源码版本：androidx1.3.2
分析场景：
RecyclerView 使用线性布局，方向为竖直方向，布局从上到下。第一次设置 LayoutManager 和 Adapter ，正常显示数据以后，滚动 RecyclerView。
先说下结论：
- 在正常滚动过程中，RecyclerView ViewHolder 的回收和复用只涉及 Recycler.mCachedViews、Recycler.mRecyclerPool。
- RecyclerView会回收滑出屏幕的View，当回收一个ViewHolder A 的时候，如果如果mCachedView缓存已达上限(默认是2)，会调用 addViewHolderToRecycledViewPool 从mCachedViews中移除最老的ViewHolder，添加到RecyclerViewPool中(每种ItemViewType的缓存池大小默认是5)。然后把A加入到Recycler.mCachedViews。
- 从 Recycler.mCachedViews 返回的 ViewHolder 是不会重新 onBindViewHolder 的。
- 从 RecyclerViewPool 返回的 ViewHolder 是会重新 onBindViewHolder 的。
其他注意的点: 在调试过程中发现，GapWorker 的 prefetchPositionWithDeadline 方法干扰到 RecyclerView 的回收和复用过程。有时候会导致 导致 Recycler.mCachedViews的 size变成了3(默认是2)。这个不是主要流程，我们先忽略，有兴趣可以仔细研究研究。
RecyclerView的缓存的 ViewHolder 的地方有 Recycler.mAttachedScrap、Recycler.mChangedScrap、Recycler.mCachedViews、Recycler.mRecyclerPool。
缓存 View 的地方有 Recycler.mViewCacheExtension、ChildHelper.mHiddenViews。 如下所示：
public final class Recycler {final ArrayList<ViewHolder> mAttachedScrap = new ArrayList<>();ArrayList<ViewHolder> mChangedScrap = null;final ArrayList<ViewHolder> mCachedViews = new ArrayList<ViewHolder>();private final List<ViewHolder>mUnmodifiableAttachedScrap = Collections.unmodifiableList(mAttachedScrap);RecycledViewPool mRecyclerPool;private ViewCacheExtension mViewCacheExtension;//...
}
class ChildHelper {private final List<View> mHiddenViews = new ArrayList<View>();//...
}
本篇文章只分析在滚动的时候的回收和复用。只涉及 Recycler.mCachedViews、Recycler.mRecyclerPool 。在后续的文章中，会进行一个总结说明每一个缓存的作用。
在滚动时候的回收
在RecyclerView滚动的时候，会调用 LinearLayoutManager 的fill方法。fill方法内部会发生View的回收和复用。
/*** @param recycler 当前关联到RecyclerView的recycler。* @param layoutState 该如何填充可用空间的配置信息。* @param state Context passed by the RecyclerView to control scroll steps.* @param stopOnFocusable 如果为true的话，遇到第一个可获取焦点的View则停止填充。* @return 返回添加的像素。*/
int fill(RecyclerView.Recycler recycler, LayoutState layoutState,RecyclerView.State state, boolean stopOnFocusable) {// max offset we should set is mFastScroll + availablefinal int start = layoutState.mAvailable;//...//注释0处，如果layoutState.mScrollingOffset不为SCROLLING_OFFSET_NaN的话，调用recycleByLayoutState方法，从这个方法名，我们可以看出来，这是一个回收View的方法。if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {//...recycleByLayoutState(recycler, layoutState);}int remainingSpace = layoutState.mAvailable + layoutState.mExtraFillSpace;LayoutChunkResult layoutChunkResult = mLayoutChunkResult;while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {layoutChunkResult.resetInternal();//注释1处，获取并添加子View，然后测量、布局子View并将分割线考虑在内。layoutChunk(recycler, state, layoutState, layoutChunkResult);//条件满足的话，跳出循环if (layoutChunkResult.mFinished) {break;}layoutState.mOffset += layoutChunkResult.mConsumed * layoutState.mLayoutDirection;//注释2处，这里也会判断是否要调用recycleByLayoutState方法。//在滚动的时候，会把滑出屏幕的View回收。if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {layoutState.mScrollingOffset += layoutChunkResult.mConsumed;if (layoutState.mAvailable < 0) {layoutState.mScrollingOffset += layoutState.mAvailable;}recycleByLayoutState(recycler, layoutState);}if (stopOnFocusable && layoutChunkResult.mFocusable) {break;}}return start - layoutState.mAvailable;
}
注释0处和注释2处，调用 recycleByLayoutState 方法，从这个方法名可以看出来，这是一个回收View的方法。接下来我们看看其中的细节。
LinearLayoutManager 的 recycleByLayoutState 方法。
