RecyclerView的回收缓存均由内部类Recycler完成
1. RecyclerView的三级缓存
通常在RecyclerView中存在着四级缓存，从低到高分别为：
-
可直接重复使用的临时缓存（
mAttachedScrap/mChangedScrap）mAttachedScrap中缓存的是屏幕中可见范围的ViewHoldermChangedScrap只能在预布局状态下重用，因为它里面装的都是即将要放到mRecyclerPool中的Holder，而mAttachedScrap则可以在非预布局状态下重用
-
可重用的缓存（
mCachedViews）：缓存滑动时即将与RecyclerView分离的ViewHolder，默认最大2个； -
自定义实现的缓存（
ViewCacheExtension）：通常忽略； -
需要重新绑定数据的缓存（
RecycledViewPool）：ViewHolder缓存池，可以支持不同的ViewType，返回的ViewHolder需要重新Bind数据；
由于绝大多数情况下无需自定义缓存，因此通常我们说RecyclerView有三级缓存
1.1 预布局（PreLayout）
- 含义：真正布局前的一次布局
- 触发时机：只有在Adapter的数据集更新的前提下，调用除了
notifyDataSetChanged以外的一系列notify方法，预布局才会生效 - 作用：当Item被删除或者添加时，预布局和真正的布局会生成不同的Item快照，从而根据两个快照执行动画
- 示例： 删除item2 ： 首先预布局就是先布局一次， 形成一个快照(
pre-layout) 如item1234然后再布局一次(post-layout) 形成另外一张快照item134这样我们其实就知道了整个动画轨迹 就可以生成动画
- 如何开启预布局：重写
LayoutManager的**supportsPredictiveItemAnimations**方法并return true，自带的三个LayoutManager已经开启来这个效果
2. 缓存机制
为了方便，我们不讨论预布局的过程，首先我们要了解三级缓存对象：mAttachedScrap，mCachedViews，mRecyclerPool，其中mAttachedScrap和mCachedViews都是ArrayList对象，而mRecyclerPool是RecyclerViewPool对象：
public static class RecycledViewPool {
private static final int DEFAULT_MAX_SCRAP = 5;
static class ScrapData {
final ArrayList mScrapHeap = new ArrayList<>();
int mMaxScrap = DEFAULT_MAX_SCRAP;
long mCreateRunningAverageNs = 0;
long mBindRunningAverageNs = 0;
}
SparseArray mScrap = new SparseArray<>();
private int mAttachCount = 0;
/**
* Acquire a ViewHolder of the specified type from the pool, or {@code null} if none are
* present.
*
* @param viewType ViewHolder type.
* @return ViewHolder of the specified type acquired from the pool, or {@code null} if none
* are present.
*/
@Nullable
public ViewHolder getRecycledView(int viewType) {
final ScrapData scrapData = mScrap.get(viewType);
if (scrapData != null && !scrapData.mScrapHeap.isEmpty()) {
final ArrayList scrapHeap = scrapData.mScrapHeap;
for (int i = scrapHeap.size() - 1; i >= 0; i--) {
if (!scrapHeap.get(i).isAttachedToTransitionOverlay()) {
return scrapHeap.remove(i);
}
}
}
return null;
}
...
