前言
上一篇,说了ItemDecoration
,这一篇,我们来说说RecyclerView
的回收复用逻辑。
问题
假如有100个item
,首屏最多展示2个半(一屏同时最多展示4个),RecyclerView
滑动时,会创建多少个viewholder
?
先别急着回答,我们写个 demo 看看
首先,是item
的布局
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical">
<TextView
android:id="@+id/tv_repeat"
android:layout_width="match_parent"
android:layout_height="200dp"
android:gravity="center" />
<TextView
android:layout_width="match_parent"
android:layout_height="2dp"
android:background="@color/colorAccent" />
</LinearLayout>
然后是RepeatAdapter
,这里使用的是原生的Adapter
public class RepeatAdapter extends RecyclerView.Adapter<RepeatAdapter.RepeatViewHolder> {
private List<String> list;
private Context context;
public RepeatAdapter(List<String> list, Context context) {
this.list = list;
this.context = context;
}
@NonNull
@Override
public RepeatViewHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int i) {
View view = LayoutInflater.from(context).inflate(R.layout.item_repeat, viewGroup, false);
Log.e("cheng", "onCreateViewHolder viewType=" + i);
return new RepeatViewHolder(view);
}
@Override
public void onBindViewHolder(@NonNull RepeatViewHolder viewHolder, int i) {
viewHolder.tv_repeat.setText(list.get(i));
Log.e("cheng", "onBindViewHolder position=" + i);
}
@Override
public int getItemCount() {
return list.size();
}
class RepeatViewHolder extends RecyclerView.ViewHolder {
public TextView tv_repeat;
public RepeatViewHolder(@NonNull View itemView) {
super(itemView);
this.tv_repeat = (TextView) itemView.findViewById(R.id.tv_repeat);
}
}
}
在Activity
中使用
List<String> list = new ArrayList<>();
for (int i = 0; i < 100; i++) {
list.add("第" + i + "个item");
}
RepeatAdapter repeatAdapter = new RepeatAdapter(list, this);
rvRepeat.setLayoutManager(new LinearLayoutManager(this));
rvRepeat.setAdapter(repeatAdapter);
当我们滑动时,log如下:
可以看到,总共执行了7次onCreateViewHolder
,也就是说,总共100个item,只创建了7个viewholder
(篇幅问题,没有截到100,有兴趣的同学可以自己试试)
WHY?
通过阅读源码,我们发现,RecyclerView
的缓存单位是viewholder
,而获取viewholder
最终调用的方法是Recycler#tryGetViewHolderForPositionByDeadline
源码如下:
@Nullable
RecyclerView.ViewHolder tryGetViewHolderForPositionByDeadline(int position, boolean dryRun, long deadlineNs) {
...省略代码...
holder = this.getChangedScrapViewForPosition(position);
...省略代码...
if (holder == null) {
holder = this.getScrapOrHiddenOrCachedHolderForPosition(position, dryRun);
}
...省略代码...
if (holder == null) {
View view = this.mViewCacheExtension.getViewForPositionAndType(this, position, type);
if (view != null) {
holder = RecyclerView.this.getChildViewHolder(view);
}
}
...省略代码...
if (holder == null) {
holder = this.getRecycledViewPool().getRecycledView(type);
}
...省略代码...
if (holder == null) {
holder = RecyclerView.this.mAdapter.createViewHolder(RecyclerView.this, type);
}
...省略代码...