private void recycleByLayoutState(RecyclerView.Recycler recycler, LayoutState layoutState) {if (!layoutState.mRecycle || layoutState.mInfinite) {return;}int scrollingOffset = layoutState.mScrollingOffset;int noRecycleSpace = layoutState.mNoRecycleSpace;if (layoutState.mLayoutDirection == LayoutState.LAYOUT_START) {//注释1处recycleViewsFromEnd(recycler, scrollingOffset, noRecycleSpace);} else {//注释2处recycleViewsFromStart(recycler, scrollingOffset, noRecycleSpace);}
}
注释1处，以默认竖直方向的LinearLayoutManager来说，就是手指从上向下滑动的时候，回收从下面滑出屏幕的View。
注释2处，以默认竖直方向的LinearLayoutManager来说，就是手指从下向上滑动的时候，回收从上面滑出屏幕的View。
LinearLayoutManager的recycleViewsFromEnd方法。
private void recycleViewsFromEnd(RecyclerView.Recycler recycler, int scrollingOffset,int noRecycleSpace) {final int childCount = getChildCount();if (scrollingOffset < 0) {return;}final int limit = mOrientationHelper.getEnd() - scrollingOffset + noRecycleSpace;if (mShouldReverseLayout) {//...} else {//从后向前遍历for (int i = childCount - 1; i >= 0; i--) {View child = getChildAt(i);if (mOrientationHelper.getDecoratedStart(child) < limit|| mOrientationHelper.getTransformedStartWithDecoration(child) < limit) {//注释1处，遇到第一个View的top坐标小于limit就停止。然后调用recycleChildren方法，回收从childCount - 1到i之间的所有View。recycleChildren(recycler, childCount - 1, i);return;}}}
}
注释1处，遇到第一个View的top坐标小于limit就停止。然后调用recycleChildren方法，回收从 [childCount - 1,i) 左闭右开之间的所有View。
LinearLayoutManager的recycleViewsFromStart方法。
private void recycleViewsFromStart(RecyclerView.Recycler recycler, int scrollingOffset,int noRecycleSpace) {if (scrollingOffset < 0) {return;}// ignore padding, ViewGroup may not clip children.final int limit = scrollingOffset - noRecycleSpace;final int childCount = getChildCount();if (mShouldReverseLayout) {//...} else {for (int i = 0; i < childCount; i++) {View child = getChildAt(i);if (mOrientationHelper.getDecoratedEnd(child) > limit|| mOrientationHelper.getTransformedEndWithDecoration(child) > limit) {//注释1处，遇到第一个View的bottom坐标大于limit就停止。然后调用recycleChildren方法，回收从0到i之间的所有View。recycleChildren(recycler, 0, i);return;}}}
}
注释1处，遇到第一个View的bottom坐标大于limit就停止。然后调用recycleChildren方法，回收从 [0,i) 左闭右开之间的所有View。
接下来我们看看recycleChildren方法。
LinearLayoutManager的recycleChildren方法。
/*** 回收指定索引之间的子View。** @param startIndex 包括* @param endIndex 不包括*/
private void recycleChildren(RecyclerView.Recycler recycler, int startIndex, int endIndex) {//startIndex == endIndex的话，直接return。比如滚动距离很短，没有View需要回收。if (startIndex == endIndex) {return;}if (endIndex > startIndex) {for (int i = endIndex - 1; i >= startIndex; i--) {removeAndRecycleViewAt(i, recycler);}} else {for (int i = startIndex; i > endIndex; i--) {removeAndRecycleViewAt(i, recycler);}}
}
调用父类RecyclerView.LayoutManager的removeAndRecycleViewAt方法。
public void removeAndRecycleViewAt(int index, @NonNull Recycler recycler) {final View view = getChildAt(index);//注释1处，调用RecyclerView.LayoutManager的removeViewAt方法移除View。removeViewAt(index);//注释2处，回收View。recycler.recycleView(view);
}
注释1处，调用RecyclerView.LayoutManager的removeViewAt方法移除View。 最终会调用 RecyclerView.this.removeViewAt(index);
注释2处，回收View。
Recycler的recycleView方法。