可以发现
-
RecyclerViewPool中有一个SparseArray对象，而ScrapData对象中的mScrapHeap就是缓存ViewHolder的ArrayList -
当从
RecyclerViewPool中获取ViewHolder时，是通过list.remove(index)方法
2.1 缓存机制起点：tryGetViewHolderForPositionByDeadline
当RecyclerView绘制的时候，会走到LayoutManager里面的next()方法，在next()里面是正式开始使用缓存机制，这里以LinearLayoutManager为例子：
onLayoutChildren -> fill -> layoutChunk -> LayoutState.next
//com.android.internal.widget.LinearLayoutManager.LayoutState#next
View next(RecyclerView.Recycler recycler) {
if (mScrapList != null) {
return nextViewFromScrapList();
}
final View view = recycler.getViewForPosition(mCurrentPosition);
mCurrentPosition += mItemDirection;
return view;
}
而getViewForPosition 又会调用到tryGetViewHolderForPositionByDeadline方法：
//com.android.internal.widget.RecyclerView.Recycler#tryGetViewHolderForPositionByDeadline
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());
}
boolean fromScrapOrHiddenOrCache = false;
ViewHolder holder = null;
// 0) If there is a changed scrap, try to find from there
if (mState.isPreLayout()) {
holder = getChangedScrapViewForPosition(position);
fromScrapOrHiddenOrCache = holder != null;
}
// 1) Find by position from scrap/hidden list/cache
if (holder == null) {
holder = getScrapOrHiddenOrCachedHolderForPosition(position, dryRun);
if (holder != null) {
if (!validateViewHolderForOffsetPosition(holder)) {
// recycle holder (and unscrap if relevant) since it can't be used
if (!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());
}
final int type = mAdapter.getItemViewType(offsetPosition);
// 2) Find from scrap/cache via stable ids, if exists
if (mAdapter.hasStableIds()) {
holder = getScrapOrCachedViewForId(mAdapter.getItemId(offsetPosition),
type, dryRun);
if (holder != null) {
// update position
holder.mPosition = offsetPosition;
fromScrapOrHiddenOrCache = true;
}
}
if (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");
} else if (holder.shouldIgnore()) {
throw new IllegalArgumentException("getViewForPositionAndType returned"
+ " a view that is ignored. You must call stopIgnoring before"
+ " returning this view.");
}
}
}
if (holder == null) { // fallback to pool
if (DEBUG) {
Log.d(TAG, "tryGetViewHolderForPositionByDeadline("
+ position + ") fetching from shared pool");
}
holder = 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 meet
return null;
}
holder = mAdapter.createViewHolder(RecyclerView.this, type);
if (ALLOW_THREAD_GAP_WORK) {
// only bother finding nested RV if prefetching
RecyclerView 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);
}
final int offsetPosition = mAdapterHelper.findPositionOffset(position);
bound = 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;
return holder;
}
2.2 根据位置从mAttachedScrap和mCachedViews中获取View Holder
-
2.2.1 getScrapOrHiddenOrCachedHolderForPosition
ViewHolder getScrapOrHiddenOrCachedHolderForPosition(int position, boolean dryRun) {
final int scrapCount = mAttachedScrap.size();
// Try first for an exact, non-invalid match from scrap.
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())) {
holder.addFlags(ViewHolder.FLAG_RETURNED_FROM_SCRAP);
return holder;
}
}
if (!dryRun) {
View view = mChildHelper.findHiddenNonRemovedView(position);
if (view != null) {
...
// 可忽略
return vh;
}
}
// Search in our first-level recycled view cache.
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 getScrapOrCachedViewForId
if (!holder.isInvalid() && holder.getLayoutPosition() == position) {
if (!dryRun) {
mCachedViews.remove(i);
}
if (DEBUG) {
Log.d(TAG, "getScrapOrHiddenOrCachedHolderForPosition(" + position
+ ") found match in cache: " + holder);
}
return holder;
}
}
return null;
}
-
第一步先从
mAttachedScrap中获取viewHolder：如果可以拿到指定的viewHolder，接着判断：- 如果
viewHolder不是从Scrap中返回的（标志位不包含FLAG_RETURNED_FROM_SCRAP）； viewHolder.mPosition == positionviewHolder为有效- 为预布局或者
viewHolder的状态不是removed
- 如果
-
如果第一步没有获得有效的
viewHolder，第二步从mChildHelper中获取Hidden状态的View，但通常RecyclerView中没有Hidden状态的View，因为在addView的时候，就将hide值置为false -
如果前两步都没有获得有效的viewHolder，则会从
mCachedView中取得viewHolder，由于dryRun参数值默认为false，所以取出来的viewHolder会被remove掉
2.1.2 检查viewHolder是否有效
#validateViewHolderForOffsetPosition：
boolean validateViewHolderForOffsetPosition(ViewHolder holder) {
// if it is a removed holder, nothing to verify since we cannot ask adapter anymore
// if it is not removed, verify the type and id.
if (holder.isRemoved()) {
if (DEBUG && !mState.isPreLayout()) {
throw new IllegalStateException("should not receive a removed view unless it"
+ " is pre layout");
}
return mState.isPreLayout();
}
if (holder.mPosition < 0 || holder.mPosition >= mAdapter.getItemCount()) {
throw new IndexOutOfBoundsException("Inconsistency detected. Invalid view holder "
+ "adapter position" + holder);
}
if (!mState.isPreLayout()) {
// don't check type if it is pre-layout.
