RecyclerView 的缓存机制还是非常值得到家参考的,先来说一下RecyclerView 关于缓存的方法, 关于RecyclerView 的缓存数据有两个级别,一个是detach ,另一个就是remove , 关于detach 就是在RecyclerView 滑动或者layout 时为了记录屏幕内条目信息而设定的,他主要的缓存的数据就是getChildCount列表所持有的数据, 至于remove 就是为了缓存我们从数据列表所删除的数据,根据这个信息我们来从代码来分析
public final class Recycler {
///这attach 相关的缓存的列表
final ArrayList<ViewHolder> mAttachedScrap = new ArrayList<>();
///和remove 相关的缓存列表
final ArrayList<ViewHolder> mCachedViews = new ArrayList<ViewHolder>();
///缓存池
RecycledViewPool mRecyclerPool;
private int mRequestedCacheMax = DEFAULT_CACHE_SIZE;
//默认的最大缓存个数
int mViewCacheMax = DEFAULT_CACHE_SIZE;
static final int DEFAULT_CACHE_SIZE = 2;
}
我们看到Recycler 中有3个和缓存的相关的列表他们分别是 mAttachedScrap mCachedViews mRecyclerPool,我们来看看都是在哪些方法对这些列表做的操作,
我们先来看看detach方法,是如何将view 暂时和Recyclerview 分离的,并看一下他是将数据缓存的哪个列表中
detachAndScrapAttachedViews
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);
}
}
我们可以看到在detachAndScrapAttachedViews方法中显示获取了getChildCount ,缓存的就是RecyclerView的所持有的展示数据,我们继续看看scrapOrRecycleView这个方法是怎么操作的
scrapOrRecycleView
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()) {
removeViewAt(index);
recycler.recycleViewHolderInternal(viewHolder);
} else {
detachViewAt(index);
recycler.scrapView(view);
mRecyclerView.mViewInfoStore.onViewDetached(viewHolder);
}
}
我们就看到根据viewHolder的状态,一部分走的是remove,我们这里暂时不管被remove的item,我们去看看scrap的方法,即recycler.scrapView(view)
recycler.scrapView(view);
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);
}
}
还是判断了一堆viewholder的状态,但是我们看到在detach时候,是将这个这个holder缓存在mAttachedScrap中,那么也就是说屏幕中的数据都放在这里,为什么要单独给展示的list单独一个缓存呢,其实他这个做是为了再下一次执行addView 的过程中从mAttachedScrap获取到数据就是已经绑定过数据的holder了而且这个holder所对应的数据还是正确的,这样可以将bindViewHolder方法所执行的时间省去了,可以加快view的绘制,
我们再来看看remove的方法
removeAndRecycleView
public void removeAndRecycleView(@NonNull View child, @NonNull Recycler recycler) {
removeView(child);
recycler.recycleView(child);
}
先执行了removeView 然后又recycleView 回收,我们来看看recycleView
recycler.recycleView(child);
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);
}
先是根据view找到这个view的holder,然后判断这个holer.isScrap ,为什么其他地方只是说一下是holder的判断,而这里却单独强调一下这个判断,我们再来说一下detach 的过程,每次我们滑动的过程中都会执行detach 将数据放入到mAttachedScrap这个list中,那么这么多的数据,这么频繁的添加数据如果不删除数据的话肯定是不行的,那么什么时候将这个数据删除呢? 对 !就是这里, holder.unScrap方法就是在remove过程中将数据从mAttachedScrap 中删除的,其他情况我们看到了再说,这里就分析一下删除的过程中的情况
holder.unScrap();
void unScrap() {
mScrapContainer.unscrapView(this);
}
在holder中他有调用了recycler的unscrapView方法,来处理接下来的逻辑,
####recycler.unscrapView(child);
void unscrapView(ViewHolder holder) {
if (holder.mInChangeScrap) {
mChangedScrap.remove(holder);
} else {
mAttachedScrap.remove(holder);
}
holder.mScrapContainer = null;
holder.mInChangeScrap = false;
holder.clearReturnedFromScrapFlag();
}
我们可以看到在unscrapView方法中,将数据移出了mAttachedScrap这个列表中的,
接下来我们继续跟踪remove过程中的recycleViewHolderInternal方法
recycleViewHolderInternal
void recycleViewHolderInternal(ViewHolder holder) {
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());
}
if (holder.isTmpDetached()) {
throw new IllegalArgumentException("Tmp detached view should be removed "
+ "from RecyclerView before it can be recycled: " + holder
+ exceptionLabel());
}
if (holder.shouldIgnore()) {
throw new IllegalArgumentException("Trying to recycle an ignored view holder. You"
+ " should first call stopIgnoringView(view) before calling recycle."
