众所周知，RecyclerView在android中实现列表是性能非常好的，那么性能好的原因在哪里呢？关键还是在它在处理view时的回收和复用。列表在滑动的时候，会进行itemView的回收和复用，那么我们就从滑动回调即onTouchEvent来入手分析。
基本概念
- ViewHolder: View的容器，一项View就对应一个ViewHolder
- Recyler：RecyclerView的内部类，主要负责View的回收和复用
- LinearLayoutManager: RecyclerView的线性布局管理器
滑动时函数调用链
四级缓存机制
| 层级 | 缓存变量 | 缓存名 | 容量 | 数据结构 | 缓存用途 |
|---|---|---|---|---|---|
| 1 | mChangeScrap与 mAttachedScrap | 可见缓存 | x | ArrayList | 用于布局过程中屏幕可见表项的回收和复用 |
| 2 | mCachedViews | 缓存列表 | 2 | ArrayList | 用于移出屏幕表项的回收和复用，不会清空数据 |
| 3 | mViewCacheExtension | 自定义缓存 | x | ||
| 4 | RecycledViewPool | 缓存池 | 5 | SparseArray | 用于移出屏幕表项的回收和复用，会将ViewHolder的数据重置 |
回收的关键方法分析
## RecyclerView.java
void recycleViewHolderInternal(ViewHolder holder) {
...
if (forceRecycle || holder.isRecyclable()) {
if (mViewCacheMax > 0
&& !holder.hasAnyOfTheFlags(ViewHolder.FLAG_INVALID
| ViewHolder.FLAG_REMOVED
| ViewHolder.FLAG_UPDATE
| ViewHolder.FLAG_ADAPTER_POSITION_UNKNOWN))
// 1) 先尝试放到cacheView中
int cachedViewSize = mCachedViews.size();
if (cachedViewSize >= mViewCacheMax && cachedViewSize > 0) {
// 如果 mCachedViews 已经满了，把第0个位置的移除并放到 缓存池中
recycleCachedViewAt(0);
cachedViewSize--;
}
if (!cached) {
// 2） 如果CacheView中没放进去，就放到 缓存池中
addViewHolderToRecycledViewPool(holder, true);
recycled = true;
}
...
}
}
复用的关键方法分析
tryGetViewHolderForPositionByDeadline
- 从一级缓存 mChangeScrap 中取
- 从二级缓存 mCachedViews中取
- 从三级缓存 mViewCacheExtension 中取
- 从四级缓存 缓存池中取
- 缓存中都没有拿到值，就直接创建
- 未绑定过时，进行绑定
ViewHolder tryGetViewHolderForPositionByDeadline(int position,
boolean dryRun, long deadlineNs) {
ViewHolder holder = null;
// 1) 从一级缓存 changed scrap 中取
if (mState.isPreLayout()) {
holder = getChangedScrapViewForPosition(position);
fromScrapOrHiddenOrCache = holder != null;
}
// 2）从二级缓存 cache中取
if (holder == null) {
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;
}
}
// 3. 从三级缓存 CacheExtension 中取
if (holder == null && mViewCacheExtension != null) {
final View view = mViewCacheExtension
.getViewForPositionAndType(this, position, type);
if (view != null) {
holder = getChildViewHolder(view);
}
}
// 4) 从四级缓存 缓存池中取
if (holder == null) { // fallback to pool
holder = getRecycledViewPool().getRecycledView(type);
if (holder != null) {
holder.resetInternal();
if (FORCE_INVALIDATE_DISPLAY_LIST) {
invalidateDisplayListInt(holder);
}
}
}
// 5）缓存中都没有拿到值，就直接创建
if (holder == 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");
}
}
}
// 6）已经 bind过了，不会再去绑定，未绑定过时，进行绑定
if (mState.isPreLayout() && holder.isBound()) {
holder.mPreLayoutPosition = position;
} else if (!holder.isBound() || holder.needsUpdate() || holder.isInvalid()) {
final int offsetPosition = mAdapterHelper.findPositionOffset(position);
// 尝试 bindView
bound = tryBindViewHolderByDeadline(holder, offsetPosition, position, deadlineNs);
}
return holder;
}
一级缓存-缓存碎片
ViewHolder getChangedScrapViewForPosition(int position) {
// If pre-layout, check the changed scrap for an exact match.
final int changedScrapSize;
if (mChangedScrap == null || (changedScrapSize = mChangedScrap.size()) == 0) {
return null;
}
// find by position
for (int i = 0; i < changedScrapSize; i++) {
final ViewHolder holder = mChangedScrap.get(i);
if (!holder.wasReturnedFromScrap() && holder.getLayoutPosition() == position) {
holder.addFlags(ViewHolder.FLAG_RETURNED_FROM_SCRAP);
return holder;
}
}
// find by id
if (mAdapter.hasStableIds()) {
final int offsetPosition = mAdapterHelper.findPositionOffset(position);
if (offsetPosition > 0 && offsetPosition < mAdapter.getItemCount()) {
final long id = mAdapter.getItemId(offsetPosition);
for (int i = 0; i < changedScrapSize; i++) {
final ViewHolder holder = mChangedScrap.get(i);
if (!holder.wasReturnedFromScrap() && holder.getItemId() == id) {
holder.addFlags(ViewHolder.FLAG_RETURNED_FROM_SCRAP);
return holder;
}
}
}
}
return null;
}
二级缓存-缓存列表
ViewHolder getScrapOrCachedViewForId(long id, int type, boolean dryRun) {
// 1) 先从mAttachedScrap中取，取到便返回
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);
}
}
}
// 2）二级缓存，从mCachedViews中取
final int cacheSize = mCachedViews.size();
for (int i = cacheSize - 1; i >= 0; i--) {
final ViewHolder holder = mCachedViews.get(i);
//从mCachedViews中取到后便返回
if (holder.getItemId() == id) {
if (type == holder.getItemViewType()) {
if (!dryRun) {
mCachedViews.remove(i);
}
return holder;
} else if (!dryRun) {
recycleCachedViewAt(i);
return null;
}
}
}
return null;
}
三级缓存-自定义缓存
略
四级缓存-缓存池
public ViewHolder getRecycledView(int viewType) {
//从mScrap中根据view的类型来取出一个
final ScrapData scrapData = mScrap.get(viewType);
if (scrapData != null && !scrapData.mScrapHeap.isEmpty()) {
final ArrayList<ViewHolder> scrapHeap = scrapData.mScrapHeap;
//从 scrapData 中拿最后一个数据，先进后出
return scrapHeap.remove(scrapHeap.size() - 1);
}
return null;
}
static class ScrapData {
//ViewHolder是作为一个List被存进来的
ArrayList<ViewHolder> mScrapHeap = new ArrayList<>();
// 缓存池中 list的大小是5，也就是每个类型的view缓存池中存储5个
int mMaxScrap = 5;
long mCreateRunningAverageNs = 0;
long mBindRunningAverageNs = 0;
}
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