Unity (Game Engine) Line Drawing with mouse using Line Renderer. Useful for painting, drawing 2d physics lines.

DrawLine.cs

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

public class DrawLine : MonoBehaviour
{

	[SerializeField]
	protected LineRenderer m_LineRenderer;
	[SerializeField]
	protected Camera m_Camera;
	protected List<Vector3> m_Points;

	public virtual LineRenderer lineRenderer
	{
		get
		{
			return m_LineRenderer;
		}
	}

	public virtual new Camera camera
	{
		get
		{
			return m_Camera;
		}
	}

	public virtual List<Vector3> points
	{
		get
		{
			return m_Points;
		}
	}

	protected virtual void Awake ()
	{
		if ( m_LineRenderer == null )
		{
			Debug.LogWarning ( "DrawLine: Line Renderer not assigned, Adding and Using default Line Renderer." );
			CreateDefaultLineRenderer ();
		}
		if ( m_Camera == null )
		{
			Debug.LogWarning ( "DrawLine: Camera not assigned, Using Main Camera or Creating Camera if main not exists." );
			CreateDefaultCamera ();
		}
		m_Points = new List<Vector3> ();
	}

	protected virtual void Update ()
	{
		if ( Input.GetMouseButtonDown ( 0 ) )
		{
			Reset ();
		}
		if ( Input.GetMouseButton ( 0 ) )
		{
			Vector3 mousePosition = m_Camera.ScreenToWorldPoint ( Input.mousePosition );
			mousePosition.z = m_LineRenderer.transform.position.z;
			if ( !m_Points.Contains ( mousePosition ) )
			{
				m_Points.Add ( mousePosition );
				m_LineRenderer.positionCount = m_Points.Count;
				m_LineRenderer.SetPosition ( m_LineRenderer.positionCount - 1, mousePosition );
			}
		}
	}

	protected virtual void Reset ()
	{
		if ( m_LineRenderer != null )
		{
			m_LineRenderer.positionCount = 0;
		}
		if ( m_Points != null )
		{
			m_Points.Clear ();
		}
	}

	protected virtual void CreateDefaultLineRenderer ()
	{
		m_LineRenderer = gameObject.AddComponent<LineRenderer> ();
		m_LineRenderer.positionCount = 0;
		m_LineRenderer.material = new Material ( Shader.Find ( "Particles/Additive" ) );
		m_LineRenderer.startColor = Color.white;
		m_LineRenderer.endColor = Color.white;
		m_LineRenderer.startWidth = 0.3f;
		m_LineRenderer.endWidth = 0.3f;
		m_LineRenderer.useWorldSpace = true;
	}

	protected virtual void CreateDefaultCamera ()
	{
		m_Camera = Camera.main;
		if ( m_Camera == null )
		{
			m_Camera = gameObject.AddComponent<Camera> ();
		}
		m_Camera.orthographic = true;
	}

}