public void recycleView(@NonNull View view) {// This public recycle method tries to make view recycle-able since layout manager// intended to recycle this view (e.g. even if it is in scrap or change cache)//获取ViewHolderViewHolder holder = getChildViewHolderInt(view);if (holder.isTmpDetached()) {removeDetachedView(view, false);}if (holder.isScrap()) {//注释1处，将ViewHolder从mChangedScrap或者mAttachedScrap中移除。holder.unScrap();} else if (holder.wasReturnedFromScrap()) {holder.clearReturnedFromScrapFlag();}//注释2处，调用recycleViewHolderInternal方法。recycleViewHolderInternal(holder);if (mItemAnimator != null && !holder.isRecyclable()) {mItemAnimator.endAnimation(holder);}
}
注释1处，如果ViewHolder是从 mChangedScrap中或者mAttachedScrap 返回的，则将ViewHolder从mChangedScrap或者mAttachedScrap中移除。也就是说滑出屏幕的ViewHolder不会缓存在mChangedScrap中或者mAttachedScrap中。
对于我们分析的这个场景，mChangedScrap中或者mAttachedScrap 是没有 ViewHolder 的。条件不满足。
注释2处，调用 Recycler 的recycleViewHolderInternal方法。
void recycleViewHolderInternal(ViewHolder holder) {//这里判断了在mChangedScrap中或者mAttachedScrap中的ViewHolder不会被回收，没有被移除的子View对应的ViewHolder也不能被回收。if (holder.isScrap() || holder.itemView.getParent() != null) {throw new IllegalArgumentException("Scrapped or attached views may not be recycled. isScrap:"+ holder.isScrap() + " isAttached:"+ (holder.itemView.getParent() != null) + exceptionLabel());}//临时从屏幕上detach的ViewHolder也不能被回收。if (holder.isTmpDetached()) {throw new IllegalArgumentException("Tmp detached view should be removed "+ "from RecyclerView before it can be recycled: " + holder+ exceptionLabel());}if (holder.shouldIgnore()) {throw new IllegalArgumentException("Trying to recycle an ignored view holder. You"+ " should first call stopIgnoringView(view) before calling recycle."+ exceptionLabel());}//是否阻止回收final boolean transientStatePreventsRecycling = holder.doesTransientStatePreventRecycling();//是否强制回收final boolean forceRecycle = mAdapter != null&& transientStatePreventsRecycling&& mAdapter.onFailedToRecycleView(holder);boolean cached = false;boolean recycled = false;if (DEBUG && mCachedViews.contains(holder)) {throw new IllegalArgumentException("cached view received recycle internal? "+ holder + exceptionLabel());}//可以回收if (forceRecycle || holder.isRecyclable()) {if (mViewCacheMax > 0&& !holder.hasAnyOfTheFlags(ViewHolder.FLAG_INVALID| ViewHolder.FLAG_REMOVED| ViewHolder.FLAG_UPDATE| ViewHolder.FLAG_ADAPTER_POSITION_UNKNOWN)) {int cachedViewSize = mCachedViews.size();if (cachedViewSize >= mViewCacheMax && cachedViewSize > 0) {//注释1处，如果mCachedView缓存已达上限，从mCachedViews中移除最老的ViewHolder到RecyclerViewPool中recycleCachedViewAt(0);cachedViewSize--;}int targetCacheIndex = cachedViewSize;if (ALLOW_THREAD_GAP_WORK&& cachedViewSize > 0&& !mPrefetchRegistry.lastPrefetchIncludedPosition(holder.mPosition)) {// when adding the view, skip past most recently prefetched viewsint cacheIndex = cachedViewSize - 1;while (cacheIndex >= 0) {int cachedPos = mCachedViews.get(cacheIndex).mPosition;if (!mPrefetchRegistry.lastPrefetchIncludedPosition(cachedPos)) {break;}cacheIndex--;}targetCacheIndex = cacheIndex + 1;}//注释2处，将要回收的ViewHolder加入mCachedViewsmCachedViews.add(targetCacheIndex, holder);cached = true;}if (!cached) {//注释3处，没有成功缓存到mCachedViews，则加入到RecycledViewPool中。addViewHolderToRecycledViewPool(holder, true);recycled = true;}} else {//...}// even if the holder is not removed, we still call this method so that it is removed// from view holder lists.//跟动画相关的ViewHolder也从mViewInfoStore移除。mViewInfoStore.removeViewHolder(holder);//...
