写在前面：今天2021年2月12日，农历正月初一，2021年春节。一个人在上海没有回家。从家里下了几个水饺吃了，然后来公司了，学会习，下午再跑个5公里。拍张照纪念一下。
我的样子.jpg
进入正题
上篇文章：RecyclerView源码分析之一
源码版本：androidx1.1.0
本文要旨：RecyclerView的回收和复用机制。
我们就以RecyclerView最简单的使用方式为例进行分析。使用线性布局，方向为竖直方向，布局从上到下。
先引用这篇文章中RecyclerView 源码分析(三) - RecyclerView的缓存机制对RecyclerView缓存的总结，感觉非常清晰非常好。
| 缓存级别 | 实际变量 | 含义 |
|---|---|---|
| 一级缓存 |
mAttachedScrap和mChangedScrap
|
优先级最高的缓存，RecyclerView在获取ViewHolder时,优先会到这两个缓存来找。其中mAttachedScrap存储的是当前还在屏幕中的ViewHolder，mChangedScrap存储的是数据被更新的ViewHolder,比如说调用了Adapter的notifyItemChanged方法。 |
| 二级缓存 | mCachedViews |
默认大小为2，在滚动的时候会存储一些ViewHolder。 |
| 三级缓存 | ViewCacheExtension |
这个是自定义缓存，一般用不到。 |
| 四级缓存 | RecyclerViewPool |
根据ViewType来缓存ViewHolder，每个ViewType的数组大小为5，可以动态的改变。 |
public final class Recycler {
final ArrayList<ViewHolder> mAttachedScrap = new ArrayList<>();
ArrayList<ViewHolder> mChangedScrap = null;
//根据postion来寻找可复用的ViewHolder
final ArrayList<ViewHolder> mCachedViews = new ArrayList<ViewHolder>();
//自定义的缓存扩展，一般不会定义吧。
private ViewCacheExtension mViewCacheExtension;
//根据type来寻找来寻找可复用的ViewHolder
RecycledViewPool mRecyclerPool;
static final int DEFAULT_CACHE_SIZE = 2;
}
public static class RecycledViewPool {
private static final int DEFAULT_MAX_SCRAP = 5;
}
还有一篇文章中，说的感觉更清晰易懂。
1、AttachedScrap: 存放可见、不需要重新绑定的ViewHolder
2、CachedViews: 存放不可见、不需要重新绑定的ViewHoler
3、ViewCacheExtension: 自定义缓存（存放不可见、不需要重新绑定）
4、RecyclerPool: 存放不可见、需要重新绑定的ViewHolder
【Android】自定义无限循环的LayoutManager
RecyclerView的回收
在布局的时候的回收
在RecyclerView的布局的第二步dispatchLayoutStep2中，会调用LinearLayoutManager的onLayoutChildren方法。
@Override
public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
//...
//注释1处，如果当前存在attach到RecyclerView的View，则临时detach，后面再复用。
detachAndScrapAttachedViews(recycler);
//...
}
注释1处，如果当前存在attach到RecyclerView的View，则临时detach，后面再复用。
RecyclerView.LayoutManager的detachAndScrapAttachedViews方法。
/**
* 临时detach和废弃当前attached的子Views。Views会被加入指定的Recycler。Recycler可能会在其他先前被回收的Views之前优先使用`scrap views`。
*
* @param recycler Recycler to scrap views into
*/
public void detachAndScrapAttachedViews(@NonNull Recycler recycler) {
final int childCount = getChildCount();
for (int i = childCount - 1; i >= 0; i--) {
final View v = getChildAt(i);
//调用scrapOrRecycleView方法
scrapOrRecycleView(recycler, i, v);
}
}
private void scrapOrRecycleView(Recycler recycler, int index, View view) {
final ViewHolder viewHolder = getChildViewHolderInt(view);
if (viewHolder.shouldIgnore()) {
return;
}
//如果ViewHolder的数据无效了 && ViewHolder对应的数据没有被从data set中移除 && 并且没有固定的id
if (viewHolder.isInvalid() && !viewHolder.isRemoved()
&& !mRecyclerView.mAdapter.hasStableIds()) {
//注释1处，移除View
removeViewAt(index);
//注释2处，回收ViewHolder
recycler.recycleViewHolderInternal(viewHolder);
} else {
//注释3处，detachView
detachViewAt(index);
//注释4处，回收View
recycler.scrapView(view);
mRecyclerView.mViewInfoStore.onViewDetached(viewHolder);
}
}
注释1处，移除View，这里会真正把子View从RecyclerView中移除。
注释2处，回收ViewHolder，正常滚动的时候也会调用Recycler的recycleViewHolderInternal方法，后面分析滚动回收的时候再看。
注释3处，临时将一个View从当前窗口detach，然后复用的时候重新attach一下就行，效率非常高。
Recycler的scrapView方法。
void scrapView(View view) {
final ViewHolder holder = getChildViewHolderInt(view);
/*
* 1. 如果ViewHolder对应的数据从data set中移除了，或者ViewHolder对应的数据无效了，需要重新绑定新的数据。
* 2. 或者ViewHolder对应的数据没有更新
* 3. 或者canReuseUpdatedViewHolder(holder)方法为true，这方法默认是true
*/
if (holder.hasAnyOfTheFlags(ViewHolder.FLAG_REMOVED | ViewHolder.FLAG_INVALID)
|| !holder.isUpdated() || canReuseUpdatedViewHolder(holder)) {
//如果ViewHolder的数据无效了 && ViewHolder对应的数据没有被从data set中移除 && 并且没有固定的id
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());
