第一篇中遗留的一个问题还没有解决：复用表项时优先级最高的scrap view是用来干嘛的？这篇文章试着通过阅读源码来解答这个问题。
scrap view对应的存储结构是final ArrayList<ViewHolder> mAttachedScrap = new ArrayList<>();。理解成员变量用途的最好办法是 “搜索它在什么时候被访问” 。对于列表结构来说就相当于 1. 在什么时候往列表添加内容？ 2. 在什么时候清空列表内容？
添加内容
全局搜索mAttachedScrap被访问的地方，其中只有一处调用了mAttachedScrap.add():
public final class Recycler { /**
* Mark an attached view as scrap.
* 回收ViewHolder到scrap集合（mAttachedScrap或mChangedScrap）
*
* <p>"Scrap" views are still attached to their parent RecyclerView but are eligible
* for rebinding and reuse. Requests for a view for a given position may return a
* reused or rebound scrap view instance.</p>
* scrap view依然依附于它的父亲。。。
*
* @param view View to scrap
*/
void scrapView(View view) { final ViewHolder holder = getChildViewHolderInt(view); if (holder.hasAnyOfTheFlags(ViewHolder.FLAG_REMOVED | ViewHolder.FLAG_INVALID)
|| !holder.isUpdated() || canReuseUpdatedViewHolder(holder)) { if (holder.isInvalid() && !holder.isRemoved() && !mAdapter.hasStableIds()) { throw new IllegalArgumentException("Called scrap view with an invalid view."
+ " Invalid views cannot be reused from scrap, they should rebound from"
+ " recycler pool." + exceptionLabel());
}
holder.setScrapContainer(this, false); //添加到mAttachedScrap集合中
mAttachedScrap.add(holder);
} else { if (mChangedScrap == null) {
mChangedScrap = new ArrayList<ViewHolder>();
}
holder.setScrapContainer(this, true); //添加到mChangedScrap集合中
mChangedScrap.add(holder);
}
}
}沿着调用链继续往上：
public abstract static class LayoutManager { private void scrapOrRecycleView(Recycler recycler, int index, View view) { final ViewHolder viewHolder = getChildViewHolderInt(view); if (viewHolder.shouldIgnore()) { if (DEBUG) {
Log.d(TAG, "ignoring view " + viewHolder);
} return;
} //删除表项并入回收池
if (viewHolder.isInvalid() && !viewHolder.isRemoved()
&& !mRecyclerView.mAdapter.hasStableIds()) {
removeViewAt(index);
recycler.recycleViewHolderInternal(viewHolder);
} //detach表项并入scrap集合
else {
detachViewAt(index);
recycler.scrapView(view);
mRecyclerView.mViewInfoStore.onViewDetached(viewHolder);
}
}
}根据viewHolder的不同状态，要么将其将其添加到mAttachedScrap集合，要么将其存入回收池。其中recycleViewHolderInternal()在RecyclerView缓存机制（回收去哪？）分析过。
沿着调用链继续向上：
public abstract static class LayoutManager { /**
* Temporarily detach and scrap all currently attached child views. Views will be scrapped
* into the given Recycler. The Recycler may prefer to reuse scrap views before
* other views that were previously recycled.
* 暂时将当可见表项进行分离并回收
*
* @param recycler Recycler to scrap views into
*/
public void detachAndScrapAttachedViews(Recycler recycler) { final int childCount = getChildCount(); //遍历所有可见表项并回收他们
for (int i = childCount - 1; i >= 0; i--) { final View v = getChildAt(i);
scrapOrRecycleView(recycler, i, v);
}
} /**
* Lay out all relevant child views from the given adapter.
* 从给定的adapter布局所有的孩子
*/
public void onLayoutChildren(Recycler recycler, State state) {
... //在填充表项之前回收所有表项
detachAndScrapAttachedViews(recycler);
... if (mAnchorInfo.mLayoutFromEnd) {
... //填充表项
fill(recycler, mLayoutState, state, false);
...
}
...
}
}public class RecyclerView extends ViewGroup implements ScrollingView, NestedScrollingChild2 { //RecyclerView布局的第二步
private void dispatchLayoutStep2() {
...
mLayout.onLayoutChildren(mRecycler, mState);
...
