NGUI ScrollView实现循环优化

List<DataUnit> dataList;

当dataList的数据很多时，如果直接把所有的item挂在UIGrid下面，效率显然是十分低下的。这时用UIWrapContent就可以实现列表的循环复用（通过循环几个item实例表现大量的数据）

在NGUI中高效优化UIScrollView之UIWrapContent的简介以及使用

UIWrapContent的大致用法：

a)UIWrapContent.onInitializeItem是列表循环复用时重设表项数据的委托：

class xxx: MonoBehaviour  
{
    public UIWrapContent _wrapScript;  
    void Awake()  
    {  
         //绑定方法  
        _wrapScript.onInitializeItem = OnUpdateItem;  
    }  
   void OnUpdateItem(GameObject go, int index, int realIndex)  
    {  
       tb.SetNumber(realIndex.ToString());  
    }  
}

b)当列表为横向时 minIndex=0,maxIndex= dataList.Count-1;当列表为纵向时 minIndex = -dataList.Count+1,maxIndex =0;

c)用代码addChild item后需要手动调用一次SortBasedOnScrollMovement()刷新界面

d)当dataList.Count==1时 minIndex==maxIndex==0 。按照UIWrapContent的逻辑，这种情况实现的是无限循环列表，不符合当前的需求，所以要对UIWrapContent.WrapContent进行改写：

/// <summary>
    /// Wrap all content, repositioning all children as needed.
    /// </summary>

    public void WrapContent ()
    {
        float extents = itemSize * mChildren.Count * 0.5f;
        Vector3[] corners = mPanel.worldCorners;
        
        for (int i = 0; i < 4; ++i)
        {
            Vector3 v = corners[i];
            v = mTrans.InverseTransformPoint(v);
            corners[i] = v;
        }
        
        Vector3 center = Vector3.Lerp(corners[0], corners[2], 0.5f);
        bool allWithinRange = true;
        float ext2 = extents * 2f;

        if (mHorizontal)
        {
            float min = corners[0].x - itemSize;
            float max = corners[2].x + itemSize;

            for (int i = 0, imax = mChildren.Count; i < imax; ++i)
            {
                Transform t = mChildren[i];
                float distance = t.localPosition.x - center.x;

                if (distance < -extents)
                {
                    Vector3 pos = t.localPosition;
                    pos.x += ext2;
                    distance = pos.x - center.x;
                    int realIndex = Mathf.RoundToInt(pos.x / itemSize);
                    if ((minIndex <= realIndex && realIndex <= maxIndex))
                    {
                        t.localPosition = pos;
                        UpdateItem(t, i);
                    }
                    else allWithinRange = false;
                }
                else if (distance > extents)
                {
                    Vector3 pos = t.localPosition;
                    pos.x -= ext2;
                    distance = pos.x - center.x;
                    int realIndex = Mathf.RoundToInt(pos.x / itemSize);
                    if ((minIndex <= realIndex && realIndex <= maxIndex))
                    //if (minIndex == maxIndex || (minIndex <= realIndex && realIndex <= maxIndex))
                    {
                        t.localPosition = pos;
                        UpdateItem(t, i);
                    }
                    else allWithinRange = false;
                }
                else if (mFirstTime) UpdateItem(t, i);

                if (cullContent)
                {
                    distance += mPanel.clipOffset.x - mTrans.localPosition.x;
                    if (!UICamera.IsPressed(t.gameObject))
                        NGUITools.SetActive(t.gameObject, (distance > min && distance < max), false);
                }
            }
        }
        else
        {
            float min = corners[0].y - itemSize;
            float max = corners[2].y + itemSize;

            for (int i = 0, imax = mChildren.Count; i < imax; ++i)
            {
                Transform t = mChildren[i];
                float distance = t.localPosition.y - center.y;

                if (distance < -extents)
                {
                    Vector3 pos = t.localPosition;
                    pos.y += ext2;
                    distance = pos.y - center.y;
                    int realIndex = Mathf.RoundToInt(pos.y / itemSize);
                    if ((minIndex <= realIndex && realIndex <= maxIndex))
                //    if (minIndex == maxIndex || (minIndex <= realIndex && realIndex <= maxIndex))
                    {
                        t.localPosition = pos;
                        UpdateItem(t, i);
                    }
                    else allWithinRange = false;
                }
                else if (distance > extents)
                {
                    Vector3 pos = t.localPosition;
                    pos.y -= ext2;
                    distance = pos.y - center.y;
                    int realIndex = Mathf.RoundToInt(pos.y / itemSize);
                    if ((minIndex <= realIndex && realIndex <= maxIndex))
           
                //    if (minIndex == maxIndex || (minIndex <= realIndex && realIndex <= maxIndex))
                    {
                        t.localPosition = pos;
                        UpdateItem(t, i);
                    }
                    else allWithinRange = false;
                }
                else if (mFirstTime) UpdateItem(t, i);

                if (cullContent)
                {
                    distance += mPanel.clipOffset.y - mTrans.localPosition.y;
                    if (!UICamera.IsPressed(t.gameObject))
                        NGUITools.SetActive(t.gameObject, (distance > min && distance < max), false);
                }
            }
        }
        mScroll.restrictWithinPanel = !allWithinRange;
    }