一：在世界空间下绘制

——当每次鼠标按下时，创建一个空物体并添加LineRenderer组件，设置当前LineRenderer的各种参数
注意需要设置LineRenderer的材质Shader(必须设置可以接收颜色的Shader，比如Sprites/Default或Legacy Shaders/Particles/Alpha Blended Premultiply)，否则修改颜色不起作用

——当鼠标按下中，将当前鼠标按下位置的每个点添加到LineRenderer的Positions数组中(LineRenderer对Positions数组中位置依次连线绘制)

——以上代码已经可以实现绘制线段
但是有一个可以优化的地方，现在当鼠标每次按下中，鼠标就算不动但仍然会一直向Positions数组中添加位置，造成不必要的性能消耗，所以我们可以添加一个判断，当鼠标移动了一定的距离后才向Positions数组中添加位置进行绘制线段

——完整代码(挂载到任意游戏物体身上)

using UnityEngine;

public class Draw : MonoBehaviour
{
    private LineRenderer curLine;//当前实例化出来的线

    private int posIndex;//当前线的Positions数组下标

    private Vector2 mouseEndPos;//鼠标结束的位置

    //线的参数
    public Color startColor = Color.black;//线开始的颜色
    public Color endColor = Color.black;//线结束的颜色
    public float width = 0.1f;//线宽度
    public int vertices = 90;//顶点数

    /// <summary>
    /// 初始化线
    /// </summary>
    private void InitLine()
    {
        posIndex = 0;

        curLine.startColor = startColor;
        curLine.endColor = endColor;
        curLine.startWidth = width;
        curLine.endWidth = width;
        curLine.numCapVertices = vertices;
        curLine.numCornerVertices = vertices;
    }

    private void Update()
    {
        if (Input.GetMouseButtonDown(0))
        {
            curLine = new GameObject("line").AddComponent<LineRenderer>();
            curLine.material = new Material(Shader.Find("Sprites/Default"));
            InitLine();
        }

        if (Input.GetMouseButton(0))
        {
            Vector3 mouseStartPos = GetMousePosInWorldSpace();
            //优化性能
            if (Vector2.Distance(mouseStartPos, mouseEndPos) >= 0.1f)
            {
                AddPositions(mouseStartPos);
            }
        }
    }

    /// <summary>
    /// 将线段添加到当前线的Positions数组中
    /// </summary>
    private void AddPositions(Vector2 pos)
    {
        curLine.positionCount = ++posIndex;

        curLine.SetPosition(posIndex - 1, pos);

        mouseEndPos = pos;
    }

    /// <summary>
    /// 得到鼠标在世界空间下的坐标
    /// </summary>
    /// <returns></returns>
    private Vector2 GetMousePosInWorldSpace()
    {
        Vector3 worldPos = Camera.main.ScreenToWorldPoint(
            new Vector3(Input.mousePosition.x, Input.mousePosition.y, -Camera.main.transform.position.z));
        return worldPos;
    }
}

二：在UI上绘制(可以指定画布区域绘制)

——将Canvas的渲染模式改为Screen Space-Camera并指定渲染的相机

——通过UGUI的事件系统进行绘制

——完整代码(挂载到可以绘制的画布物体上，不是Canvas根物体)

using UnityEngine;
using UnityEngine.EventSystems;

public class Draw : MonoBehaviour, IPointerEnterHandler, IPointerExitHandler, IPointerDownHandler, IPointerUpHandler
{
    private LineRenderer curLine;//当前实例化出来的线

    private int posIndex;//当前线的Positions数组下标

    private Vector2 mouseEndPos;//鼠标结束的位置

    //线的参数
    public Color startColor = Color.black;//线开始的颜色
    public Color endColor = Color.black;//线结束的颜色
    public float width = 0.1f;//线宽度
    public int vertices = 90;//顶点数

    private bool withinCanvas;//是否在画布里

    private void Update()
    {
        if (curLine != null && withinCanvas)
        {
            Vector3 mouseStartPos = GetMousePosInWorldSpace();
            if (Vector2.Distance(mouseStartPos, mouseEndPos) >= 0.1f)
            {
                AddPositions(mouseStartPos);
            }
        }
    }

    /// <summary>
    /// 初始化线
    /// </summary>
    private void InitLine()
    {
        posIndex = 0;

        curLine.startColor = startColor;
        curLine.endColor = endColor;
        curLine.startWidth = width;
        curLine.endWidth = width;
        curLine.numCapVertices = vertices;
        curLine.numCornerVertices = vertices;
    }

    /// <summary>
    /// 将线段添加到当前线的Positions数组中
    /// </summary>
    private void AddPositions(Vector2 pos)
    {
        curLine.positionCount = ++posIndex;

        curLine.SetPosition(posIndex - 1, pos);

        mouseEndPos = pos;
    }

    /// <summary>
    /// 得到鼠标在世界空间下的坐标
    /// </summary>
    /// <returns></returns>
    private Vector2 GetMousePosInWorldSpace()
    {
        Vector3 worldPos = Camera.main.ScreenToWorldPoint(
            new Vector3(Input.mousePosition.x, Input.mousePosition.y, -Camera.main.transform.position.z));
        return worldPos;
    }

    public void OnPointerEnter(PointerEventData eventData)
    {
        withinCanvas = true;
    }

    public void OnPointerExit(PointerEventData eventData)
    {
        withinCanvas = false;
    }

    public void OnPointerDown(PointerEventData eventData)
    {
        curLine = new GameObject("line").AddComponent<LineRenderer>();
        curLine.material = new Material(Shader.Find("Sprites/Default"));
        InitLine();
    }

    public void OnPointerUp(PointerEventData eventData)
    {
        curLine = null;
    }
}