[UGUI]滑动列表优化(循环利用)

需要注意的有下面几点：

1.

区分好表现上的index和逻辑上的index。表现上的index是指这个go是go列表中的第几项，但实际上这个index的意义并不大，因为在滚动的过程中go列表是轮转的；逻辑上的index是指这个go对应数据中的第几项，在滑动的过程中不断地更新逻辑上的index，然后取对应的数据去刷新显示即可。在一般的滑动列表中，有几项数据就生成几个go，因此表现上的index和逻辑上的index是一致的；而在循环利用的循环列表中，这两个是不一致的。

那么，在实现上，就是需要知道每个go对应的逻辑index是多少了。而这个可以简化为，只需要知道第一个对应的逻辑index是多少，因为后面的就是依次递增的。

2.

做好缓存策略。对于循环利用的列表，需要生成的个数等于能显示的最大个数加上2-3个的缓存个数，防止滑动过快时出现穿帮。

3.

关于循环利用的实现。其实就是在滑动过程中，收集被移除显示的go，然后对这些go重新调整位置，并刷新go上的控件显示。那么如何收集呢，就是将go对应的逻辑index和当前的逻辑index范围进行比较，将不在这个范围内的go收集即可。然后在需要的时候取出来，刷新这些go即可。

代码如下：

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

 [RequireComponent(typeof(ScrollRect))]
 public class LoopScrollView : MonoBehaviour {

     private List<GameObject> goList;//当前显示的go列表
     private Queue<GameObject> freeGoQueue;//空闲的go队列，存放未显示的go
     private Dictionary<GameObject, int> goIndexDic;//key:所有的go value:真实索引
     private ScrollRect scrollRect;
     private RectTransform contentRectTra;
     private Vector2 scrollRectSize;
     private Vector2 cellSize;
     private int startIndex;//起始索引
     private int maxCount;//创建的最大数量
     private int createCount;//当前显示的数量

     private const int cacheCount = ;//缓存数目
     private const int invalidStartIndex = -;//非法的起始索引

     private int dataCount;
     private GameObject prefabGo;
     private Action<GameObject, int> updateCellCB;
     private float cellPadding;

     //初始化SV并刷新
     public void Show(int dataCount, GameObject prefabGo, Action<GameObject, int> updateCellCB, float cellPadding = 0f)
     {
         //数据和组件初始化
         this.dataCount = dataCount;
         this.prefabGo = prefabGo;
         this.updateCellCB = updateCellCB;
         this.cellPadding = cellPadding;

         goList = new List<GameObject>();
         freeGoQueue = new Queue<GameObject>();
         goIndexDic = new Dictionary<GameObject, int>();
         scrollRect = GetComponent<ScrollRect>();
         contentRectTra = scrollRect.content;
         scrollRectSize = scrollRect.GetComponent<RectTransform>().sizeDelta;
         cellSize = prefabGo.GetComponent<RectTransform>().sizeDelta;
         startIndex = ;
         maxCount = GetMaxCount();
         createCount = ;

         if (scrollRect.horizontal)
         {
             contentRectTra.anchorMin = new Vector2(, );
             contentRectTra.anchorMax = new Vector2(, );
         }
         else
         {
             contentRectTra.anchorMin = new Vector2(, );
             contentRectTra.anchorMax = new Vector2(, );
         }
         scrollRect.onValueChanged.RemoveAllListeners();
         scrollRect.onValueChanged.AddListener(OnValueChanged);
         ResetSize(dataCount);
     }

     //重置数量
     public void ResetSize(int dataCount)
     {
         this.dataCount = dataCount;
         contentRectTra.sizeDelta = GetContentSize();

         //回收显示的go
         for (int i = goList.Count - ; i >= ; i--)
         {
             GameObject go = goList[i];
             RecoverItem(go);
         }

         //创建或显示需要的go
         createCount = Mathf.Min(dataCount, maxCount);
         for (int i = ; i < createCount; i++)
         {
             CreateItem(i);
         }

         //刷新数据
         startIndex = -;
         contentRectTra.anchoredPosition = Vector3.zero;
         OnValueChanged(Vector2.zero);
     }

     //更新当前显示的列表
     public void UpdateList()
     {
         for (int i = ; i < goList.Count; i++)
         {
             GameObject go = goList[i];
             int index = goIndexDic[go];
             updateCellCB(go, index);
         }
     }

     //创建或显示一个item
     private void CreateItem(int index)
     {
         GameObject go;
         if (freeGoQueue.Count > )//使用原来的
         {
             go = freeGoQueue.Dequeue();
             goIndexDic[go] = index;
             go.SetActive(true);
         }
         else//创建新的
         {
             go = Instantiate<GameObject>(prefabGo);
             goIndexDic.Add(go, index);
             go.transform.SetParent(contentRectTra.transform);

