PageScrollRect

//UGUI ScrollRect CenterOnChild

using UnityEngine;
using System.Collections;
using UnityEngine.EventSystems;
using UnityEngine.UI;
using System.Collections.Generic;
using DG.Tweening;
public class PageScrollRect : MonoBehaviour, IDragHandler,IEndDragHandler{

    public ScrollRect scrollRect;
    public RectTransform viewPointTransform;
    public RectTransform targetTransform;
    public int curCenterIndex;
   
    void OnGUI()
    {
        if(GUILayout.Button("DoCenter"))
        {
            CenterOnItem(targetTransform);
        }

        if(GUILayout.Button("NextPage"))
        {
            FlipPage(true);
        }

        if (GUILayout.Button("PrevPage"))
        {
            FlipPage(false);
        }

    }

    void CenterChildByIndex(int index)
    {
        if(index>=0 && index<scrollRect.content.transform.childCount)
        {
            curCenterIndex = index;
            CenterOnItem(scrollRect.content.transform.GetChild(index).GetComponent<RectTransform>());
        }
    }


    public void  FlipPage(bool nextPage)
    {
         if(nextPage)
         {
             CenterChildByIndex(curCenterIndex + 1);
         }
         else
         {
             CenterChildByIndex(curCenterIndex -1);
         }
    }

    /// <summary>
    /// 指定一个 item让其定位到ScrollRect中间
    /// </summary>
    /// <param name="target">需要定位到的目标</param>
    public void CenterOnItem(RectTransform target)
    {
        Debug.Log("Going to center on " + target.name);
        // Item is here
        var itemCenterPositionInScroll = GetWorldPointInWidget(scrollRect.GetComponent<RectTransform>(), GetWidgetWorldPoint(target));
        Debug.Log("Item Anchor Pos In Scroll: " + itemCenterPositionInScroll);
        // But must be here
        var targetPositionInScroll = GetWorldPointInWidget(scrollRect.GetComponent<RectTransform>(), GetWidgetWorldPoint(viewPointTransform));
        Debug.Log("Target Anchor Pos In Scroll: " + targetPositionInScroll);
        // So it has to move this distance
        var difference = targetPositionInScroll - itemCenterPositionInScroll;
        difference.z = 0f;

        RectTransform contentTransform = scrollRect.content;

        var newNormalizedPosition = new Vector2(difference.x / (contentTransform.rect.width - viewPointTransform.rect.width),
            difference.y / (contentTransform.rect.height - viewPointTransform.rect.height));

        newNormalizedPosition = scrollRect.normalizedPosition - newNormalizedPosition;

        
       
        newNormalizedPosition.x = Mathf.Clamp01(newNormalizedPosition.x);
        newNormalizedPosition.y = Mathf.Clamp01(newNormalizedPosition.y);
       
       // scrollRect.normalizedPosition = new Vector2(scrollRect.normalizedPosition.x, newNormalizedPosition.y);
        if (scrollRect.vertical)
        {
            DOTween.To(() => scrollRect.normalizedPosition, y => scrollRect.normalizedPosition = y, newNormalizedPosition, 1f);
        }
        else if(scrollRect.horizontal)
        {
            DOTween.To(() => scrollRect.normalizedPosition, x => scrollRect.normalizedPosition = x, newNormalizedPosition, 1f);
        }
    }

    Vector3 GetWidgetWorldPoint(RectTransform target)
    {
        //pivot position + item size has to be included
        var pivotOffset = new Vector3(
            (0.5f - target.pivot.x) * target.rect.size.x,
            (0.5f - target.pivot.y) * target.rect.size.y,
            0f);
        var localPosition = target.localPosition + pivotOffset;
        return target.parent.TransformPoint(localPosition);
    }

    Vector3 GetWorldPointInWidget(RectTransform target, Vector3 worldPoint)
    {
        return target.InverseTransformPoint(worldPoint);
    }

    public void OnDrag(PointerEventData eventData)
    {
        //throw new System.NotImplementedException();
    }

    public void OnEndDrag(PointerEventData eventData)
    {
        //throw new System.NotImplementedException();
        CenterToClosestChild(scrollRect.content);
    }


    void CenterToClosestChild(Transform parentChild)
    {
        Transform closestChild=null;
        float dist=float.MaxValue;
        
        var currentPos = GetWorldPointInWidget(scrollRect.GetComponent<RectTransform>(), GetWidgetWorldPoint(viewPointTransform));
        for(int i=0;i<parentChild.childCount;i++)
        {
            Transform child = parentChild.GetChild(i);
            RectTransform rect = child.GetComponent<RectTransform>();
            var itemCenterPositionInScroll = GetWorldPointInWidget(scrollRect.GetComponent<RectTransform>(), GetWidgetWorldPoint(rect));
            Debug.Log("Item Anchor Pos In Scroll: " + itemCenterPositionInScroll);
            float distance =Mathf.Abs(currentPos.y - itemCenterPositionInScroll.y);
           if(distance<dist)
           {
               dist = distance;
               closestChild = child;
               curCenterIndex = i;
           }
        }

        if(closestChild)
        {
            CenterOnItem(closestChild.GetComponent<RectTransform>());
        }
        else
        {
            Debug.LogError("Got nothing to center on ");
        }

    }

}