+ exceptionLabel());
}
final boolean transientStatePreventsRecycling = holder
.doesTransientStatePreventRecycling();
final boolean forceRecycle = mAdapter != null
&& transientStatePreventsRecycling
&& mAdapter.onFailedToRecycleView(holder);
boolean cached = false;
boolean recycled = false;
if (forceRecycle || holder.isRecyclable()) {
if (mViewCacheMax > 0
&& !holder.hasAnyOfTheFlags(ViewHolder.FLAG_INVALID | ViewHolder.FLAG_REMOVED
| ViewHolder.FLAG_UPDATE | ViewHolder.FLAG_ADAPTER_POSITION_UNKNOWN)) {
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)) {
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;
}
} else {
}
mViewInfoStore.removeViewHolder(holder);
if (!cached && !recycled && transientStatePreventsRecycling) {
holder.mOwnerRecyclerView = null;
}
}
这段代码看起来很长,但是逻辑非常简单,前面很长的抛异常,我们可以略过,主要从forceRecycle || holder.isRecyclable() 这个判断开始,这个判断的意思就是我们通过代码设置希望对holder回收,在最大回收个数大于0的情况下,对mCachedViews 进行操作,但是比较有意思的地方就是,如果现在的mCachedViews 满了,我们需要将mCachedViews 列表中的数据放入到RecycledViewPool 中,代码如下
if (cachedViewSize >= mViewCacheMax && cachedViewSize > 0) {
recycleCachedViewAt(0);
cachedViewSize--;
}
void recycleCachedViewAt(int cachedViewIndex) {
if (DEBUG) {
Log.d(TAG, "Recycling cached view at index " + cachedViewIndex);
}
ViewHolder viewHolder = mCachedViews.get(cachedViewIndex);
if (DEBUG) {
Log.d(TAG, "CachedViewHolder to be recycled: " + viewHolder);
}
addViewHolderToRecycledViewPool(viewHolder, true);
mCachedViews.remove(cachedViewIndex);
}
void addViewHolderToRecycledViewPool(@NonNull ViewHolder holder, boolean dispatchRecycled) {
clearNestedRecyclerViewIfNotNested(holder);
if (holder.hasAnyOfTheFlags(ViewHolder.FLAG_SET_A11Y_ITEM_DELEGATE)) {
holder.setFlags(0, ViewHolder.FLAG_SET_A11Y_ITEM_DELEGATE);
ViewCompat.setAccessibilityDelegate(holder.itemView, null);
}
if (dispatchRecycled) {
dispatchViewRecycled(holder);
}
holder.mOwnerRecyclerView = null;
getRecycledViewPool().putRecycledView(holder);
}
在为mCachedViews 清理出来数据后,再将我们需要remove的holder放入到mCachedViews 中,而mCachedViews 中的数据其实也是缓存的绑定完数据的holder,也就是如果这个holder是从mCachedViews 中匹配出来的,那么也不需要经过bindViewHolder 这个过程,从上面我们看到mCachedViews 他的最大缓存这个是2个,那是不是越大越好呢,肯定不是的,只要缓存的个数够用其实就是可以了,但是我们什么时候需要对他扩容来帮助我们来提高运行速度呢,我们来做一个假设下面图片这种情况,
image.png
如果我们使用的是一个GridLayoutManager 这个manager,每一行有3条数据,那么最大2个的缓存池是肯定需要重新bindViewHoder的,这种情况我们就可以适当的将这个最大值扩充一下,至于说是3还是6,这个就仁者见仁,智者见智了,