}
注释1处，如果mCachedView缓存已达上限，从mCachedViews中移除最老的ViewHolder到RecyclerViewPool中。
Recycler的recycleCachedViewAt方法。
void recycleCachedViewAt(int cachedViewIndex) {//从mCachedViews中获取ViewHolderViewHolder viewHolder = mCachedViews.get(cachedViewIndex);//加入到RecycledViewPooladdViewHolderToRecycledViewPool(viewHolder, true);//mCachedViews移除对应位置上的ViewHoldermCachedViews.remove(cachedViewIndex);
}
内部调用了Recycler的addViewHolderToRecycledViewPool方法。
void addViewHolderToRecycledViewPool(@NonNull ViewHolder holder, boolean dispatchRecycled) {clearNestedRecyclerViewIfNotNested(holder);View itemView = holder.itemView;//...if (dispatchRecycled) {dispatchViewRecycled(holder);}holder.mOwnerRecyclerView = null;//注释1处，调用RecycledViewPool的putRecycledView方法。getRecycledViewPool().putRecycledView(holder);
}
注释1处，调用RecycledViewPool的putRecycledView方法。
/*** 将废弃的ViewHolder加入到缓存池。* 如果ViewHolder对应的ViewType类型的缓存池已经满了，就直接将ViewHolder丢弃。** @param scrap ViewHolder to be added to the pool.*/
public void putRecycledView(ViewHolder scrap) {final int viewType = scrap.getItemViewType();final ArrayList <ViewHolder> scrapHeap = getScrapDataForType(viewType).mScrapHeap;if(mScrap.get(viewType).mMaxScrap <= scrapHeap.size()) {//注释1处，如果超过最大限制(默认是5)，直接丢弃。PoolingContainer.callPoolingContainerOnRelease(scrap.itemView);return;}if(sDebugAssertionsEnabled && scrapHeap.contains(scrap)) {throw new IllegalArgumentException("this scrap item already exists");}scrap.resetInternal();//加入到缓存池scrapHeap.add(scrap);
}
Recycler的 recycleViewHolderInternal 方法注释2处，先将要回收的ViewHolder加入mCachedViews中。
注释3处，没有成功缓存到 mCachedViews，则加入到RecycledViewPool中。比如说我们设置 mCachedViews 的大小为0的时候，那么就不会缓存到 mCachedViews 中，直接加入到 RecycledViewPool 中。
或者调用 notifyDataSetChanged，notifyItemChanged , notifyItemRemoved 的时候也不会不会缓存到 mCachedViews 中。因为此时，这个条件不满足。
!holder.hasAnyOfTheFlags(ViewHolder.FLAG_INVALID| ViewHolder.FLAG_REMOVED| ViewHolder.FLAG_UPDATE| ViewHolder.FLAG_ADAPTER_POSITION_UNKNOWN)
在滚动时候的复用
LinearLayoutManager 的 layoutChunk 方法。
void layoutChunk(RecyclerView.Recycler recycler, RecyclerView.State state,LayoutState layoutState, LayoutChunkResult result) {//注释1处，获取子View，可能是从缓存中或者新创建的View。View view = layoutState.next(recycler);if (view == null) {//注释2处，如果获取到的子View为null，将LayoutChunkResult的mFinished置为true，用于跳出循环然后直接return。result.mFinished = true;return;}RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) view.getLayoutParams();if (layoutState.mScrapList == null) {if (mShouldReverseLayout == (layoutState.mLayoutDirection== LayoutState.LAYOUT_START)) {//注释3处addView(view); } else {//注释4处addView(view, 0);}} //...}
上篇文章 RecyclerView 源码分析之一 我们分析了layoutChunk方法。在注释1处，获取子View，可能是从缓存中或者新创建的View。我们现在就看一看其中细节。
View view = layoutState.next(recycler);
LayoutState 的 next 方法。
View next(RecyclerView.Recycler recycler) {if (mScrapList != null) {//在我们的分析场景中为nullreturn nextViewFromScrapList();}//注释1处，调用Recycler的getViewForPosition方法获取View，mCurrentPosition是当前的位置。final View view = recycler.getViewForPosition(mCurrentPosition);mCurrentPosition += mItemDirection;return view;
