效果

1.界面设置

2.涉及两个脚本UIDraw.cs和UIDrawLine.cs

UIDraw.cs

using System;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.EventSystems;
using UnityEngine.UI;

public class UIDraw : MonoBehaviour, IPointerEnterHandler, IPointerExitHandler , IPointerDownHandler, IPointerUpHandler
{
    public Color DrawColor = Color.red;
    [Range(3,10)]
    public int DrawWidth = 3;
    public Color BgColor = new Color(0, 0, 0, 0);

    Vector2 lastPosition;
    public UIDrawLine UIDrawLine;

    List<Vector2> Vertics = new List<Vector2>();
    List<List<Vector2>> CachVertics = new List<List<Vector2>>();
    void Awake()
    {
    }
    public RectTransform Drawer;

    void Update()
    {
        if (canDraw)
        {
            Drawer.position = Input.mousePosition;
            DrawPoint(Drawer.anchoredPosition);

#if UNITY_ANDROID||UNITY_IPHONE 

#endif
        }
    }

    public void OnPointerDown(PointerEventData eventData)
    {
        canDraw = true;
    }
    public void OnPointerUp(PointerEventData eventData)
    {
        canDraw = false;
        //添加结尾
        lastPosition = Vector2.zero;
        AddPoint(lastPosition);
        //添加到缓存
        AddToCach();
    }
    bool canDraw;
    public void OnPointerEnter(PointerEventData eventData)
    {

    }
    public void OnPointerExit(PointerEventData eventData)  
    {
        lastPosition = Vector2.zero;
        AddPoint(lastPosition);
    }

    public void DrawPoint(Vector2 point)
    {
        if (lastPosition == Vector2.zero)
        {
            lastPosition = point;
        }

        UpdateColor(lastPosition, point, DrawWidth, DrawColor);
    }

    public void UpdateColor(Vector2 start_point, Vector2 end_point, int width, Color color)
    {
        float distance = Vector2.Distance(start_point, end_point);

        if (distance < 0.1f) return;


        AddPoint(end_point) ;

        lastPosition = end_point;

    }

    void AddPoint(Vector2 v)
    {
        Vertics.Add(v);
        UIDrawLine.Vertices.Add(v);
        UIDrawLine.SetAllDirty();
    }
    private void AddToCach()
    {
        List<Vector2> list = new List<Vector2>();
        list.AddRange(Vertics);
        CachVertics.Add(list);
        Vertics.Clear();
    }
    public void UnDo()
    {
        if (CachVertics.Count == 0) return;

        CachVertics.RemoveAt(CachVertics.Count-1);

        UIDrawLine.Vertices.Clear();

        for (int i = 0; i < CachVertics.Count; i++)
        {
            UIDrawLine.Vertices.AddRange(CachVertics[i]);
        }
        UIDrawLine.SetAllDirty();
    }
    public void Send()
    {
        //UIDrawLine.Vertices;
        Clear();
        //gameObject.SetActive(false);
    }
    internal void Clear()
    {
        UIDrawLine.Vertices.Clear();
        UIDrawLine.SetAllDirty();

        Vertics.Clear();
        CachVertics.Clear();
    }
    void OnDisable() 
    {
        Clear();
    }


}

UIDrawLine.cs

using System;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

[AddComponentMenu("UI/Extensions/UI Line")]
public class UIDrawLine : MaskableGraphic
{
        [SerializeField]
        Texture m_Texture;

        public float thickness = 5;

        public List<Vector2> Vertices=new List<Vector2> ();

        private float size = 0;

        public override Texture mainTexture
        {
            get
            {
                return m_Texture == null ? s_WhiteTexture : m_Texture;
            }
        }
        public Texture texture
        {
            get
            {
                return m_Texture;
            }
            set
            {
                if (m_Texture == value) return;
                m_Texture = value;
                SetVerticesDirty();
                SetMaterialDirty();
            }
        }
        protected override void OnEnable()
        {
            Vertices.Clear();
        }
        void Update()
        {

        }
        protected UIVertex[] SetVbo(Vector2[] vertices, Vector2[] uvs)
        {
            UIVertex[] vbo = new UIVertex[4];
            for (int i = 0; i < vertices.Length; i++)
            {
                var vert = UIVertex.simpleVert;
                vert.color = color;
                vert.position = vertices[i];
                vert.uv0 = uvs[i];
                vbo[i] = vert;
            }
            return vbo;
        }
        //生成方格的四个点
        protected override void OnPopulateMesh(VertexHelper vh)
        {
            vh.Clear();

            Vector2 uv0 = new Vector2(0, 1);
            Vector2 uv1 = new Vector2(1, 1);
            Vector2 uv2 = new Vector2(1, 0);
            Vector2 uv3 = new Vector2(0, 0);

            Vector2 lastPos0 = Vector2.zero;
            Vector2 lastPos1 = Vector2.zero;
            Vector2 lastPos2 = Vector2.zero;
            Vector2 lastPos3 = Vector2.zero;

        for (int i = 1; i < Vertices.Count; i++)
        {
            //当有0是说明是结束点，开始下个画线
            if (Vertices[i - 1]==Vector2.zero || Vertices[i]== Vector2.zero)
            {
                lastPos0 = Vector2.zero;
                continue;
            }
  
            Vector2 v = Rotate90(Vertices[i] - Vertices[i - 1]).normalized;

            Vector2 pos0 = Vertices[i - 1]- v * thickness * 0.5f;
            Vector2 pos1 = Vertices[i]- v * thickness * 0.5f;

            Vector2 pos2 = pos1 + v * thickness;
            Vector2 pos3 = pos0 + v * thickness;

            vh.AddUIVertexQuad(SetVbo(new[] { pos0, pos1, pos2, pos3 }, new[] { uv0, uv1, uv2, uv3 }));

            //角
            if (lastPos0!=Vector2.zero)
            {
                Vector2 p0 = pos0;
                Vector2 p1 = lastPos1;
                Vector2 p2 = pos3;
                Vector2 p3 = lastPos2;
                vh.AddUIVertexQuad(SetVbo(new[] { p0, p1, p2, p3 }, new[] { uv0, uv1, uv2, uv3 }));
            }

            lastPos0 = pos0;
            lastPos1 = pos1;
            lastPos2 = pos2;
            lastPos3 = pos3;
        }
    }
    private Vector2 Rotate90(Vector2 p)
    {
        return new Vector2(p.y, -p.x);
    }

}