Vector2 touchPos = Camera.main.ScreenToWorldPoint(Input.touches[0].position);

public GameObject linePrefab;
private Line activeLine;

private void Update()
{
    if (Input.touchCount > 0)
    {
        if (Input.GetTouch(0).phase == TouchPhase.Began)
        {
            GameObject lineGO = Instantiate(linePrefab);
            activeLine = lineGO.GetComponent<Line>();
        }

        if (Input.GetTouch(0).phase == TouchPhase.Ended)
        {
            activeLine = null;
        }
    }

    if (activeLine != null)
    {
        Vector2 touchPos = Camera.main.ScreenToWorldPoint(Input.touches[0].position);
        activeLine.UpdateLine(touchPos);
    }
}