final int type = mAdapter.getItemViewType(holder.mPosition);
if (type != holder.getItemViewType()) {
return false;
}
}
if (mAdapter.hasStableIds()) {
return holder.getItemId() == mAdapter.getItemId(holder.mPosition);
}
return true;
}
如果步骤1 返回null，那么会进行步骤2.3：
2.3 根据Id从mAttachedScrap和mCachedViews中获取viewHolder
// 2) Find from scrap/cache via stable ids, if exists
if (mAdapter.hasStableIds()) {
holder = getScrapOrCachedViewForId(mAdapter.getItemId(offsetPosition),
type, dryRun);
if (holder != null) {
// update position
holder.mPosition = offsetPosition;
fromScrapOrHiddenOrCache = true;
}
}
- 可以看到，当Adapter有固定ID时，我们可以根据Id来获取viewHolder，但是默认情况下，需要我们进行以下两个步骤才会生效：
-
-
设置
Apapter.setHasStableIds(true) -
重写
Adapter.getItemId(position)方法
-
//com.android.internal.widget.RecyclerView.Recycler#getScrapOrCachedViewForId
ViewHolder getScrapOrCachedViewForId(long id, int type, boolean dryRun) {
// Look in our attached views first
final int count = mAttachedScrap.size();
for (int i = count - 1; i >= 0; i--) {
final ViewHolder holder = mAttachedScrap.get(i);
if (holder.getItemId() == id && !holder.wasReturnedFromScrap()) {
if (type == holder.getItemViewType()) {
holder.addFlags(ViewHolder.FLAG_RETURNED_FROM_SCRAP);
if (holder.isRemoved()) {
// this might be valid in two cases:
// > item is removed but we are in pre-layout pass
// >> do nothing. return as is. make sure we don't rebind
// > item is removed then added to another position and we are in
// post layout.
// >> remove removed and invalid flags, add update flag to rebind
// because item was invisible to us and we don't know what happened in
// between.
if (!mState.isPreLayout()) {
holder.setFlags(ViewHolder.FLAG_UPDATE, ViewHolder.FLAG_UPDATE
| ViewHolder.FLAG_INVALID | ViewHolder.FLAG_REMOVED);
}
}
return holder;
} else if (!dryRun) {
// if we are running animations, it is actually better to keep it in scrap
// but this would force layout manager to lay it out which would be bad.
// Recycle this scrap. Type mismatch.
mAttachedScrap.remove(i);
removeDetachedView(holder.itemView, false);
quickRecycleScrapView(holder.itemView);
}
}
}
// Search the first-level cache
final int cacheSize = mCachedViews.size();
for (int i = cacheSize - 1; i >= 0; i--) {
final ViewHolder holder = mCachedViews.get(i);
if (holder.getItemId() == id) {
if (type == holder.getItemViewType()) {
if (!dryRun) {
mCachedViews.remove(i);
}
return holder;
} else if (!dryRun) {
recycleCachedViewAt(i);
return null;
}
}
}
return null;
}
根据Id从mAttachedScrap和mCachedViews中获取viewHolder，如果获取到了并且ItemViewType一致，则直接返回
2.4 从mRecyclerPool中获取viewHolder
//3.1 获取viewHolder
holder = getRecycledViewPool().getRecycledView(type);
if (holder != null) {
// 3.2 重置viewHolder
holder.resetInternal();
if (FORCE_INVALIDATE_DISPLAY_LIST) {
invalidateDisplayListInt(holder);
}
}
2.4.1 获取viewHolder：getRecycledView
//com.android.internal.widget.RecyclerView.RecycledViewPool#getRecycledView
public ViewHolder getRecycledView(int viewType) {
// 根据viewType获取对应的viewHolder缓存
final ScrapData scrapData = mScrap.get(viewType);
if (scrapData != null && !scrapData.mScrapHeap.isEmpty()) {
final ArrayList scrapHeap = scrapData.mScrapHeap;
return scrapHeap.remove(scrapHeap.size() - 1);
}