}
从上到下,依次是mChangedScrap
、mAttachedScrap
、mCachedViews
、mViewCacheExtension
、mRecyclerPool
最后才是createViewHolder
ArrayList<RecyclerView.ViewHolder> mChangedScrap = null;
final ArrayList<RecyclerView.ViewHolder> mAttachedScrap = new ArrayList();
final ArrayList<RecyclerView.ViewHolder> mCachedViews = new ArrayList();
private RecyclerView.ViewCacheExtension mViewCacheExtension;
RecyclerView.RecycledViewPool mRecyclerPool;
mChangedScrap
完整源码如下:
if (RecyclerView.this.mState.isPreLayout()) {
holder = this.getChangedScrapViewForPosition(position);
fromScrapOrHiddenOrCache = holder != null;
}
由于isPreLayout
方法取决于mInPreLayout
,而mInPreLayout
默认为false
,即mChangedScrap不参与回收复用逻辑。
-
mAttachedScrap
完整源码如下:
RecyclerView.ViewHolder getScrapOrHiddenOrCachedHolderForPosition(int position, boolean dryRun) {
int scrapCount = this.mAttachedScrap.size();
int cacheSize;
RecyclerView.ViewHolder vh;
for(cacheSize = 0; cacheSize < scrapCount; ++cacheSize) {
vh = (RecyclerView.ViewHolder)this.mAttachedScrap.get(cacheSize);
if (!vh.wasReturnedFromScrap() && vh.getLayoutPosition() == position && !vh.isInvalid() && (RecyclerView.this.mState.mInPreLayout || !vh.isRemoved())) {
vh.addFlags(32);
return vh;
}
}
}
这段代码什么时候会生效呢,那得找找什么时候将viewholder
添加到mAttachedScrap
的
我们在源码中全局搜索mAttachedScrap.add
,发现是Recycler#scrapView()
方法
void scrapView(View view) {
...省略代码...
this.mAttachedScrap.add(holder);
...省略代码...
}
什么时候调用scrapView()
方法呢?
继续全局搜索,发现最终是Recycler#detachAndScrapAttachedViews()
方法,这个方法又是什么时候会被调用的呢?
答案是LayoutManager#onLayoutChildren()
。我们知道onLayoutChildren
负责item的布局工作(这部分后面再说),所以,mAttachedScrap应该存放是当前屏幕上显示的viewhoder
,我们来看下detachAndScrapAttachedViews
的源码
public void detachAndScrapAttachedViews(@NonNull RecyclerView.Recycler recycler) {
int childCount = this.getChildCount();
for(int i = childCount - 1; i >= 0; --i) {
View v = this.getChildAt(i);
this.scrapOrRecycleView(recycler, i, v);
}
}
其中,childCount
即为屏幕上显示的item数量。那同学们就要问了,mAttachedScrap
有啥用?
答案当然是有用的,比如说,拖动排序,比如说第1个item和第2个item 互换,这个时候,mAttachedScrap就派上了用场,直接从这里通过position
拿viewholder
,都不用经过onCreateViewHolder
和onBindViewHolder
。
mCachedViews
完整代码如下:
cacheSize = this.mCachedViews.size();
for(int i = 0; i < cacheSize; ++i) {
RecyclerView.ViewHolder holder = (RecyclerView.ViewHolder)this.mCachedViews.get(i);
if (!holder.isInvalid() && holder.getLayoutPosition() == position) {
if (!dryRun) {
this.mCachedViews.remove(i);
}
return holder;
}
}
我们先来找找viewholder
是在什么时候添加进mCachedViews
?是在Recycler#recycleViewHolderInternal()
方法
void recycleViewHolderInternal(RecyclerView.ViewHolder holder) {
if (!holder.isScrap() && holder.itemView.getParent() == null) {
if (holder.isTmpDetached()) {
throw new IllegalArgumentException("Tmp detached view should be removed from RecyclerView before it can be recycled: " + holder + RecyclerView.this.exceptionLabel());
} else if (holder.shouldIgnore()) {
throw new IllegalArgumentException("Trying to recycle an ignored view holder. You should first call stopIgnoringView(view) before calling recycle." + RecyclerView.this.exceptionLabel());
} else {
boolean transientStatePreventsRecycling = holder.doesTransientStatePreventRecycling();
boolean forceRecycle = RecyclerView.this.mAdapter != null && transientStatePreventsRecycling && RecyclerView.this.mAdapter.onFailedToRecycleView(holder);
boolean cached = false;
boolean recycled = false;
if (forceRecycle || holder.isRecyclable()) {
