RecyclerView緩存機制（scrap view）

這是RecyclerView緩存機制系列文章的第四篇材蛛，系列文章的目錄如下：

第一篇中遺留的一個問題還沒有解決：復(fù)用表項時優(yōu)先級最高的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問題毅整。

scrap view對應(yīng)的存儲結(jié)構(gòu)是final ArrayList<ViewHolder> mAttachedScrap = new ArrayList<>();。理解成員變量用途的最好辦法是 “搜索它在什么時候被訪問” 绽左。對于列表結(jié)構(gòu)來說就相當于 1. 在什么時候往列表添加內(nèi)容悼嫉？ 2. 在什么時候清空列表內(nèi)容？

添加內(nèi)容

全局搜索mAttachedScrap被訪問的地方拼窥，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戏蔑。。鲁纠。
         *
         * @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;
            }
            //刪除表項并入回收池
            if (viewHolder.isInvalid() && !viewHolder.isRemoved()
                    && !mRecyclerView.mAdapter.hasStableIds()) {
                removeViewAt(index);
                recycler.recycleViewHolderInternal(viewHolder);
            }
            //detach表項并入scrap集合
            else {
                detachViewAt(index);
                recycler.scrapView(view);
                mRecyclerView.mViewInfoStore.onViewDetached(viewHolder);
            }
        }
}

根據(jù)viewHolder的不同狀態(tài)总棵，要么將其將其添加到mAttachedScrap集合，要么將其存入回收池改含。其中recycleViewHolderInternal()在RecyclerView緩存機制（回收去哪情龄？）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.
         * 暫時將當可見表項進行分離并回收
         *
         * @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) {
            ...
            //在填充表項之前回收所有表項
            detachAndScrapAttachedViews(recycler);
            ...
            if (mAnchorInfo.mLayoutFromEnd) {
                ...
                //填充表項
                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);
        ...
    }
}

在將表項一個個填充到列表之前會先將其先回收到mAttachedScrap中候味，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隔心。
注釋中“暫時將當可見表項進行分離并回收”白群，既然是“暫時回收”，那待會必然會發(fā)生“復(fù)用”硬霍。復(fù)用邏輯可移步RecyclerView緩存機制（咋復(fù)用帜慢？）
至此可以得出結(jié)論：mAttachedScrap用于屏幕中可見表項的回收和復(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布局開始唯卖，終止于RecyclerView布局結(jié)束粱玲。

總結(jié)

經(jīng)過四篇文章的分析，RecyclerVeiw的四級緩存都分析完了拜轨，總結(jié)如下：

Recycler有4個層次用于緩存ViewHolder對象抽减，優(yōu)先級從高到底依次為ArrayList<ViewHolder> mAttachedScrap、ArrayList<ViewHolder> mCachedViews橄碾、ViewCacheExtension mViewCacheExtension卵沉、RecycledViewPool mRecyclerPool。如果四層緩存都未命中法牲，則重新創(chuàng)建并綁定ViewHolder對象
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	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：沒有大小限制史汗，但最多包含屏幕可見表項。
- mCachedViews：默認大小限制為2拒垃，放不下時停撞，按照先進先出原則將最先進入的ViewHolder存入回收池以騰出空間。
- mRecyclerPool：對ViewHolder按viewType分類存儲（通過SparseArray）悼瓮，同類ViewHolder存儲在默認大小為5的ArrayList中戈毒。
緩存用途：
- mAttachedScrap：用于布局過程中屏幕可見表項的回收和復(fù)用艰猬。
- mCachedViews：用于移出屏幕表項的回收和復(fù)用，且只能用于指定位置的表項副硅，有點像“回收池預(yù)備隊列”姥宝，即總是先回收到mCachedViews，當它放不下的時候恐疲，按照先進先出原則將最先進入的ViewHolder存入回收池腊满。
- mRecyclerPool：用于移出屏幕表項的回收和復(fù)用，且只能用于指定viewType的表項
緩存結(jié)構(gòu)：
- mAttachedScrap：ArrayList<ViewHolder>
- mCachedViews：ArrayList<ViewHolder>
- mRecyclerPool：對ViewHolder按viewType分類存儲在SparseArray<ScrapData>中培己，同類ViewHolder存儲在ScrapData中的ArrayList中