Android从源码分析RecyclerView四级缓存复用机制一（缓存ViewHolder）

//一级缓存中用来存储屏幕中显示的ViewHolder
final ArrayList mAttachedScrap = new ArrayList<>();
ArrayList mChangedScrap = null;
//二级缓存中用来存储屏幕外的ViewHolder
final ArrayList mCachedViews = new ArrayList();
//暂可忽略 mAttachedScrap的不可变视图
private final List
mUnmodifiableAttachedScrap = Collections.unmodifiableList(mAttachedScrap);

private int mRequestedCacheMax = DEFAULT_CACHE_SIZE;
//mCachedViews屏幕外缓存的存储上限默认为DEFAULT_CACHE_SIZE也就是2，可变
int mViewCacheMax = DEFAULT_CACHE_SIZE;
//四级缓存当屏幕外缓存的大小大于2，便放入mRecyclerPool中缓存
RecycledViewPool mRecyclerPool;
//三级缓存自定义缓存，自己定义的缓存规则
private ViewCacheExtension mViewCacheExtension;
//默认屏幕外缓存大小
static final int DEFAULT_CACHE_SIZE = 2;
//…

2.再看一下主要的方法调用流程，从RecyclerView的onMeasure方法开始一直到三个存储的地方一级，二级和四级缓存，别问为啥没有mViewCacheExtension，问就是这个你需要自己去存

3.源码分析，本文对重点方法和重点代码进行分析（流程最好自己去跟一下）。

/**

• 5.RecyclerView.scrapOrRecycleView
  */
  private void scrapOrRecycleView(Recycler recycler, int index, View view) {
  final ViewHolder viewHolder = getChildViewHolderInt(view);
  //只展示重点代码…
  if (viewHolder.isInvalid() && !viewHolder.isRemoved()
  && !mRecyclerView.mAdapter.hasStableIds()) {
  removeViewAt(index);
  //这里调用到二级和四级缓存
  recycler.recycleViewHolderInternal(viewHolder);
  } else {
  detachViewAt(index);
  //这里调用到一级缓存
  recycler.scrapView(view);
  mRecyclerView.mViewInfoStore.onViewDetached(viewHolder);
  }
  }

① 分析一级缓存

/**

• 13.RecyclerView.scrapView
  */
  void scrapView(View view) {
  final ViewHolder holder = getChildViewHolderInt(view);
  if (holder.hasAnyOfTheFlags(ViewHolder.FLAG_REMOVED | ViewHolder.FLAG_INVALID)
  || !holder.isUpdated() || canReuseUpdatedViewHolder(holder)) {
  //…
  holder.setScrapContainer(this, false);
  //缓存adapter其他notify系列方法(包括notifyDataSetChanged)被移除的ViewHolder
  mAttachedScrap.add(holder);
  } else {
  if (mChangedScrap == null) {
  mChangedScrap = new ArrayList();
  }
  holder.setScrapContainer(this, true);
  //缓存adapter的notifyItemRangeChanged被移除的ViewHolder
  mChangedScrap.add(holder);
  }
  }

② 分析二级缓存

/**

• 6.RecyclerView.recycleViewHolderInternal
  */
  void recycleViewHolderInternal(ViewHolder holder) {
  //…一系列是否需要二级回收的判断
  if (forceRecycle || holder.isRecyclable()) {
  if (mViewCacheMax > 0
  && !holder.hasAnyOfTheFlags(ViewHolder.FLAG_INVALID
  | ViewHolder.FLAG_REMOVED
  | ViewHolder.FLAG_UPDATE
  | ViewHolder.FLAG_ADAPTER_POSITION_UNKNOWN)) {
  // Retire oldest cached view
  int cachedViewSize = mCachedViews.size();
  //判断mCachedViews的大小是否大于2
  if (cachedViewSize >= mViewCacheMax && cachedViewSize > 0) {
  //重点分析一
  recycle

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CachedViewAt(0);
cachedViewSize–;
}
//…计算targetCacheIndex的下标 让mCachedViews满足队列先进先出原则
mCachedViews.add(targetCacheIndex, holder);
cached = true;
}
if (!cached) {
//…如果二级缓存没有存储则添加到四级缓存
addViewHolderToRecycledViewPool(holder, true);
recycled = true;
}
} else {
//…
}
//…
}
/**

• 重点分析一：7.RecyclerView.recycleViewHolderInternal
• 作用：如果mCachedViews的大小大于2则内部调用addViewHolderToRecycledViewPool方法添加到RecycledViewPool中
  */
  void recycleCachedViewAt(int cachedViewIndex) {

ViewHolder viewHolder = mCachedViews.get(cachedViewIndex);
//Viewholder存储到四级缓存
addViewHolderToRecycledViewPool(viewHolder, true);
//Viewholder在四级缓存存储后移除mCachedViews中对应的Viewholder
mCachedViews.remove(cachedViewIndex);
}

③分析四级缓存

/**

• 8.RecyclerView.recycleViewHolderInternal
  hedViews中对应的Viewholder
  mCachedViews.remove(cachedViewIndex);
  }

③分析四级缓存

/**

• 8.RecyclerView.recycleViewHolderInternal

标签：viewHolder,ViewHolder,holder,缓存,RecyclerView,源码,mCachedViews
来源： https://blog.csdn.net/m0_64603929/article/details/121709392