jump-out-of-the-world/Assets/Scripts/BackgroundMovementController.cs

/*
    zlib License
    
    跳出一方天地 (C) 2022-2023 Lucas
    
    This software is provided 'as-is', without any express or implied
    warranty.  In no event will the authors be held liable for any damages
    arising from the use of this software.

    Permission is granted to anyone to use this software for any purpose,
    including commercial applications, and to alter it and redistribute it
    freely, subject to the following restrictions:

    1.The origin of this software must not be misrepresented; you must not
    claim that you wrote the original software. If you use this software
    in a product, an acknowledgment in the product documentation would be
    appreciated but is not required.
    2. Altered source versions must be plainly marked as such, and must not be
    misrepresented as being the original software.
    3. This notice may not be removed or altered from any source distribution.
*/
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class BackgroundMovementController : MonoBehaviour
{
    public GameObject backgroundObject;
    private GameObject backgroundObject1;
    private GameObject backgroundObject2;
    // Update: Triple moving background -> boundless transform
    private GameObject backgroundObject3;
    private float distance;
    private Rigidbody2D brg2D1;
    private Rigidbody2D brg2D2;
    private Rigidbody2D brg2D3;
    public float speed;
    private float leftBound;
    // Start is called before the first frame update
    void Start()
    {
        backgroundObject1 = Instantiate(backgroundObject);
        backgroundObject2 = Instantiate(backgroundObject);
        backgroundObject3 = Instantiate(backgroundObject);
        
        brg2D1 = backgroundObject1.GetComponent<Rigidbody2D>();
        brg2D2 = backgroundObject2.GetComponent<Rigidbody2D>();
        brg2D3 = backgroundObject3.GetComponent<Rigidbody2D>();
        
        Vector2 speedVector = new(speed, 0);
        brg2D1.velocity = speedVector;
        brg2D2.velocity = speedVector;
        brg2D3.velocity = speedVector;
        
        Vector2 topLeft = Camera.main!.ViewportToWorldPoint(new Vector2(0, 1));
        leftBound = topLeft.x;

        var sRenderer = backgroundObject1.GetComponent<SpriteRenderer>();
        distance = sRenderer.bounds.extents.x * 2;
        var position = backgroundObject2.transform.position;
        position = new Vector3(position.x + distance,
            position.y, position.z);
        backgroundObject2.transform.position = position;
        position = new Vector3(position.x + distance,
            position.y, position.z);
        backgroundObject3.transform.position = position;
    }

    // Update is called once per frame
    void Update()
    {

        if (brg2D1.transform.position.x <= leftBound)
        {
            var position = brg2D3.transform.position;
            Vector3 newPosition = new(position.x + distance,
                position.y,
                position.z);
            brg2D1.transform.position = newPosition;
        }
        else if (brg2D2.transform.position.x <= leftBound)
        {
            var position = brg2D1.transform.position;
            Vector3 newPosition = new(position.x + distance,
                position.y,
                position.z);
            brg2D2.transform.position = newPosition;
        }
        else if (brg2D3.transform.position.x <= leftBound)
        {
            var position = brg2D2.transform.position;
            Vector3 newPosition = new(position.x + distance,
                position.y,
                position.z);
            brg2D3.transform.position = newPosition;
        }
    }

}