}
//调用setScrapContainer方法，第二个参数是isChangeScrap的意思，这里为false
holder.setScrapContainer(this, false);
//将ViewHolder回收到mAttachedScrap
mAttachedScrap.add(holder);
} else {
if (mChangedScrap == null) {
mChangedScrap = new ArrayList<ViewHolder>();
}
//调用setScrapContainer方法，第二个参数是isChangeScrap的意思，这里为true
holder.setScrapContainer(this, true);
//将ViewHolder回收到mChangedScrap
mChangedScrap.add(holder);
}
}
这里会根据ViewHolder的状态，决定将其加入到mAttachedScrap还是mChangedScrap。mAttachedScrap还是mChangedScrap的区别引用这篇文章中的叙述RecyclerView 源码分析(三) - RecyclerView的缓存机制
这是优先级最高的缓存，RecyclerView在获取ViewHolder时,优先会到这两个缓存来找。其中mAttachedScrap存储的是当前还在屏幕中的ViewHolder，mChangedScrap存储的是数据被更新的ViewHolder,比如说调用了Adapter的notifyItemChanged方法。
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 + available
final 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。
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，如果layoutState.mScrollingOffset不为SCROLLING_OFFSET_NaN的话，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到i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到i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) {
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);
}
RecyclerView.LayoutManager的removeAndRecycleViewAt方法。
public void removeViewAt(int index) {
final View child = getChildAt(index);
if (child != null) {
//这里调用RecyclerView的mChildHelper进行移除。
mChildHelper.removeViewAt(index);
}
}
在RecyclerView的initChildrenHelper方法中，初始化了mChildHelper，并传入了一个ChildHelper.Callback对象。最终就是调用这个ChildHelper.Callback对象的removeViewAt方法来移除的。
@Override
public void removeViewAt(int index) {
final View child = RecyclerView.this.getChildAt(index);
if (child != null) {
dispatchChildDetached(child);
// Clear any android.view.animation.Animation that may prevent the item from
// detaching when being removed. If a child is re-added before the
// lazy detach occurs, it will receive invalid attach/detach sequencing.
child.clearAnimation();
}
//注释1处，RecyclerView自身调用ViewGroup的removeViewAt方法来移除子View。
RecyclerView.this.removeViewAt(index);
}
注释1处，RecyclerView自身调用ViewGroup的removeViewAt方法来移除子View。
然后我们回到RecyclerView.LayoutManager的removeAndRecycleViewAt方法的注释2处，回收移除掉的View。
recycler.recycleView(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ViewHolder
ViewHolder holder = getChildViewHolderInt(view);
if (holder.isTmpDetached()) {
//清除FLAG_TMP_DETACHED临时分离的标记
removeDetachedView(view, false);
}
//注释1处，将ViewHolder从mChangedScrap或者mAttachedScrap中移除。
if (holder.isScrap()) {
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在mChangedScrap中或者mAttachedScrap中，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。
注释2处，调用recycleViewHolderInternal方法。
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());
}
//不需要回收的ViewHolder
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());
}
if (forceRecycle || holder.isRecyclable()) {
if (mViewCacheMax > 0
&& !holder.hasAnyOfTheFlags(ViewHolder.FLAG_INVALID
| ViewHolder.FLAG_REMOVED
| ViewHolder.FLAG_UPDATE
| ViewHolder.FLAG_ADAPTER_POSITION_UNKNOWN)) {
//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
int cachedViewSize = mCachedViews.size();
if (cachedViewSize >= mViewCacheMax && cachedViewSize > 0) {
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;
}
//注释2处，将要回收的ViewHolder加入mCachedViews
mCachedViews.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 {
// NOTE: A view can fail to be recycled when it is scrolled off while an animation
// runs. In this case, the item is eventually recycled by
// ItemAnimatorRestoreListener#onAnimationFinished.