if (this.mViewCacheMax > 0 && !holder.hasAnyOfTheFlags(526)) {
int cachedViewSize = this.mCachedViews.size();
if (cachedViewSize >= this.mViewCacheMax && cachedViewSize > 0) {
this.recycleCachedViewAt(0);
--cachedViewSize;
}
int targetCacheIndex = cachedViewSize;
if (RecyclerView.ALLOW_THREAD_GAP_WORK && cachedViewSize > 0 && !RecyclerView.this.mPrefetchRegistry.lastPrefetchIncludedPosition(holder.mPosition)) {
int cacheIndex;
for(cacheIndex = cachedViewSize - 1; cacheIndex >= 0; --cacheIndex) {
int cachedPos = ((RecyclerView.ViewHolder)this.mCachedViews.get(cacheIndex)).mPosition;
if (!RecyclerView.this.mPrefetchRegistry.lastPrefetchIncludedPosition(cachedPos)) {
break;
}
}
targetCacheIndex = cacheIndex + 1;
}
this.mCachedViews.add(targetCacheIndex, holder);
cached = true;
}
if (!cached) {
this.addViewHolderToRecycledViewPool(holder, true);
recycled = true;
}
}
RecyclerView.this.mViewInfoStore.removeViewHolder(holder);
if (!cached && !recycled && transientStatePreventsRecycling) {
holder.mOwnerRecyclerView = null;
}
}
} else {
throw new IllegalArgumentException("Scrapped or attached views may not be recycled. isScrap:" + holder.isScrap() + " isAttached:" + (holder.itemView.getParent() != null) + RecyclerView.this.exceptionLabel());
}
}
最上层是RecyclerView#removeAndRecycleViewAt
方法
public void removeAndRecycleViewAt(int index, @NonNull RecyclerView.Recycler recycler) {
View view = this.getChildAt(index);
this.removeViewAt(index);
recycler.recycleView(view);
}
这个方法是在哪里调用的呢?答案是LayoutManager
,我们写个demo效果看着比较直观
定义MyLayoutManager
,并重写removeAndRecycleViewAt
,然后添加log
class MyLayoutManager extends LinearLayoutManager {
public MyLayoutManager(Context context) {
super(context);
}
@Override
public void removeAndRecycleViewAt(int index, @NonNull RecyclerView.Recycler recycler) {
super.removeAndRecycleViewAt(index, recycler);
Log.e("cheng", "removeAndRecycleViewAt index=" + index);
}
}
将其设置给RecyclerView
,然后滑动,查看日志输出情况
可以看到,每次有item滑出屏幕时,都会调用removeAndRecycleViewAt()
方法,需要注意的是,此index
表示的是该item
在chlid
中的下标,也就是在当前屏幕中的下标,而不是在RecyclerView
的。
事实是不是这样的呢?让我们来看看源码,以LinearLayoutManager
为例,默认是垂直滑动的,此时控制其滑动距离的方法是scrollVerticallyBy()
,其调用的是scrollBy()
方法
int scrollBy(int dy, Recycler recycler, State state) {
if (this.getChildCount() != 0 && dy != 0) {
this.mLayoutState.mRecycle = true;
this.ensureLayoutState();
int layoutDirection = dy > 0 ? 1 : -1;
int absDy = Math.abs(dy);
this.updateLayoutState(layoutDirection, absDy, true, state);
int consumed = this.mLayoutState.mScrollingOffset + this.fill(recycler, this.mLayoutState, state, false);
if (consumed < 0) {
return 0;
} else {
int scrolled = absDy > consumed ? layoutDirection * consumed : dy;
this.mOrientationHelper.offsetChildren(-scrolled);
this.mLayoutState.mLastScrollDelta = scrolled;
return scrolled;
}
} else {
return 0;
}
}
关键代码是fill()
方法中的recycleByLayoutState()
,判断滑动方向,从第一个还是最后一个开始回收。
private void recycleByLayoutState(Recycler recycler, LinearLayoutManager.LayoutState layoutState) {
if (layoutState.mRecycle && !layoutState.mInfinite) {
if (layoutState.mLayoutDirection == -1) {
this.recycleViewsFromEnd(recycler, layoutState.mScrollingOffset);
} else {
this.recycleViewsFromStart(recycler, layoutState.mScrollingOffset);
}
}
}
扯的有些远了,让我们回顾下recycleViewHolderInternal()
方法,当cachedViewSize >= this.mViewCacheMax
时,会移除第1个,也就是最先加入的viewholder
,mViewCacheMax
是多少呢?
