RecyclerView的回收缓存均由内部类Recycler完成(3)，2024年最新HarmonyOS鸿
return scrapHeap.remove(i);}
}
}
return null;
}
…
可以发现
[*]RecyclerViewPool中有一个SparseArray<ScrapData>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 缓存机制出发点：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 == position
[*]viewHolder为有效
[*]为预布局大概viewHolder的状态不是removed
[*]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参数值默以为false，以是取出来的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;
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