[UnityUI]循环滑动列表

效果图：

使用的是UGUI和DOTween

当中比較关键的是循环滑动和层次排序：

1.循环滑动：这里先如果显示五张图片。分别标记为0,1,2,3,4，那么当向左滑动时，序列就变为1,2,3,4,0，这里先保存4的位置，然后从4開始，4的位置和大小向3渐变，3的位置和大小向2渐变。2的位置和大小向1渐变。1的位置和大小向0渐变。0的位置直接变为原来保存的4的位置。

也就是说，当向左滑动时，最左端的那张图片特殊处理，其余的向左推进。当向右滑动时，最右端的那张图片特殊处理，其余的向右推进。

2.层次排序：因为使用的是UGUI。UI的排序跟在Hierarchy的位置有关。假设图片缩放得越小，就觉得它越远离屏幕。因此就越靠前。会被后面的图片遮住。注意的是在缩放动画播放时，localScale是不确定的，因此要直接将当前位置的下一个位置的localScale传入，从而计算图片的"深度"。还有就是使用SetSiblingIndex时要全然确定好全部图片的排序。

using UnityEngine;
using System.Collections.Generic;
using DG.Tweening;

public class ScrollView : MonoBehaviour {

	public int xOffset = 1;//x轴偏移
    public int yOffset = 0;//y轴偏移
	public float scale = 0.8f;//缩放倍数
	public float time = 0.5f;//偏移与缩放动画的播放时间

    private int left;//最左端的编号
    private int right;//最右端的编号
	public int itemAmount = 5;//展示的图片数
	public Vector3 middlePos;//最中间的位置

	public GameObject itemPrefab;
    private GameObject canvas;
    private GameObject[] sortArray;
	private List<GameObject> list = new List<GameObject>();

	private void InstantiateItem(Vector3 pos,float scale)
	{
		GameObject go = Instantiate(itemPrefab) as GameObject;
        go.transform.SetParent(canvas.transform);
        go.transform.localPosition = pos;
		go.transform.localScale *= scale;

        InsertToSortArray(go, go.transform.localScale.x);
		list.Add(go);
	}

	public void Init()
	{
        left = 0;
        right = itemAmount - 1;
        canvas = GameObject.Find("Canvas");
        sortArray = new GameObject[itemAmount];

		int oneSideAmount = (itemAmount - 1) / 2;

		for(int i = oneSideAmount;i >= 1;i--)
		{
			Vector3 pos = middlePos + new Vector3(i * xOffset,i * yOffset,0) * -1;
			InstantiateItem(pos,Mathf.Pow(scale,i));
		}

		InstantiateItem(middlePos,1);

		for(int i = 1;i <= oneSideAmount;i++)
		{
			Vector3 pos = middlePos + new Vector3(i * xOffset,i * yOffset,0);
			InstantiateItem(pos,Mathf.Pow(scale,i));
		}

        Sort();
	}

    /// <summary>
    /// 依据缩放倍数计算深度
    /// </summary>
    /// <param name="scaleNum"></param>
    /// <returns></returns>
    private int CalculateDepth(float scaleNum)
    {
        float num = 0;
        int i = 0;
        while (true)
        {
            num = Mathf.Pow(scale, i);
            if (num != scaleNum) i++;
            else break;
        }
        return i;
    }

    /// <summary>
    /// 插入到排序数组中。数组序号越低。则越远离屏幕
    /// </summary>
    /// <param name="go"></param>
    /// <param name="localScaleX"></param>
    private void InsertToSortArray(GameObject go, float localScaleX)
    {
        int depth = CalculateDepth(localScaleX);
        depth = itemAmount / 2 - depth;

        if (depth == itemAmount / 2)
            sortArray[depth * 2] = go;
        else if (sortArray[depth] == null)
            sortArray[depth] = go;
        else
            sortArray[depth + itemAmount / 2] = go;   
    }

	private void Sort()
	{
        for (int i = 0; i < itemAmount; i++)
        {
            sortArray[i].transform.SetSiblingIndex(i);
        }
        sortArray = new GameObject[itemAmount];
	}

	public void Move(int direction)
	{
		if(direction == -1)//向左滑动
		{
			int startIndex = left;
			int lastIndex = right;
            Vector3 lastPos = list[lastIndex].transform.position;

            InsertToSortArray(list[startIndex], list[startIndex].transform.localScale.x);

            for (int i = 0; i < itemAmount - 1;i++ )
            {
                int index = (lastIndex - i + itemAmount) % itemAmount;
                int preIndex = (index - 1 + itemAmount) % itemAmount;
                list[index].transform.DOMove(list[preIndex].transform.position,time);
                list[index].transform.DOScale(list[preIndex].transform.localScale,time);

                InsertToSortArray(list[index], list[preIndex].transform.localScale.x);
            }
            			
			list[startIndex].transform.DOMove(lastPos,time);
            
            left = (left + 1) % itemAmount;
            right = (right + 1) % itemAmount;
		}
        else if (direction == 1)//向右滑动
		{
            int startIndex = right;
            int lastIndex = left;
            Vector3 lastPos = list[lastIndex].transform.position;

            InsertToSortArray(list[startIndex], list[startIndex].transform.localScale.x);

            for (int i = 0; i < itemAmount - 1; i++)
            {
                int index = (lastIndex + i + itemAmount) % itemAmount;
                int preIndex = (index + 1 + itemAmount) % itemAmount;
                list[index].transform.DOMove(list[preIndex].transform.position, time);
                list[index].transform.DOScale(list[preIndex].transform.localScale, time);

                InsertToSortArray(list[index], list[preIndex].transform.localScale.x);
            }

            list[startIndex].transform.DOMove(lastPos, time);

            left = (left - 1 + itemAmount) % itemAmount;
            right = (right - 1 + itemAmount) % itemAmount;
		}

		Sort();
	}
}

不足之处：

1.仅仅适用于展示图片数为奇数的场合

2.等等...