2D Vertical scrolling infinite background

I've faced something like that and I found two ways to handle it. You can use an gameObject like a barrier to collide with BG elements and reposition it again or you can use the camera view to reposition elements when they are outside the viewport. The method I will use here is about the barrier collision.

First or all you have to put your BG objects inside a parent BG object. I have accomplished it on my hierarchy window like this:

01 - Background

• bg01

• bg02

• bg03

• bg04

• bg05

You can put how many objects you want to, I've used 5, maybe you should use more or less, its up to you.

Second you have to make all elements scrolling. Its the easy part, just make a script to move the parent BG object and all objects inside will move as well.

Attach your move script or my own snippet on your parent BG:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class MoveBackground : MonoBehaviour {
 
     public float speed = 2f;
     public float horizontalDirection = -1; //move from right to left
   
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
         //move the root BG from right to left
         transform.Translate(new Vector3(speed * horizontalDirection * Time.deltaTime, 0, 0));
     }
 
 }

You should change the speed variable to fit your project.

Third part is the funniest one. When the first BG object pass though our invisible barrier, the OnTriggerExit2D will raise up and now the code to change the BG position to make the illusion of an infinite scrolling background will raise up. So the first BG will pass to the last position, the second BG will be the first and so on. We can accomplish it using a game object to trigger the collision (I used a sprite). Make sure this object is the same Z position as the BG objects and it must be positioned outside the camera viewport to the left side.

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 
 
 public class BarrierCheck : MonoBehaviour
 {
 
     public Transform rootBG;
     private List<Transform> bgList;
 
     void Awake()
     {
         bgList = new List<Transform>();
 
         //get all BGs from the parent BG and put those on the list
         for (var i = 0; i < rootBG.childCount; i++)
         {
             bgList.Add(rootBG.GetChild(i));
         }
 
         //ordem them all (left to right)
         bgList = bgList.OrderBy(t => t.position.x).ToList();
     }
 
     // Use this for initialization
     void Start()
     {
 
     }
 
     // Update is called once per frame
     void Update()
     {
 
     }
 
     void OnTriggerExit2D(Collider2D c)
     {
         //when the first BG on the list exit the barrier trigger, this snippet is called
         Transform firstBG = bgList.FirstOrDefault();
         Transform lastBG = bgList.LastOrDefault();
 
         firstBG.Translate(
             new Vector3(lastBG.transform.position.x + firstBG.renderer.bounds.size.x, 0, firstBG.transform.position.z));
 
         bgList.Remove(firstBG);
         bgList.Add(firstBG);
     }
 
 }

Now we have to attach the parent BG on rootBG to get the children elements. OnTriggerExit2D will be called when some BG object exit the barrier bounds. Make sure all BG objects and barrir object have a Box Collider 2D and Rigidbody 2D with Gravity Scale property to 0.

If you make this it will work.

Sorry about my english I hope it's clear enough.