return null;
}
2.4.2 重置viewHolder状态
void resetInternal() {
mFlags = 0;
mPosition = NO_POSITION;
mOldPosition = NO_POSITION;
mItemId = NO_ID;
mPreLayoutPosition = NO_POSITION;
mIsRecyclableCount = 0;
mShadowedHolder = null;
mShadowingHolder = null;
clearPayload();
mWasImportantForAccessibilityBeforeHidden = View.IMPORTANT_FOR_ACCESSIBILITY_AUTO;
mPendingAccessibilityState = PENDING_ACCESSIBILITY_STATE_NOT_SET;
clearNestedRecyclerViewIfNotNested(this);
}
2.5 创建新的viewHolder
如果前三步得到的holder为空，则需要新建一个viewHolder：
holder = mAdapter.createViewHolder(RecyclerView.this, type);
2.6 根据viewHolder状态确定是否需要调用bindViewHolder
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);
}
final int offsetPosition = mAdapterHelper.findPositionOffset(position);
// 重写bindView
bound = tryBindViewHolderByDeadline(holder, offsetPosition, position, deadlineNs);
}
private boolean tryBindViewHolderByDeadline(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 meet
return false;
}
// 熟悉的bindViewHolder
mAdapter.bindViewHolder(holder, offsetPosition);
long endBindNs = getNanoTime();
mRecyclerPool.factorInBindTime(holder.getItemViewType(), endBindNs - startBindNs);
attachAccessibilityDelegate(holder.itemView);
if (mState.isPreLayout()) {
holder.mPreLayoutPosition = position;
}
return true;
}
-
回收机制
3.1 回收方法
LayoutManager提供了各种回收方法:
detachAndScrapView(View child, Recycler recycler)
detachAndScrapViewAt(int index, Recycler recycler)
detachAndScrapAttachedViews(Recycler recycler)
removeAndRecycleView(View child, Recycler recycler)
removeAndRecycleViewAt(int index, Recycler recycler)
removeAndRecycleAllViews(Recycler recycler)
-
前三个方法负责将View回收到一级缓存（
Recycler.mAttachedMap）中，而一级缓存只是一个临时缓存，用于初始化或者数据集变化时，将所有的View放到临时放到缓存中，即只在布局（调用****onLayoutChildren）时才会调用（detachAndScrapAttachedViews） ，detachAndScrapView/detachAndScrapViewAt没有看到有调用的地方）。 -
后三个方法负责将View回收到二级缓存（
mCachedViews）或者四级缓存(RecyclerViewPool)中，mCachedViews默认大小为2（但目前存在mPrefetchMaxCountObserved参数，值为1 ，所以mCachedViews大小可能为3）
3.2 回收时机
3.2.1 回收到一级缓存的时机
只有在Adapter数据集发生变化，调用各种notify方法，因此重新布局后才会对屏幕内的View回收到一级缓存（mAttachedMap）中，此时由onLayoutChildren发起，调用detachAndScrapAttachedViews方法，进而调用到scrapOrRecycleView方法：
private void scrapOrRecycleView(Recycler recycler, int index, View view) {
final ViewHolder viewHolder = getChildViewHolderInt(view);
...
if (viewHolder.isInvalid() && !viewHolder.isRemoved()
&& !mRecyclerView.mAdapter.hasStableIds()) {
removeViewAt(index);
recycler.recycleViewHolderInternal(viewHolder);
} else {
detachViewAt(index);
// 回收到一级缓存
recycler.scrapView(view);
mRecyclerView.mViewInfoStore.onViewDetached(viewHolder);
}
}
当viewHolder满足 无效/没有被移除/Adapter没有stableIds 时，会回收到二级缓存或四级缓存中，否则回收到一级缓存
3.2.2 回收到二级缓存或者四级缓存的时机
- 数据集发生变化，调用
LayoutManage.onLayoutChildren->LayoutManage.detachAndScrapAttachedViews->recycler.recycleViewHolderInternal进行回收； - item滑出屏幕时，调用
scrollBy->fill->recycleByLayoutState进行回收
由于上述两种情况下最终调用的地方有重合，我们直接以第二种情况来做说明：
整体时序图如下：
用户在滑动列表时，会使用LinearLayoutManage.scrollBy函数：
int scrollBy(int delta, RecyclerView.Recycler recycler, RecyclerView.State state) {
...
updateLayoutState(layoutDirection, absDelta, true, state);
// 关键点：调用fill函数
final int consumed = mLayoutState.mScrollingOffset
+ fill(recycler, mLayoutState, state, false);
...