             RectTransform rect = go.GetComponent<RectTransform>();
             rect.pivot = new Vector2(, );
             rect.anchorMin = new Vector2(, );
             rect.anchorMax = new Vector2(, );
         }
         goList.Add(go);
         go.transform.localPosition = GetPosition(index);
         updateCellCB(go, index);
     }

     //回收一个item
     private void RecoverItem(GameObject go)
     {
         go.SetActive(false);
         goList.Remove(go);
         freeGoQueue.Enqueue(go);
         goIndexDic[go] = invalidStartIndex;
     }

     //滑动回调
     private void OnValueChanged(Vector2 vec)
     {
         int curStartIndex = GetStartIndex();
         //Debug.LogWarning(curStartIndex);

         if ((startIndex != curStartIndex) && (curStartIndex > invalidStartIndex))
         {
             startIndex = curStartIndex;

             //收集被移出去的go
             //索引的范围:[startIndex, startIndex + createCount - 1]
             for (int i = goList.Count - ; i >= ; i--)
             {
                 GameObject go = goList[i];
                 int index = goIndexDic[go];
                 if (index < startIndex || index > (startIndex + createCount - ))
                 {
                     RecoverItem(go);
                 }
             }

             //对移除出的go进行重新排列
             for (int i = startIndex; i < startIndex + createCount; i++)
             {
                 if (i >= dataCount)
                 {
                     break;
                 }

                 bool isExist = false;
                 for (int j = ; j < goList.Count; j++)
                 {
                     GameObject go = goList[j];
                     int index = goIndexDic[go];
                     if (index == i)
                     {
                         isExist = true;
                         break;
                     }
                 }
                 if (isExist)
                 {
                     continue;
                 }

                 CreateItem(i);
             }
         }
     }

     //获取需要创建的最大prefab数目
     private int GetMaxCount()
     {
         if (scrollRect.horizontal)
         {
             return Mathf.CeilToInt(scrollRectSize.x / (cellSize.x + cellPadding)) + cacheCount;
         }
         else
         {
             return Mathf.CeilToInt(scrollRectSize.y / (cellSize.y + cellPadding)) + cacheCount;
         }
     }

     //获取起始索引
     private int GetStartIndex()
     {
         if (scrollRect.horizontal)
         {
             return Mathf.FloorToInt(-contentRectTra.anchoredPosition.x / (cellSize.x + cellPadding));
         }
         else
         {
             return Mathf.FloorToInt(contentRectTra.anchoredPosition.y / (cellSize.y + cellPadding));
         }
     }

     //获取索引所在位置
     private Vector3 GetPosition(int index)
     {
         if (scrollRect.horizontal)
         {
             return new Vector3(index * (cellSize.x + cellPadding), , );
         }
         else
         {
             return new Vector3(, index * -(cellSize.y + cellPadding), );
         }
     }

     //获取内容长宽
     private Vector2 GetContentSize()
     {
         if (scrollRect.horizontal)
         {
             return new Vector2(cellSize.x * dataCount + cellPadding * (dataCount - ), contentRectTra.sizeDelta.y);
         }
         else
         {
             return new Vector2(contentRectTra.sizeDelta.x, cellSize.y * dataCount + cellPadding * (dataCount - ));
         }
     }
 }

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;

 public class TestLoopScrollView : MonoBehaviour {

     public LoopScrollView loopScrollView;
     public LoopScrollView loopScrollView2;
     public GameObject prefabGo;
     public GameObject prefabGo2;

     private void Start ()
     {

     }

     private void Update()
     {
         if (Input.GetKeyDown(KeyCode.Q))
         {
             loopScrollView.Show(, prefabGo, UpdateSV);
         }
         else if(Input.GetKeyDown(KeyCode.W))
         {
             loopScrollView.ResetSize();
         }
         else if (Input.GetKeyDown(KeyCode.E))
         {
             loopScrollView.ResetSize();
         }
         else if (Input.GetKeyDown(KeyCode.R))
         {
             loopScrollView.UpdateList();
         }

         if (Input.GetKeyDown(KeyCode.A))
         {
             loopScrollView2.Show(, prefabGo2, UpdateSV);
         }
         else if (Input.GetKeyDown(KeyCode.S))
         {
             loopScrollView2.ResetSize();
         }
         else if (Input.GetKeyDown(KeyCode.D))
         {
             loopScrollView2.ResetSize();
         }
         else if (Input.GetKeyDown(KeyCode.F))
         {
             loopScrollView.UpdateList();
         }
     }

     private void UpdateSV(GameObject go, int index)
     {
         Text text = go.transform.Find("Text").GetComponent<Text>();
         text.text = index.ToString();
     }
 }

效果：