}
注释1处，调用Recycler的getViewForPosition方法获取View，mCurrentPosition是当前的位置。
@NonNull
public View getViewForPosition(int position) {return getViewForPosition(position, false);
}View getViewForPosition(int position, boolean dryRun) {return tryGetViewHolderForPositionByDeadline(position, dryRun, FOREVER_NS).itemView;
}
tryGetViewHolderForPositionByDeadline方法里面的逻辑就是先从缓存里面取ViewHolder进行复用，如果没有可复用的ViewHolder，则进行创建。
@Nullable
ViewHolder tryGetViewHolderForPositionByDeadline(int position,boolean dryRun, long deadlineNs) {if (position < 0 || position >= mState.getItemCount()) {throw new IndexOutOfBoundsException("Invalid item position " + position+ "(" + position + "). Item count:" + mState.getItemCount()+ exceptionLabel());}boolean fromScrapOrHiddenOrCache = false;ViewHolder holder = null;// 注释0处，预布局的时候，从mChangedScrap中获取ViewHolder。 正常滚动的时候，不是预布局状态，不会从这里找。if (mState.isPreLayout()) {holder = getChangedScrapViewForPosition(position);fromScrapOrHiddenOrCache = holder != null;}// 1) Find by position from scrap/hidden list/cache//注释1处，从Recycler.mAttachedScrap、ChildHelper.mHiddenViews、Recycler.mCachedViews中获取ViewHolderif (holder == null) {holder = getScrapOrHiddenOrCachedHolderForPosition(position, dryRun);if (holder != null) {if (!validateViewHolderForOffsetPosition(holder)) {// recycle holder (and unscrap if relevant) since it can't be usedif (!dryRun) {// we would like to recycle this but need to make sure it is not used by// animation logic etc.holder.addFlags(ViewHolder.FLAG_INVALID);if (holder.isScrap()) {removeDetachedView(holder.itemView, false);holder.unScrap();} else if (holder.wasReturnedFromScrap()) {holder.clearReturnedFromScrapFlag();}recycleViewHolderInternal(holder);}holder = null;} else {fromScrapOrHiddenOrCache = true;}}}if (holder == null) {final int offsetPosition = mAdapterHelper.findPositionOffset(position);if (offsetPosition < 0 || offsetPosition >= mAdapter.getItemCount()) {throw new IndexOutOfBoundsException("Inconsistency detected. Invalid item "+ "position " + position + "(offset:" + offsetPosition + ")."+ "state:" + mState.getItemCount() + exceptionLabel());}//获取ViewTypefinal int type = mAdapter.getItemViewType(offsetPosition);// 2) Find from scrap/cache via stable ids, if exists//注释2处，通过stable ids从mAttachedScrap、mCachedViews中获取ViewHolder。if (mAdapter.hasStableIds()) {holder = getScrapOrCachedViewForId(mAdapter.getItemId(offsetPosition),type, dryRun);if (holder != null) {// update positionholder.mPosition = offsetPosition;fromScrapOrHiddenOrCache = true;}}//注释3处，从我们自定义的缓存扩展mViewCacheExtension中获取ViewHolderif (holder == null && mViewCacheExtension != null) {// We are NOT sending the offsetPosition because LayoutManager does not// know it.final View view = mViewCacheExtension.getViewForPositionAndType(this, position, type);if (view != null) {holder = getChildViewHolder(view);if (holder == null) {throw new IllegalArgumentException("getViewForPositionAndType returned"+ " a view which does not have a ViewHolder"+ exceptionLabel());} else if (holder.shouldIgnore()) {throw new IllegalArgumentException("getViewForPositionAndType returned"+ " a view that is ignored. You must call stopIgnoring before"+ " returning this view." + exceptionLabel());}}}if (holder == null) { // fallback to poolif (DEBUG) {Log.d(TAG, "tryGetViewHolderForPositionByDeadline("+ position + ") fetching from shared pool");}//注释4处，从RecycledViewPool中获取ViewHolderholder = getRecycledViewPool().getRecycledView(type);if (holder != null) {holder.resetInternal();if (FORCE_INVALIDATE_DISPLAY_LIST) {invalidateDisplayListInt(holder);}}}if (holder == null) {long start = getNanoTime();if (deadlineNs != FOREVER_NS&& !mRecyclerPool.willCreateInTime(type, start, deadlineNs)) {// abort - we have a deadline we can't meetreturn null;}//注释5处，从适配器中创建一个新的ViewHolderholder = mAdapter.createViewHolder(RecyclerView.this, type);if (ALLOW_THREAD_GAP_WORK) {// only bother finding nested RV if prefetchingRecyclerView innerView = findNestedRecyclerView(holder.itemView);if (innerView != null) {holder.mNestedRecyclerView = new WeakReference<>(innerView);}}long end = getNanoTime();mRecyclerPool.factorInCreateTime(type, end - start);if (DEBUG) {Log.d(TAG, "tryGetViewHolderForPositionByDeadline created new ViewHolder");}}}// This is very ugly but the only place we can grab this information// before the View is rebound and returned to the LayoutManager for post layout ops.// We don't need this in pre-layout since the VH is not updated by the LM.if (fromScrapOrHiddenOrCache && !mState.isPreLayout() && holder.hasAnyOfTheFlags(ViewHolder.FLAG_BOUNCED_FROM_HIDDEN_LIST)) {holder.setFlags(0, ViewHolder.FLAG_BOUNCED_FROM_HIDDEN_LIST);if (mState.mRunSimpleAnimations) {int changeFlags = ItemAnimator.buildAdapterChangeFlagsForAnimations(holder);changeFlags |= ItemAnimator.FLAG_APPEARED_IN_PRE_LAYOUT;final ItemHolderInfo info = mItemAnimator.recordPreLayoutInformation(mState,holder, changeFlags, holder.getUnmodifiedPayloads());recordAnimationInfoIfBouncedHiddenView(holder, info);}}boolean bound = false;if (mState.isPreLayout() && holder.isBound()) {// do not update unless we absolutely have to.holder.mPreLayoutPosition = position;} else if (!holder.isBound() || holder.needsUpdate() || holder.isInvalid()) {if (DEBUG && holder.isRemoved()) {throw new IllegalStateException("Removed holder should be bound and it should"+ " come here only in pre-layout. Holder: " + holder+ exceptionLabel());}final int offsetPosition = mAdapterHelper.findPositionOffset(position);//注释6处，绑定ViewHolderbound = tryBindViewHolderByDeadline(holder, offsetPosition, position, deadlineNs);}final ViewGroup.LayoutParams lp = holder.itemView.getLayoutParams();final LayoutParams rvLayoutParams;if (lp == null) {rvLayoutParams = (LayoutParams) generateDefaultLayoutParams();holder.itemView.setLayoutParams(rvLayoutParams);} else if (!checkLayoutParams(lp)) {rvLayoutParams = (LayoutParams) generateLayoutParams(lp);holder.itemView.setLayoutParams(rvLayoutParams);} else {rvLayoutParams = (LayoutParams) lp;}rvLayoutParams.mViewHolder = holder;rvLayoutParams.mPendingInvalidate = fromScrapOrHiddenOrCache && bound;//最终返回ViewHolder对象。return holder;
}
注释0处，预布局的时候，从mChangedScrap中获取ViewHolder。 正常滚动的时候，不是预布局状态，不会从这里找。
注释1处，从Recycler.mAttachedScrap、ChildHelper.mHiddenViews、Recycler.mCachedViews中获取ViewHolder。
holder = getScrapOrHiddenOrCachedHolderForPosition(position, dryRun);
RecyclerView.Recycler的getScrapOrHiddenOrCachedHolderForPosition方法。
ViewHolder getScrapOrHiddenOrCachedHolderForPosition(int position, boolean dryRun) {final int scrapCount = mAttachedScrap.size();//注释1处，先从mAttachedScrap查找for (int i = 0; i < scrapCount; i++) {final ViewHolder holder = mAttachedScrap.get(i);if (!holder.wasReturnedFromScrap() && holder.getLayoutPosition() == position&& !holder.isInvalid() && (mState.mInPreLayout || !holder.isRemoved())) {//添加标志位FLAG_RETURNED_FROM_SCRAPholder.addFlags(ViewHolder.FLAG_RETURNED_FROM_SCRAP);return holder;}}if (!dryRun) {//注释2处，从mHiddenViews中查找View，然后根据View获取ViewHolderView view = mChildHelper.findHiddenNonRemovedView(position);if (view != null) {// This View is good to be used. We just need to unhide, detach and move to the// scrap list.final ViewHolder vh = getChildViewHolderInt(view);//将获取到的View从mHiddenViews移除。mChildHelper.unhide(view);int layoutIndex = mChildHelper.indexOfChild(view);if (layoutIndex == RecyclerView.NO_POSITION) {throw new IllegalStateException("layout index should not be -1 after "+ "unhiding a view:" + vh + exceptionLabel());}mChildHelper.detachViewFromParent(layoutIndex);//注释3处，将View对应的ViewHolder加入到mAttachedScrap或者mChangedScrap中scrapView(view);//添加标记位，多了一个FLAG_BOUNCED_FROM_HIDDEN_LIST的标记位vh.addFlags(ViewHolder.FLAG_RETURNED_FROM_SCRAP| ViewHolder.FLAG_BOUNCED_FROM_HIDDEN_LIST);return vh;}}//注释4处，从mCachedViews中获取final int cacheSize = mCachedViews.size();for (int i = 0; i < cacheSize; i++) {final ViewHolder holder = mCachedViews.get(i);// invalid view holders may be in cache if adapter has stable ids as they can be// retrieved via getScrapOrCachedViewForIdif (!holder.isInvalid() && holder.getLayoutPosition() == position&& !holder.isAttachedToTransitionOverlay()) {if (!dryRun) {//找到ViewHolder后，从mCachedViews中移除。mCachedViews.remove(i);}if (DEBUG) {Log.d(TAG, "getScrapOrHiddenOrCachedHolderForPosition(" + position+ ") found match in cache: " + holder);}return holder;}}return null;
}
注释1处，先从mAttachedScrap查找。正常在滚动状态的时候， mAttachedScrap 是空的。
注释2处，从mHiddenViews中查找View，然后根据View获取ViewHolder。正常在滚动状态的时候 mHiddenViews 也是空的。
注释4处，从mCachedViews中获取。正常在这里可以找到。从mCachedViews中获取的ViewHolder 不需要重新绑定。
