本文参考自：【进阶】RecyclerView源码解析(二)——缓存机制 https://www.jianshu.com/p/e44961f8add5

—起因：由notifyItemRemoved引起的刷新，ui缓冲，加载holder。

• 管理缓存的内部类：

  public final class Recycler {
  
      * 1.一级缓存：mAttachedScrap
      * 2.二级缓存：mCacheViews
      * 3.三级缓存：mViewCacheExtension       （自定义缓存）
      * 4.四级缓存：mRecyclerPool
  
          final ArrayList<RecyclerView.ViewHolder> mAttachedScrap = new ArrayList();
          ArrayList<RecyclerView.ViewHolder> mChangedScrap = null;
          final ArrayList<RecyclerView.ViewHolder> mCachedViews = new ArrayList();      
          RecyclerView.RecycledViewPool mRecyclerPool;
  ｝
  

   

• recyclerview缓存holder的大致步骤

• 缓存入口 ：

  final View view = recycler.getViewForPosition(mCurrentPosition);

•  View getViewForPosition(int position, boolean dryRun) {
              return this.tryGetViewHolderForPositionByDeadline(position, dryRun, 9223372036854775807L).itemView;
          }
  
  
  RecyclerView.ViewHolder tryGetViewHolderForPositionByDeadline(int position, boolean dryRun, long deadlineNs) {
  ..........
  
  1. 默认为false，只有有动画的时候为true ， holder 从Recycler类 mChangeScrap 中获取
       但是这个 并不在 正常的四级缓存中，，因为大多数时候都是false。
  
          if (RecyclerView.this.mState.isPreLayout()) {
                      holder = this.getChangedScrapViewForPosition(position);
                      fromScrapOrHiddenOrCache = holder != null;
                  }
  
  2.第一次正式获取缓存   //看名字 就知道 分三步  scrap hidden  cache
     holder = this.getScrapOrHiddenOrCachedHolderForPosition(position, dryRun);
       
     RecyclerView.ViewHolder getScrapOrHiddenOrCachedHolderForPosition(int position, boolean dryRun) {
  
     **尝试从一级缓存mAttachedScrap 获取缓存
      for(cacheSize = 0; cacheSize < scrapCount; ++cacheSize) {
                  vh = (RecyclerView.ViewHolder)this.mAttachedScrap.get(cacheSize);
                  ......
              }
     **   。。。。有个hiddenView
  
          if (!dryRun) {
              //从HiddenView中获得，这里获得是View
              View view = mChildHelper.findHiddenNonRemovedView(position);
              if (view != null) {
                 
                  final ViewHolder vh = getChildViewHolderInt(view);
                  
              //添加到scrap中？
                  scrapView(view);
                 
              }
          }
     **尝试从二级缓存获取
         final int cacheSize = mCachedViews.size();
         for (int i = 0; i < cacheSize; i++) {
              final ViewHolder holder = mCachedViews.get(i);
            
              //holder是有效的，并且position相同
              if (!holder.isInvalid() && holder.getLayoutPosition() == position) {
                  if (!dryRun) {
                      mCachedViews.remove(i);
                  }
                  return holder;
              }
          }
  
  
  ｝

   

2.第二次正式获取缓存  //从scrap cache  通过 id 获取

if (holder == null) {
                    .....
                    type = RecyclerView.this.mAdapter.getItemViewType(offsetPosition);
                    if (RecyclerView.this.mAdapter.hasStableIds()) {
                        holder = this.getScrapOrCachedViewForId(RecyclerView.this.mAdapter.getItemId(offsetPosition), type, dryRun);
                        ...
                    }

RecyclerView.ViewHolder getScrapOrCachedViewForId(long id, int type, boolean dryRun) {

 *** 尝试从一级缓存attachedScrap  获取
     for(i = count - 1; i >= 0; --i) {
                RecyclerView.ViewHolder holder = (RecyclerView.ViewHolder)this.mAttachedScrap.get(i);
                if (type == holder.getItemViewType()) {
                    holder.addFlags(ViewHolder.FLAG_RETURNED_FROM_SCRAP);
                    .....

                    if (!dryRun) {
                        this.mAttachedScrap.remove(i);
                        .....
                    }
                }
  ***尝试从二级缓存cacheView获取
        i = this.mCachedViews.size();
        for(int ix = i - 1; ix >= 0; --ix) {
                RecyclerView.ViewHolder holderx = (RecyclerView.ViewHolder)this.mCachedViews.get(ix);
                if (holderx.getItemId() == id) {
                    if (type == holderx.getItemViewType()) {
                        if (!dryRun) {
                            this.mCachedViews.remove(ix);
                        }
            }
｝

•  第三次正式获取缓存     （自定义缓存  ，e）
•  
• 第四次正式获取缓存  —-是完全的针对pool 的  和之前的 性质不一样

holder = this.getRecycledViewPool().getRecycledView(type);

public static class RecycledViewPool {
    private static final int DEFAULT_MAX_SCRAP = 5;
    static class ScrapData {
        ArrayList<ViewHolder> mScrapHeap = new ArrayList<>();
        int mMaxScrap = DEFAULT_MAX_SCRAP;
      ....
    }
    SparseArray<ScrapData> mScrap = new SparseArray<>();
    ...
    // 可以看到 SparseArray 的 key 是ViewType,  value 是 ArrayList<ViewHolder>。
    public ViewHolder getRecycledView(int viewType) {
        final ScrapData scrapData = mScrap.get(viewType);
        if (scrapData != null && !scrapData.mScrapHeap.isEmpty()) {
            final ArrayList<ViewHolder> scrapHeap = scrapData.mScrapHeap;
            return scrapHeap.remove(scrapHeap.size() - 1);
        }
        return null;
    }
    ......
  }

•  第五次“获取”缓存  —-当以上4步都没有得到 缓存的holder，通过自己创建holder

  holder = RecyclerView.this.mAdapter.createViewHolder(RecyclerView.this, type);