using UnityEngine;
using System.Collections;
using System.Collections.Generic;
 
public class DrawLine : MonoBehaviour
{
  //线段预制
  [Tooltip("Line renderer used for the line drawing.")]
  public LineRenderer linePrefab;
 
  //线段相关保存和下标
  private List linesDrawn = new List();
  private LineRenderer currentLine;
  private int lineVertexIndex = 2;
 
  void Update()
  {
    //删除最近一笔
    if (Input.GetKeyDown(KeyCode.U))
    {
      // U-key means Undo
      DeleteLastLine();
    }
 
    if (currentLine == null &&
      Input.GetMouseButton(0))
    {
      // 鼠标按下，开始画线
      currentLine = Instantiate(linePrefab).GetComponent();
      currentLine.name = "Line" + linesDrawn.Count;
      currentLine.transform.parent = transform;
 
      Vector3 cursorPos = Input.mousePosition;
      cursorPos.z = 0f;
 
      //将鼠标按下的屏幕坐标转换成世界坐标
      Vector3 cursorSpacePos = Camera.main.ScreenToWorldPoint(cursorPos);
      cursorSpacePos.z = 0f;
      currentLine.SetPosition(0, cursorSpacePos);
      currentLine.SetPosition(1, cursorSpacePos);
 
      lineVertexIndex = 2;
      linesDrawn.Add(currentLine.gameObject);
 
      StartCoroutine(DrawLines());
    }
 
    if (currentLine != null &&
      Input.GetMouseButtonUp(0))
    {
      // 鼠标左键抬起结束当前笔画
      currentLine = null;
    }
  }
 
  //撤销最后一笔
  public void DeleteLastLine()
  {
    if (linesDrawn.Count > 0)
    {
      GameObject goLastLine = linesDrawn[linesDrawn.Count - 1];
      linesDrawn.RemoveAt(linesDrawn.Count - 1);
      Destroy(goLastLine);
    }
  }
 
  //持续画线
  IEnumerator DrawLines()
  {
    while (Input.GetMouseButton(0))
    {
      yield return new WaitForEndOfFrame();
 
      if (currentLine != null)
      {
        lineVertexIndex++;
        currentLine.SetVertexCount(lineVertexIndex);
 
        Vector3 cursorPos = Input.mousePosition;
        cursorPos.z = 0f;
 
        Vector3 cursorSpacePos = Camera.main.ScreenToWorldPoint(cursorPos);
        cursorSpacePos.z = 0f;
        currentLine.SetPosition(lineVertexIndex - 1, cursorSpacePos);
      }
    }
  }
}