public static class RectTransformExtensions
 {
  public static bool Overlaps(this RectTransform a, RectTransform b)
  {
   return a.WorldRect().Overlaps(b.WorldRect());
  }
  public static bool Overlaps(this RectTransform a, RectTransform b, bool allowInverse)
  {
   return a.WorldRect().Overlaps(b.WorldRect(), allowInverse);
  }
 
  public static Rect WorldRect(this RectTransform rectTransform)
  {
   Vector2 sizeDelta = rectTransform.sizeDelta;
   float rectTransformWidth = sizeDelta.x * rectTransform.lossyScale.x;
   float rectTransformHeight = sizeDelta.y * rectTransform.lossyScale.y;
 
   Vector3 position = rectTransform.position;
 
   return new Rect(
    position.x - rectTransformWidth * rectTransform.pivot.x,
    position.y - rectTransformHeight * rectTransform.pivot.y,
    rectTransformWidth,
    rectTransformHeight);
  }
  /// <summary>
  /// 
  /// </summary>
  /// <param name="rectTransform"></param>
  /// <param name="pos">世界坐標的position</param>
  /// <returns></returns>
  public static Rect WorldRect2(this RectTransform rectTransform, Vector3 pos)
  {
   Rect rect = new Rect();
   Vector2 sizeDelta = rectTransform.sizeDelta;
   float rectTransformWidth = sizeDelta.x * rectTransform.lossyScale.x;
   float rectTransformHeight = sizeDelta.y * rectTransform.lossyScale.y;
 
   Vector3 position = pos;
 
   rect.x = position.x - rectTransformWidth * rectTransform.pivot.x;
   rect.y = position.y - rectTransformHeight * rectTransform.pivot.y;
   rect.width = rectTransformWidth;
   rect.height = rectTransformHeight;
 
   return rect;
  }
 }

private void OnScrollRectValueChanged(Vector2 arg0)
  {
   Dictionary<int, DynamicRect> inOverlaps = new Dictionary<int, DynamicRect>();
 
   mRectMask = 遮罩物體的RectTransform.WorldRect();
   //m_DynamicRectDic這個字典保存的是你所有UI需要放置的位置數據，
   //判斷所有UI哪個是可見哪個不可見 ，保存起來
   foreach (DynamicRect dR in m_DynamicRectDic.Values)
   {
    tmpTra.localPosition = dR.localPos;
    //獲取每個位置UI的世界坐標Rect
    Rect rect = m_LevelItemPrefabRT.WorldRect2(tmpTra.position);
    if (rect.Overlaps(mRectMask))
    {
     inOverlaps.Add(dR.Index, dR);
    }
   }
   //m_LevelItemList是保存你實例化后的UI列表，比如你這個遮罩頁面最多顯示3個UI，你需要實例化4個UI，然后動態(tài)修改gameobject的顯示與隱藏
   int len = m_LevelItemList.Count;
   for (int i = 0; i < len; ++i)
   {
    //LevelItem是UI上掛載的腳本，用于更新UI界面的顯示和數據存儲的
    LevelItem item = m_LevelItemList[i];
    if (item.DRect != null && !inOverlaps.ContainsKey(item.DRect.Index))
    {
     //item的DRect為null時，隱藏物體，否則顯示物體
     item.DRect = null;
    }
   }
   //判斷哪些可以重復利用的UI，然后賦予新的數據與位置
   foreach (DynamicRect dR in inOverlaps.Values)
   {
    if (GetDynmicItem(dR) == null)
    {
     LevelItem item = GetNullDynmicItem();
 
     if (item == null) continue;
 
     item.DRect = dR;
     //更新UI的位置和顯示（自己計算，每種顯示不一樣）
     _UpdateChildTransformPos(item.gameObject, dR.Index);
    }
   }
  }
 
  /// <summary>
  /// 通過動態(tài)格子獲得動態(tài)渲染器
  /// </summary>
  /// <param name="rect"></param>
  /// <returns></returns>
  private LevelItem GetDynmicItem(DynamicRect rect)
  {
   int len = m_LevelItemList.Count;
   for (int i = 0; i < len; ++i)
   {
    LevelItem item = m_LevelItemList[i];
    if (item.DRect == null)
     continue;
    if (rect.Index == item.DRect.Index)
     return item;
   }
   return null;
  }
  /// <summary>
  /// 獲得待渲染的渲染器
  /// </summary>
  /// <returns></returns>
  private LevelItem GetNullDynmicItem()
  {
   int len = m_LevelItemList.Count;
   for (int i = 0; i < len; ++i)
   {
    LevelItem item = m_LevelItemList[i];
    if (item.DRect == null)
     return item;
   }
 
   return null;
  }
 
 public class DynamicRect
 {
  /// <summary>
  /// 本地坐標
  /// </summary>
  public Vector3 localPos;
  /// <summary>
  /// 格子索引
  /// </summary>
  public int Index;
 
  public DynamicRect(int index, Vector3 localPos)
  {
   this.Index = index;
   this.localPos = localPos;
  }
 }