说完了数据缓存,我们再来看看我们是如何将缓存的数据取出来的,
recycler.getViewForPosition(i);
getViewForPosition就是我们获取到这个view 的入口,既然我么分析过了保存数据的过程,其实就可以猜测这个获取的过程,肯定是最开始从detach 的mAttachedScrap 这个list里面获取,因为他是缓存的屏幕上面的数据,也就是我么需要渲染的数据,其次是mCachedViews 这个list,mCachedViews 的缓存list的size比较小,所以不能缓存太多数据,然后是 RecycledViewPool 这个缓存池从这个里面获取的holder可能是没有绑定数据,也可能绑定错误的数据,所以需要重新绑定数据,最后才是new ,我么来具体看一下实现方法,由于这个里面的代码比较长,我会删掉一些不影响逻辑的方法
@NonNull
public View getViewForPosition(int position) {
return getViewForPosition(position, false);
}
View getViewForPosition(int position, boolean dryRun) {
return tryGetViewHolderForPositionByDeadline(position, dryRun, FOREVER_NS).itemView;
}
@Nullable
ViewHolder tryGetViewHolderForPositionByDeadline(int position,
boolean dryRun, long deadlineNs) {
....
boolean fromScrapOrHiddenOrCache = false;
ViewHolder holder = null;
if (mState.isPreLayout()) {
holder = getChangedScrapViewForPosition(position);
fromScrapOrHiddenOrCache = holder != null;
}
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) {
holder.addFlags(ViewHolder.FLAG_INVALID);
if (holder.isScrap()) {
removeDetachedView(holder.itemView, false);
holder.unScrap();
}
....
}
....
}
....
}
}
if (holder == null) {
final int offsetPosition = mAdapterHelper.findPositionOffset(position);
final int type = mAdapter.getItemViewType(offsetPosition);
if (mAdapter.hasStableIds()) {
holder = getScrapOrCachedViewForId(mAdapter.getItemId(offsetPosition), type, dryRun);
....
}
if (holder == null && mViewCacheExtension != null) {
final View view = mViewCacheExtension.getViewForPositionAndType(this, position, type);
....
}
if (holder == null) {
holder = getRecycledViewPool().getRecycledView(type);
....
}
if (holder == null) {
holder = mAdapter.createViewHolder(RecyclerView.this, type);
...
}
}
.....
return holder;
}
虽然我输出了一大部分代码,但是剩下的代码还是比较多,逻辑非常清晰,我们一点一点来看,
我们先来看第一次获取viewholder
if (mState.isPreLayout()) {
holder = getChangedScrapViewForPosition(position);
fromScrapOrHiddenOrCache = holder != null;
}
我只知道isPreLayout和执行动画有关,但是他们之间到底什么是什么关系就不是很清楚了,这个过程还没有分析,我们就先越过这个过程
再看第二个判断,
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) {
holder.addFlags(ViewHolder.FLAG_INVALID);
if (holder.isScrap()) {
removeDetachedView(holder.itemView, false);
holder.unScrap();
}
....
}
....
}
....
