一、缓存
缓存流程图
二、缓存 mCachedViews
2.1、第一种进入缓存方式
执行LinearLayoutManager.onLayoutChildren方法
public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
...
detachAndScrapAttachedViews(recycler);
...
}
public void detachAndScrapAttachedViews(@NonNull Recycler recycler) {
final int childCount = getChildCount();
for (int i = childCount - 1; i >= 0; i--) {
final View v = getChildAt(i);
scrapOrRecycleView(recycler, i, v);
}
}
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;
}
if (viewHolder.isInvalid() && !viewHolder.isRemoved()
&& !mRecyclerView.mAdapter.hasStableIds()) {
//将index的view从mHiddenViews移除的动作
removeViewAt(index);
//向 mCachedViews 或 Pool 中添加
recycler.recycleViewHolderInternal(viewHolder);
} else {
//设置RecyclerView这个位置的view的parent为null， 并标记ViewHolder为FLAG_TMP_DETACHED
detachViewAt(index);
//添加到mAttachedScrap 或 mCachedViews集合中
recycler.scrapView(view);
mRecyclerView.mViewInfoStore.onViewDetached(viewHolder);
}
}
2.2、第二种进入缓存方式
进入RecyclerView内部类Recycler#scrapView中
--》 fill
--》 recycleByLayoutState
--》 recycleViewsFromStart
--》 recycleChildren
--》 removeAndRecycleViewAt
--》 recycler.recycleView
--》 recycleViewHolderInternal // 关键代码，（cacheView、pool）
我们直接以LinearLayoutManager#recycleViewsFromStart开始
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);
}
}
private void recycleViewsFromStart(RecyclerView.Recycler recycler, int scrollingOffset,
int noRecycleSpace) {
if (scrollingOffset < 0) {
if (DEBUG) {
Log.d(TAG, "Called recycle from start with a negative value. This might happen"
+ " during layout changes but may be sign of a bug");
}
return;
}
// ignore padding, ViewGroup may not clip children.
final int limit = scrollingOffset - noRecycleSpace;
final int childCount = getChildCount();
if (mShouldReverseLayout) {
for (int i = childCount - 1; i >= 0; i--) {
View child = getChildAt(i);
if (mOrientationHelper.getDecoratedEnd(child) > limit
|| mOrientationHelper.getTransformedEndWithDecoration(child) > limit) {
// stop here
recycleChildren(recycler, childCount - 1, i);
return;
}
}
} else {
for (int i = 0; i < childCount; i++) {
View child = getChildAt(i);
if (mOrientationHelper.getDecoratedEnd(child) > limit
|| mOrientationHelper.getTransformedEndWithDecoration(child) > limit) {
// stop here
recycleChildren(recycler, 0, i);
return;
}
}
}
}
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);
}
}
}
执行RecyclerView内部类LayoutManager#removeAndRecycleViewAt
public void removeAndRecycleViewAt(int index, @NonNull Recycler recycler) {
final View view = getChildAt(index);
removeViewAt(index);
recycler.recycleView(view);
}
执行RecyclerView内部类Recycler#recycleView
public void recycleView(@NonNull View view) {
ViewHolder holder = getChildViewHolderInt(view);
if (holder.isTmpDetached()) {
removeDetachedView(view, false);
}
if (holder.isScrap()) {
holder.unScrap();
} else if (holder.wasReturnedFromScrap()) {
holder.clearReturnedFromScrapFlag();
}
//向 mCachedViews 或 Pool 中添加
recycleViewHolderInternal(holder);
if (mItemAnimator != null && !holder.isRecyclable()) {
mItemAnimator.endAnimation(holder);
}
}
2.1、 mCachedViews 或 Pool 缓存
在RecyclerView的集合中缓存
void recycleViewHolderInternal(RecyclerView.ViewHolder holder) {
final boolean transientStatePreventsRecycling = holder
.doesTransientStatePreventRecycling();
@SuppressWarnings("unchecked")
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());
}
if (forceRecycle || holder.isRecyclable()) {
if (mViewCacheMax > 0
&& !holder.hasAnyOfTheFlags(RecyclerView.ViewHolder.FLAG_INVALID