mLayoutState.mLastScrollDelta = scrolled;
return scrolled;
}
继续深入fill函数：
//androidx.recyclerview.widget.LinearLayoutManager#fill
int fill(RecyclerView.Recycler recycler, LayoutState layoutState,
RecyclerView.State state, boolean stopOnFocusable) {
if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {
// TODO ugly bug fix. should not happen
if (layoutState.mAvailable < 0) {
layoutState.mScrollingOffset += layoutState.mAvailable;
}
// 1. 回收到二级缓存或四级缓存的起点
recycleByLayoutState(recycler, layoutState);
}
int remainingSpace = layoutState.mAvailable + layoutState.mExtraFillSpace;
LayoutChunkResult layoutChunkResult = mLayoutChunkResult;
while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {
layoutChunkResult.resetInternal();
...
layoutChunk(recycler, state, layoutState, layoutChunkResult);
...
layoutState.mOffset += layoutChunkResult.mConsumed * layoutState.mLayoutDirection;
if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {
layoutState.mScrollingOffset += layoutChunkResult.mConsumed;
if (layoutState.mAvailable < 0) {
layoutState.mScrollingOffset += layoutState.mAvailable;
}
recycleByLayoutState(recycler, layoutState);
}
}
return start - layoutState.mAvailable;
}
#recycleByLayoutState
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) {
recycleViewsFromEnd(recycler, scrollingOffset, noRecycleSpace);
} else {
recycleViewsFromStart(recycler, scrollingOffset, noRecycleSpace);
}
}
根据滑动的方向，选择调用recycleViewsFromEnd还是recycleViewsFromStart，最终调用方法一致，我们以recycleViewsFromEnd 为例进行说明
// androidx.recyclerview.widget.LinearLayoutManager#recycleViewsFromEnd
private void recycleViewsFromEnd(RecyclerView.Recycler recycler, int scrollingOffset,
int noRecycleSpace) {
final int childCount = getChildCount();
if (mShouldReverseLayout) {
for (int i = 0; i < childCount; i++) {
View child = getChildAt(i);
if (mOrientationHelper.getDecoratedStart(child) < limit
|| mOrientationHelper.getTransformedStartWithDecoration(child) < limit) {
// stop here
recycleChildren(recycler, 0, i);
return;
}
}
} else {
for (int i = childCount - 1; i >= 0; i--) {
View child = getChildAt(i);
if (mOrientationHelper.getDecoratedStart(child) < limit
|| mOrientationHelper.getTransformedStartWithDecoration(child) < limit) {
// stop here
recycleChildren(recycler, childCount - 1, i);
return;
}
}
}
}
不管mShouldReverseLayout是什么值（一般为false），都会调用到recycleChildren：
private void recycleChildren(RecyclerView.Recycler recycler, int startIndex, int endIndex) {
if (startIndex == endIndex) {
return;
}
if (DEBUG) {
Log.d(TAG, "Recycling " + Math.abs(startIndex - endIndex) + " items");
}
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);
}
}
}
至此，我们看到了开头讲的那几个回收方法中的一个：removeAndRecycleViewAt ，之后便会调用到Recycler.recyclerView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)
ViewHolder holder = getChildViewHolderInt(view);
if (holder.isTmpDetached()) {
removeDetachedView(view, false);
}
if (holder.isScrap()) {
holder.unScrap();
} else if (holder.wasReturnedFromScrap()) {
holder.clearReturnedFromScrapFlag();
}
recycleViewHolderInternal(holder);
}
紧接着调用Recycler 内部的recycleViewHolderInternal 方法：
void recycleViewHolderInternal(ViewHolder holder) {
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();
if (cachedViewSize >= mViewCacheMax && cachedViewSize > 0) {
// mCachedViews中满了之后，调用addViewHolderToRecycledViewPool，将第一个ViewHolder放入RecycledViewPool
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 views
int cacheIndex = cachedViewSize - 1;
while (cacheIndex >= 0) {
int cachedPos = mCachedViews.get(cacheIndex).mPosition;
if (!mPrefetchRegistry.lastPrefetchIncludedPosition(cachedPos)) {
break;
}
cacheIndex--;
}
targetCacheIndex = cacheIndex + 1;
}
mCachedViews.add(targetCacheIndex, holder);
cached = true;
}
if (!cached) {
addViewHolderToRecycledViewPool(holder, true);
recycled = true;
}
}
通常会将viewHolder添加进二级缓存mCachedViews中，mCachedViews中满员后，会将第一个viewHolder通过addViewHolderToRecycledViewPool函数添加进四级缓存RecycledViewPool