unity ui sorting

BillBoard2D.cs

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

public class BillBoard2D : MonoBehaviour {

    public Camera m_Camera;

    void Update()
    {
        if (!m_Camera)
            m_Camera = Camera.main;
        transform.LookAt(transform.position + m_Camera.transform.rotation * Vector3.forward,
            m_Camera.transform.rotation * Vector3.up);
    }
    
}

FollowTarget.cs

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

public class FollowTarget : MonoBehaviour {

    public Transform target;

    public Vector3 offset;
    public float smoothTime = 0.1f;

    private float velocity;
    private Transform myTransform;
    
    // Use this for initialization
    void Awake () {
        myTransform = GetComponent<Transform>();
	}
	
	// Update is called once per frame
	void Update () {
        if (target == null) return;

        Vector3 pos = target.position + offset;
        Vector3 now = myTransform.position;

        if (pos.Equals(now)) return;

        float distance = Vector3.Distance(pos, now);

        float v = Mathf.SmoothDamp(0, distance, ref velocity, smoothTime);
        myTransform.position =  Vector3.MoveTowards(now, pos, v);
	}
}

UISortByCamera.cs

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

public class UISortByCamera : MonoBehaviour {

    private Transform myTransform;
    private List<Pair> childs = new List<Pair>();
    public Camera camera;

    public struct Pair
    {
        public Transform target;
        public float distance;
        public int index;
    }

    private void Awake()
    {
        myTransform = GetComponent<Transform>();
        if (camera == null)
            camera = Camera.main;
    }

    // Update is called once per frame
    void Update () {
        if (camera == null) return;

        Vector3 cameraPos = camera.transform.position;
        Vector3 cameraForward = camera.transform.forward;
        float near = camera.nearClipPlane;
        childs.Clear();
        for(int i = 0, l = myTransform.childCount; i < l; i++)
        {
            Transform target = myTransform.GetChild(i);
            float distance = Vector3.Dot(target.position - cameraPos, cameraForward);
            childs.Add(new Pair()
            {
                target = target,
                distance = distance,
                index = i
            });
        }
        childs.Sort(SortComparerr);
        for (int i = 0, l = childs.Count; i < l; i++)
        {
            if (childs[i].distance >= near)
                childs[i].target.SetAsLastSibling();
        }
	}

    private int SortComparerr(Pair a, Pair b)
    {
        if (a.distance > b.distance)
            return -1;
        if (a.distance < b.distance)
            return 1;
        // List.Sort is unstable
        // so make stable sort with index
        return a.index - b.index;
    }
}