| RecyclerView.ViewHolder.FLAG_REMOVED
| RecyclerView.ViewHolder.FLAG_UPDATE
| RecyclerView.ViewHolder.FLAG_ADAPTER_POSITION_UNKNOWN)) {
// Retire oldest cached view
int cachedViewSize = mCachedViews.size();
//大于缓存的最大值 mViewCacheMax = 2， CacheView 把老的移除集合，放入 pool 中
if (cachedViewSize >= mViewCacheMax && cachedViewSize > 0) {
//取出集合的第一个元素放入 pool 中
recycleCachedViewAt(0);
//缓存的个数减一
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;
}
//加入 mCachedViews 集合
mCachedViews.add(targetCacheIndex, holder);
cached = true;
}
if (!cached) {
// 添加到Pool中
addViewHolderToRecycledViewPool(holder, true);
recycled = true;
}
} else {
if (DEBUG) {
Log.d(TAG, "trying to recycle a non-recycleable holder. Hopefully, it will "
+ "re-visit here. We are still removing it from animation lists"
+ exceptionLabel());
}
}
mViewInfoStore.removeViewHolder(holder);
if (!cached && !recycled && transientStatePreventsRecycling) {
holder.mOwnerRecyclerView = null;
}
}
- 接下来看向 pool 中添加的步骤
void addViewHolderToRecycledViewPool(@NonNull RecyclerView.ViewHolder holder, boolean dispatchRecycled) {
clearNestedRecyclerViewIfNotNested(holder);
View itemView = holder.itemView;
if (mAccessibilityDelegate != null) {
AccessibilityDelegateCompat itemDelegate = mAccessibilityDelegate.getItemDelegate();
AccessibilityDelegateCompat originalDelegate = null;
if (itemDelegate instanceof RecyclerViewAccessibilityDelegate.ItemDelegate) {
originalDelegate =
((RecyclerViewAccessibilityDelegate.ItemDelegate) itemDelegate)
.getAndRemoveOriginalDelegateForItem(itemView);
}
// Set the a11y delegate back to whatever the original delegate was.
ViewCompat.setAccessibilityDelegate(itemView, originalDelegate);
}
if (dispatchRecycled) {
dispatchViewRecycled(holder);
}
holder.mOwnerRecyclerView = null;
getRecycledViewPool().putRecycledView(holder);
}
public void putRecycledView(RecyclerView.ViewHolder scrap) {
final int viewType = scrap.getItemViewType();
final ArrayList<RecyclerView.ViewHolder> scrapHeap = getScrapDataForType(viewType).mScrapHeap;
if (mScrap.get(viewType).mMaxScrap <= scrapHeap.size()) {
return;
}
if (DEBUG && scrapHeap.contains(scrap)) {
throw new IllegalArgumentException("this scrap item already exists");
}
//进行漂白，即清除一些标志信息个数据
scrap.resetInternal();
//保存 ViewHolder
scrapHeap.add(scrap);
}
调用RecyclerView.RecycledViewPool中的方法getScrapDataForType
private ScrapData getScrapDataForType(int viewType) {
ScrapData scrapData = mScrap.get(viewType);
if (scrapData == null) {
scrapData = new ScrapData();
mScrap.put(viewType, scrapData);
}
return scrapData;
}
- RecyclerView#ScrapData
static class ScrapData {
@UnsupportedAppUsage
//pool 的实际缓存对象，一种 ViewType 对应一个 ScrapData
ArrayList<ViewHolder> mScrapHeap = new ArrayList<>();
int mMaxScrap = DEFAULT_MAX_SCRAP;
long mCreateRunningAverageNs = 0;
long mBindRunningAverageNs = 0;
}
SparseArray<ScrapData> mScrap = new SparseArray<>();
resetInternal重置数据
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);
}
2.2、mAttachedScrap 或 mCachedViews 缓存
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);
mAttachedScrap.add(holder);
} else {
if (mChangedScrap == null) {
mChangedScrap = new ArrayList<ViewHolder>();
}
holder.setScrapContainer(this, true);
mChangedScrap.add(holder);
}
}
二、总结
1、从缓存池（RecycledViewPool）复用 ViewHolder 需要调用 onBindViewHodler。
2、从屏幕外缓存（mCachedViews）复用 ViewHolder 不需要调用 onBindViewHodler。
3、从 mCachedViews 中没有拿到 ViewHolder，需要调用 onCreateViewHolder 和 onBindViewHodler。
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