RecyclerView.Recycler的tryGetViewHolderForPositionByDeadline方法的注释3处，从我们自定义的缓存扩展mViewCacheExtension中获取ViewHolder，这个自定义的ViewCacheExtension就先忽略了。
（你自定义过ViewCacheExtension吗？，没有。你自定义过？我也没有？正常人谁自定义ViewCacheExtension呀。是，自定义ViewCacheExtension的叫正常人吗？哈哈，纯属搞笑。）
RecyclerView.Recycler的tryGetViewHolderForPositionByDeadline方法的注释4处，从RecycledViewPool中获取ViewHolder。 从 RecycledViewPool 中获取的 ViewHolder，需要重新 bindViewHolder。
holder = getRecycledViewPool().getRecycledView(type);
RecycledView.RecycledViewPool类，这里提一下，RecycledViewPool可以用来在多个RecyclerView之间来复用View。
/*** RecycledViewPool可以让你在多个RecyclerViews之间复用Views。 lets you share Views between multiple RecyclerViews.* <p>* 如果你想在多个RecyclerView之间复用Views，可以创建一个RecycledViewPool实例，然后调用 RecyclerView的 setRecycledViewPool(RecycledViewPool) 方法设置。* <p>* 如果你不提供RecycledViewPool实例，RecyclerView会自动创建一个。*/
public static class RecycledViewPool {//每个viewType类型的ViewHolder默认缓存5个private static final int DEFAULT_MAX_SCRAP = 5;static class ScrapData {final ArrayList<ViewHolder> mScrapHeap = new ArrayList<>();int mMaxScrap = DEFAULT_MAX_SCRAP;long mCreateRunningAverageNs = 0;long mBindRunningAverageNs = 0;}//使用SparseArray缓存不ViewType类型的ViewHolder，SparseArray的key就是ViewType类SparseArray<ScrapData> mScrap = new SparseArray<>();@Nullablepublic ViewHolder getRecycledView(int viewType) {//注释1处，先根据ViewType类获取ScrapData，然后从ScrapData获取ViewHolder。final ScrapData scrapData = mScrap.get(viewType);if (scrapData != null && !scrapData.mScrapHeap.isEmpty()) {final ArrayList<ViewHolder> scrapHeap = scrapData.mScrapHeap;for (int i = scrapHeap.size() - 1; i >= 0; i--) {if (!scrapHeap.get(i).isAttachedToTransitionOverlay()) {return scrapHeap.remove(i);}}}return null;}//...
}
注释1处，从RecycledViewPool中获取ViewHolder，先根据ViewType类获取ScrapData，然后从ScrapData获取ViewHolder。
回到 RecyclerView.Recycler的tryGetViewHolderForPositionByDeadline方法的注释5处，无法从缓存中获取ViewHolder，就使用适配器中创建一个新的ViewHolder。
RecyclerView.Recycler的tryGetViewHolderForPositionByDeadline方法的注释6处，绑定ViewHolder。
bound = tryBindViewHolderByDeadline(holder, offsetPosition, position, deadlineNs);
Recycler的tryBindViewHolderByDeadline方法。
private boolean tryBindViewHolderByDeadline(@NonNull ViewHolder holder, int offsetPosition,int position, long deadlineNs) {holder.mOwnerRecyclerView = RecyclerView.this;final int viewType = holder.getItemViewType();long startBindNs = getNanoTime();if (deadlineNs != FOREVER_NS&& !mRecyclerPool.willBindInTime(viewType, startBindNs, deadlineNs)) {// abort - we have a deadline we can't meetreturn false;}//适配器绑定ViewHoldermAdapter.bindViewHolder(holder, offsetPosition);long endBindNs = getNanoTime();mRecyclerPool.factorInBindTime(holder.getItemViewType(), endBindNs - startBindNs);attachAccessibilityDelegateOnBind(holder);if (mState.isPreLayout()) {holder.mPreLayoutPosition = position;}return true;
}
内部调用了适配器的 bindViewHolder 方法。
参考链接：
- RecyclerView 源码分析之一
--中断概述)