public Recycler() {
this.mUnmodifiableAttachedScrap = Collections.unmodifiableList(this.mAttachedScrap);
this.mRequestedCacheMax = 2;
this.mViewCacheMax = 2;
}
mViewCacheMax
为2,也就是mCachedViews的初始化大小为2,超过这个大小时,viewholer
将会被移除,放到哪里去了呢?带着这个疑问我们继续往下看
-
mViewCacheExtension
ViewCacheExtension
这个类需要使用者通过setViewCacheExtension()
方法传入,RecyclerView
自身并不会实现它,一般正常的使用也用不到。 -
mRecyclerPool
我们带着之前的疑问,继续看源码,之前提到mCachedViews初始大小为2,超过这个大小,最先放入的会被移除,移除的viewholder
到哪里去了呢?我们来看recycleCachedViewAt()
方法的源码
void recycleCachedViewAt(int cachedViewIndex) {
RecyclerView.ViewHolder viewHolder = (RecyclerView.ViewHolder)this.mCachedViews.get(cachedViewIndex);
this.addViewHolderToRecycledViewPool(viewHolder, true);
this.mCachedViews.remove(cachedViewIndex);
}
addViewHolderToRecycledViewPool()
方法
void addViewHolderToRecycledViewPool(@NonNull RecyclerView.ViewHolder holder, boolean dispatchRecycled) {
RecyclerView.clearNestedRecyclerViewIfNotNested(holder);
if (holder.hasAnyOfTheFlags(16384)) {
holder.setFlags(0, 16384);
ViewCompat.setAccessibilityDelegate(holder.itemView, (AccessibilityDelegateCompat)null);
}
if (dispatchRecycled) {
this.dispatchViewRecycled(holder);
}
holder.mOwnerRecyclerView = null;
this.getRecycledViewPool().putRecycledView(holder);
}
可以看到,该viewholder
被添加到mRecyclerPool
中
我们继续看看RecycledViewPool
的源码
public static class RecycledViewPool {
private static final int DEFAULT_MAX_SCRAP = 5;
SparseArray<RecyclerView.RecycledViewPool.ScrapData> mScrap = new SparseArray();
private int mAttachCount = 0;
public RecycledViewPool() {
}
...省略代码...
}
static class ScrapData {
final ArrayList<RecyclerView.ViewHolder> mScrapHeap = new ArrayList();
int mMaxScrap = 5;
long mCreateRunningAverageNs = 0L;
long mBindRunningAverageNs = 0L;
ScrapData() {
}
}
可以看到,其内部有一个SparseArray
用来存放viewholder
。
总结
- 总共有
mAttachedScrap
、mCachedViews
、mViewCacheExtension
、mRecyclerPool
4级缓存,其中mAttachedScrap
只保存布局时,屏幕上显示的viewholder
,一般不参与回收、复用(拖动排序时会参与); -
mCachedViews
主要保存刚移除屏幕的viewholder
,初始大小为2; -
mViewCacheExtension
为预留的缓存池,需要自己去实现; -
mRecyclerPool
则是最后一级缓存,当mCachedViews
满了之后,viewholder
会被存放在mRecyclerPool
,继续复用。
其中,mAttachedScrap
、mCachedViews
为精确匹配,即为对应position
的viewholder
才会被复用;mRecyclerPool
为模糊匹配,只匹配viewType
,所以复用时,需要调用onBindViewHolder
为其设置新的数据。
回答之前的疑问
当滑出第6个item时,这时mCachedViews
中存放着第1、2个item,屏幕上显示的是第3、4、5、6个item,再滑出第7个item时,不存在能复用的viewholder
,所以调用onCreateViewHolder
创建了一个新的viewholder
,并且把第1个viewholder
放入mRecyclerPool
,以备复用。