多个矩形组合而成
箭头：一个三角形，一个矩形组合而成。
空心椭圆：获取两个圆上的顶点，按照顺序绘制三角形。
椭圆绘制思路:已知圆心,半径
将圆划分多份，利用三角函数和弧度获取x坐标，y坐标。
将x，y坐标缩放，可得到椭圆上的点。