// TODO: consider cancelling an animation when an item is removed scrollBy,
// to return it to the pool faster
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());
}
}
// 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);
if (!cached && !recycled && transientStatePreventsRecycling) {
holder.mOwnerRecyclerView = null;
}
}
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ViewHolder
ViewHolder viewHolder = mCachedViews.get(cachedViewIndex);
//加入到RecycledViewPool
addViewHolderToRecycledViewPool(viewHolder, true);
//mCachedViews移除对应位置上的ViewHolder
mCachedViews.remove(cachedViewIndex);
}
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。
我们接下来看看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);
}
RecycledViewPool的putRecycledView方法。
/**
* 将废弃的ViewHolder加入到缓存池。
* <p>
* 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();
//根据viewType获取缓存池，就是一个ArrayList<ViewHolder>
final ArrayList<ViewHolder> scrapHeap = getScrapDataForType(viewType).mScrapHeap;
//缓存池已经满了，不回收，直接return。
if (mScrap.get(viewType).mMaxScrap <= scrapHeap.size()) {
return;
}
//ViewHolder做一下清除操作。
scrap.resetInternal();
//缓存ViewHolder
scrapHeap.add(scrap);
}
RecyclerView的复用
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。后面分析缓存相关的点的时候再看。
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) {
//如果需要布局特殊的View是，可以将mScrapList赋值为不为null的值，然后就会从mScrapList获取View，这里可以忽略
if (mScrapList != null) {
return nextViewFromScrapList();
}
//注释1处，调用Recycler的getViewForPosition方法获取View
final View view = recycler.getViewForPosition(mCurrentPosition);
mCurrentPosition += mItemDirection;
return view;
}
注释1处，正常调用Recycler的getViewForPosition方法获取View。RecyclerView.Recycler就是处理RecyclerView的缓存复用相关逻辑的类。
@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;
// 0) If there is a changed scrap, try to find from there
//注释0处，从mChangedScrap中获取ViewHolder，注意只有LinearLayoutManager没有开启自动测量的情况下，mState.isPreLayout()才可能是是true，我们这里就忽略mState.isPreLayout()为true的情况。
if (mState.isPreLayout()) {
holder = getChangedScrapViewForPosition(position);
fromScrapOrHiddenOrCache = holder != null;
}
// 1) Find by position from scrap/hidden list/cache
//注释1处，从mAttachedScrap、mHiddenViews、mCachedViews中获取ViewHolder
if (holder == null) {
holder = getScrapOrHiddenOrCachedHolderForPosition(position, dryRun);
if (holder != null) {
//是否可以在该位置上使用holder，
if (!validateViewHolderForOffsetPosition(holder)) {
// 如果holder不能被使用，回收之。
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();
}
//回收
recycleViewHolderInternal(holder);
}
holder = null;
} else {
fromScrapOrHiddenOrCache = true;
}
}
}
//holder为null，继续进行查找
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ViewType
final 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 position
holder.mPosition = offsetPosition;
fromScrapOrHiddenOrCache = true;
}
}
//注释3处，从我们自定义的缓存扩展mViewCacheExtension中获取ViewHolder
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"
+ 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 pool
if (DEBUG) {
Log.d(TAG, "tryGetViewHolderForPositionByDeadline("
+ position + ") fetching from shared pool");
}
//注释4处，从RecycledViewPool中获取ViewHolder
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;
}
//注释5处，从适配器中创建一个新的ViewHolder
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
+ exceptionLabel());
}
final int offsetPosition = mAdapterHelper.findPositionOffset(position);
//注释6处，绑定ViewHolder
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;
//最终返回ViewHolder对象。
return holder;
}
注释0处，从mChangedScrap中获取ViewHolder。
if (mState.isPreLayout()) {//开启了预布局
holder = getChangedScrapViewForPosition(position);
fromScrapOrHiddenOrCache = holder != null;
}
注意只有LinearLayoutManager没有开启自动测量的情况下，mState.isPreLayout()才可能是是true，并不是常规行为，我们这里就忽略这种情况。
注释1处，从mAttachedScrap、mHiddenViews、mCachedViews中获取ViewHolder。传入的dryRun为false。这个是用来标记找到一个ViewHolder以后，是否从缓存它的数据集中移除掉。
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_SCRAP
holder.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ViewHolder
View 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 getScrapOrCachedViewForId
if (!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。
注释2处，从mHiddenViews中查找View，然后根据View获取ViewHolder。获取到的View会从mHiddenViews移除，然后注释3处，将View对应的ViewHolder加入到mAttachedScrap或者mChangedScrap中。
这里顺便提一下，mHiddenViews存储的是在动画过程中需要隐藏的Views。
注释4处，从mCachedViews中获取。
然后我们回到RecyclerView.Recycler的tryGetViewHolderForPositionByDeadline方法的注释2处。通过stable ids从mAttachedScrap、mCachedViews中获取ViewHolder。
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。
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<>();
@Nullable
public 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。
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 meet
return false;
}
//适配器绑定ViewHolder，内部最终会调用我们平常写的onBindViewHolder方法。
mAdapter.bindViewHolder(holder, offsetPosition);
long endBindNs = getNanoTime();
mRecyclerPool.factorInBindTime(holder.getItemViewType(), endBindNs - startBindNs);
attachAccessibilityDelegateOnBind(holder);
if (mState.isPreLayout()) {
holder.mPreLayoutPosition = position;
}
return true;
}
结尾：关于RecyclerView的动画相关的内容，并没有进行分析。主要还是关注RecyclerView的回收和复用的大致逻辑。
参考链接：
