這是RecyclerView緩存機(jī)制系列文章的第四篇，系列文章的目錄如下：
- RecyclerView緩存機(jī)制（咋復(fù)用？）
- RecyclerView緩存機(jī)制（回收些啥？）
- RecyclerView緩存機(jī)制（回收去哪？）
- RecyclerView緩存機(jī)制（scrap view）
第一篇中遺留的一個(gè)問題還沒有解決：復(fù)用表項(xiàng)時(shí)優(yōu)先級(jí)最高的scrap view是用來干嘛的？這篇文章試著通過閱讀源碼來解答這個(gè)問題。
scrap view對(duì)應(yīng)的存儲(chǔ)結(jié)構(gòu)是final ArrayList<ViewHolder> mAttachedScrap = new ArrayList<>();。理解成員變量用途的最好辦法是 “搜索它在什么時(shí)候被訪問” 。對(duì)于列表結(jié)構(gòu)來說就相當(dāng)于 1. 在什么時(shí)候往列表添加內(nèi)容？ 2. 在什么時(shí)候清空列表內(nèi)容？
添加內(nèi)容
全局搜索mAttachedScrap被訪問的地方，其中只有一處調(diào)用了mAttachedScrap.add():
public final class Recycler {
/**
* Mark an attached view as scrap.
* 回收ViewHolder到scrap集合（mAttachedScrap或mChangedScrap）
*
* <p>"Scrap" views are still attached to their parent RecyclerView but are eligible
* for rebinding and reuse. Requests for a view for a given position may return a
* reused or rebound scrap view instance.</p>
* scrap view依然依附于它的父親。。。
*
* @param view View to scrap
*/
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);
//添加到mAttachedScrap集合中
mAttachedScrap.add(holder);
} else {
if (mChangedScrap == null) {
mChangedScrap = new ArrayList<ViewHolder>();
}
holder.setScrapContainer(this, true);
//添加到mChangedScrap集合中
mChangedScrap.add(holder);
}
}
}
沿著調(diào)用鏈繼續(xù)往上：
public abstract static class LayoutManager {
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;
}
//刪除表項(xiàng)并入回收池
if (viewHolder.isInvalid() && !viewHolder.isRemoved()
&& !mRecyclerView.mAdapter.hasStableIds()) {
removeViewAt(index);
recycler.recycleViewHolderInternal(viewHolder);
}
//detach表項(xiàng)并入scrap集合
else {
detachViewAt(index);
recycler.scrapView(view);
mRecyclerView.mViewInfoStore.onViewDetached(viewHolder);
}
}
}
根據(jù)viewHolder的不同狀態(tài)，要么將其將其添加到mAttachedScrap集合，要么將其存入回收池。其中recycleViewHolderInternal()在RecyclerView緩存機(jī)制（回收去哪？）分析過。
沿著調(diào)用鏈繼續(xù)向上：
public abstract static class LayoutManager {
/**
* Temporarily detach and scrap all currently attached child views. Views will be scrapped
* into the given Recycler. The Recycler may prefer to reuse scrap views before
* other views that were previously recycled.
* 暫時(shí)將當(dāng)可見表項(xiàng)進(jìn)行分離并回收
*
* @param recycler Recycler to scrap views into
*/
public void detachAndScrapAttachedViews(Recycler recycler) {
final int childCount = getChildCount();
//遍歷所有可見表項(xiàng)并回收他們
for (int i = childCount - 1; i >= 0; i--) {
final View v = getChildAt(i);
scrapOrRecycleView(recycler, i, v);
}
}
/**
* Lay out all relevant child views from the given adapter.
* 從給定的adapter布局所有的孩子
*/
public void onLayoutChildren(Recycler recycler, State state) {
...
//在填充表項(xiàng)之前回收所有表項(xiàng)
detachAndScrapAttachedViews(recycler);
...
if (mAnchorInfo.mLayoutFromEnd) {
...
//填充表項(xiàng)
fill(recycler, mLayoutState, state, false);
...
}
...
}
}
public class RecyclerView extends ViewGroup implements ScrollingView, NestedScrollingChild2 {
//RecyclerView布局的第二步
private void dispatchLayoutStep2() {
...
mLayout.onLayoutChildren(mRecycler, mState);
...