}
}在将表项一个个填充到列表之前会先将其先回收到
mAttachedScrap中，回收数据的来源是LayoutManager的孩子，而LayoutManager的孩子都是屏幕上可见的表项。注释中“暂时将当可见表项进行分离并回收”，既然是“暂时回收”，那待会必然会发生“复用”。复用逻辑可移步RecyclerView缓存机制（咋复用？）
至此可以得出结论：
mAttachedScrap用于屏幕中可见表项的回收和复用
清空内容
全局搜索mAttachedScrap被访问的地方，其中只有一处调用了mAttachedScrap.clear():
public final class Recycler { void clearScrap() {
mAttachedScrap.clear(); if (mChangedScrap != null) {
mChangedScrap.clear();
}
}
}public abstract static class LayoutManager { /**
* Recycles the scrapped views.
* 回收所有scrapped view
*/
void removeAndRecycleScrapInt(Recycler recycler) { final int scrapCount = recycler.getScrapCount(); // Loop backward, recycler might be changed by removeDetachedView()
//遍历搜有scrap view并重置ViewHolder状态
for (int i = scrapCount - 1; i >= 0; i--) { final View scrap = recycler.getScrapViewAt(i); final ViewHolder vh = getChildViewHolderInt(scrap); if (vh.shouldIgnore()) { continue;
}
vh.setIsRecyclable(false); if (vh.isTmpDetached()) {
mRecyclerView.removeDetachedView(scrap, false);
} if (mRecyclerView.mItemAnimator != null) {
mRecyclerView.mItemAnimator.endAnimation(vh);
}
vh.setIsRecyclable(true);
recycler.quickRecycleScrapView(scrap);
} //清空scrap view集合
recycler.clearScrap(); if (scrapCount > 0) {
mRecyclerView.invalidate();
}
}
}沿着调用链向上：
public class RecyclerView extends ViewGroup implements ScrollingView, NestedScrollingChild2 { //RecyclerView布局的最后一步
private void dispatchLayoutStep3() {
...
mLayout.removeAndRecycleScrapInt(mRecycler);
...
}至此可以得出结论：mAttachedScrap生命周期起始于RecyclerView布局开始，终止于RecyclerView布局结束。
总结
经过四篇文章的分析，RecyclerVeiw的四级缓存都分析完了，总结如下：
Recycler有4个层次用于缓存ViewHolder对象，优先级从高到底依次为ArrayList<ViewHolder> mAttachedScrap、ArrayList<ViewHolder> mCachedViews、ViewCacheExtension mViewCacheExtension、RecycledViewPool mRecyclerPool。如果四层缓存都未命中，则重新创建并绑定ViewHolder对象缓存性能：
缓存 重新创建 ViewHolder重新绑定数据 mAttachedScrap false false mCachedViews false false mRecyclerPool false true
缓存容量：
mAttachedScrap：没有大小限制，但最多包含屏幕可见表项。mCachedViews：默认大小限制为2，放不下时，按照先进先出原则将最先进入的ViewHolder存入回收池以腾出空间。mRecyclerPool：对ViewHolder按viewType分类存储（通过SparseArray），同类ViewHolder存储在默认大小为5的ArrayList中。
缓存用途：
mAttachedScrap：用于布局过程中屏幕可见表项的回收和复用。mCachedViews：用于移出屏幕表项的回收和复用，且只能用于指定位置的表项，有点像“回收池预备队列”，即总是先回收到mCachedViews，当它放不下的时候，按照先进先出原则将最先进入的ViewHolder存入回收池。mRecyclerPool：用于移出屏幕表项的回收和复用，且只能用于指定viewType的表项
缓存结构：
mAttachedScrap：ArrayList<ViewHolder>mCachedViews：ArrayList<ViewHolder>mRecyclerPool：对ViewHolder按viewType分类存储在SparseArray<ScrapData>中，同类ViewHolder存储在ScrapData中的ArrayList中
作者：唐子玄
链接：https://www.jianshu.com/p/1184cdc5dda8
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