}
}
我们可以看到这个holder是通过getScrapOrHiddenOrCachedHolderForPosition 这个方法来找到,我们进去看看他具体的实现逻辑,
在这之前来说一下我们在detach的时候会将数据添加到mAttachedScrap这个list,我们知道了在remove的时候我们会将这个holder移出mAttachedScrap,从上面这个方法中我们看到同样进行了holder.unScrap();,也就是在如果在mAttachedScrap找到这个holder也会将它在list移除
getScrapOrHiddenOrCachedHolderForPosition
ViewHolder getScrapOrHiddenOrCachedHolderForPosition(int position, boolean dryRun) {
final int scrapCount = mAttachedScrap.size();
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) {
final ViewHolder vh = getChildViewHolderInt(view);
mChildHelper.unhide(view);
int layoutIndex = mChildHelper.indexOfChild(view);
mChildHelper.detachViewFromParent(layoutIndex);
scrapView(view);
vh.addFlags(ViewHolder.FLAG_RETURNED_FROM_SCRAP
| ViewHolder.FLAG_BOUNCED_FROM_HIDDEN_LIST);
return vh;
}
}
final int cacheSize = mCachedViews.size();
for (int i = 0; i < cacheSize; i++) {
final ViewHolder holder = mCachedViews.get(i);
if (!holder.isInvalid() && holder.getLayoutPosition() == position) {
if (!dryRun) {
mCachedViews.remove(i);
}
return holder;
}
}
return null;
}
我们看到在这个getScrapOrHiddenOrCachedHolderForPosition 方法中首先遍历mAttachedScrap,如果mAttachedScrap这个position 和我们需要查找的position相同则返回,如果没有找到则根据我们根据dryRun来判断,我们看到在View getViewForPosition(int position)方法中这个dryRun向下传递的是false,那么这里就是调用的从mChildHelper.findHiddenNonRemovedView 这个方法,
View findHiddenNonRemovedView(int position) {
final int count = mHiddenViews.size();
for (int i = 0; i < count; i++) {
final View view = mHiddenViews.get(i);
RecyclerView.ViewHolder holder = mCallback.getChildViewHolder(view);
if (holder.getLayoutPosition() == position
&& !holder.isInvalid()
&& !holder.isRemoved()) {
return view;
}
}
return null;
}
这就是第一次查找,如果第一次查找我们没有找到,那么就应该去下一个地方继续查找
final int offsetPosition = mAdapterHelper.findPositionOffset(position);
final int type = mAdapter.getItemViewType(offsetPosition);
if (mAdapter.hasStableIds()) {
holder = getScrapOrCachedViewForId(mAdapter.getItemId(offsetPosition),
type, dryRun);
if (holder != null) {
// update position
holder.mPosition = offsetPosition;
fromScrapOrHiddenOrCache = true;
}
}
先找到offsetPosition ,根绝offsetPosition 查找type,最后根据itemdid 和 type 去getScrapOrCachedViewForId方法查找
getScrapOrCachedViewForId
ViewHolder getScrapOrCachedViewForId(long id, int type, boolean dryRun) {
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()) {
if (!mState.isPreLayout()) {
holder.setFlags(ViewHolder.FLAG_UPDATE, ViewHolder.FLAG_UPDATE
| ViewHolder.FLAG_INVALID | ViewHolder.FLAG_REMOVED);
}
}
return holder;
} else if (!dryRun) {
mAttachedScrap.remove(i);
removeDetachedView(holder.itemView, false);
quickRecycleScrapView(holder.itemView);
}
}
}
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;
}
从上面的方法我们看到同样的还是遍历mAttachedScrap 这个list,只不过这次判断的逻辑是 itemid 和type,同时相同才可以,如果itemid相同,type不同那么则将这个holder 放入到mCachedViews 中,接下来就是遍历mCachedViews,继续寻找,
若果上面两种情况都没找到,就会继续去mViewCacheExtension 中寻找,但是mViewCacheExtension这个需要我们自己去实现,这里就不多说了,继续看下面
holder = getRecycledViewPool().getRecycledView(type);
这个就很简单了,从缓存池根据type获取,
最后一个就是new
holder = mAdapter.createViewHolder(RecyclerView.this, type);
整个过程我们就分析完成了,一个非常巧妙的设计,
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