}
}
- 在將表項(xiàng)一個(gè)個(gè)填充到列表之前會(huì)先將其先回收到
mAttachedScrap中，回收數(shù)據(jù)的來源是LayoutManager的孩子，而LayoutManager的孩子都是屏幕上可見的表項(xiàng)。 - 注釋中“暫時(shí)將當(dāng)可見表項(xiàng)進(jìn)行分離并回收”，既然是“暫時(shí)回收”，那待會(huì)必然會(huì)發(fā)生“復(fù)用”。復(fù)用邏輯可移步RecyclerView緩存機(jī)制（咋復(fù)用？）
- 至此可以得出結(jié)論：
mAttachedScrap用于屏幕中可見表項(xiàng)的回收和復(fù)用
清空內(nèi)容
全局搜索mAttachedScrap被訪問的地方，其中只有一處調(diào)用了mAttachedScrap.clear():
public final class Recycler {
void clearScrap() {
mAttachedScrap.clear();
if (mChangedScrap != null) {
mChangedScrap.clear();
}
}
}
public abstract static class LayoutManager {
/**
* Recycles the scrapped views.
* 回收所有scrapped view
*/
void removeAndRecycleScrapInt(Recycler recycler) {
final int scrapCount = recycler.getScrapCount();
// Loop backward, recycler might be changed by removeDetachedView()
//遍歷搜有scrap view并重置ViewHolder狀態(tài)
for (int i = scrapCount - 1; i >= 0; i--) {
final View scrap = recycler.getScrapViewAt(i);
final ViewHolder vh = getChildViewHolderInt(scrap);
if (vh.shouldIgnore()) {
continue;
}
vh.setIsRecyclable(false);
if (vh.isTmpDetached()) {
mRecyclerView.removeDetachedView(scrap, false);
}
if (mRecyclerView.mItemAnimator != null) {
mRecyclerView.mItemAnimator.endAnimation(vh);
}
vh.setIsRecyclable(true);
recycler.quickRecycleScrapView(scrap);
}
//清空scrap view集合
recycler.clearScrap();
if (scrapCount > 0) {
mRecyclerView.invalidate();
}
}
}
沿著調(diào)用鏈向上：
public class RecyclerView extends ViewGroup implements ScrollingView, NestedScrollingChild2 {
//RecyclerView布局的最后一步
private void dispatchLayoutStep3() {
...
mLayout.removeAndRecycleScrapInt(mRecycler);
...
}
至此可以得出結(jié)論：mAttachedScrap生命周期起始于RecyclerView布局開始，終止于RecyclerView布局結(jié)束。
總結(jié)
經(jīng)過四篇文章的分析，RecyclerVeiw的四級(jí)緩存都分析完了，總結(jié)如下：
Recycler有4個(gè)層次用于緩存ViewHolder對(duì)象，優(yōu)先級(jí)從高到底依次為ArrayList<ViewHolder> mAttachedScrap、ArrayList<ViewHolder> mCachedViews、ViewCacheExtension mViewCacheExtension、RecycledViewPool mRecyclerPool。如果四層緩存都未命中，則重新創(chuàng)建并綁定ViewHolder對(duì)象-
緩存性能：
緩存 重新創(chuàng)建 ViewHolder重新綁定數(shù)據(jù) mAttachedScrap false false mCachedViews false false mRecyclerPool false true
-
緩存容量：
-
mAttachedScrap：沒有大小限制，但最多包含屏幕可見表項(xiàng)。 -
mCachedViews：默認(rèn)大小限制為2，放不下時(shí)，按照先進(jìn)先出原則將最先進(jìn)入的ViewHolder存入回收池以騰出空間。 -
mRecyclerPool：對(duì)ViewHolder按viewType分類存儲(chǔ)（通過SparseArray），同類ViewHolder存儲(chǔ)在默認(rèn)大小為5的ArrayList中。
-
-
緩存用途：
-
mAttachedScrap：用于布局過程中屏幕可見表項(xiàng)的回收和復(fù)用。 -
mCachedViews：用于移出屏幕表項(xiàng)的回收和復(fù)用，且只能用于指定位置的表項(xiàng)，有點(diǎn)像“回收池預(yù)備隊(duì)列”，即總是先回收到mCachedViews，當(dāng)它放不下的時(shí)候，按照先進(jìn)先出原則將最先進(jìn)入的ViewHolder存入回收池。 -
mRecyclerPool：用于移出屏幕表項(xiàng)的回收和復(fù)用，且只能用于指定viewType的表項(xiàng)
-
-
緩存結(jié)構(gòu)：
-
mAttachedScrap：ArrayList<ViewHolder> -
mCachedViews：ArrayList<ViewHolder> -
mRecyclerPool：對(duì)ViewHolder按viewType分類存儲(chǔ)在SparseArray<ScrapData>中，同類ViewHolder存儲(chǔ)在ScrapData中的ArrayList中
-
