RecyclerView回收复用机制探索

aopmeta 10/3/2021 源码分析

# 缓存获取流程

# 入口一

onTouchEvent -> scrollByInternal -> scrollStep -> mLayout.scrollVerticallyBy -> scrollBy -> fill -> layoutChunk -> layoutState.next -> addView

layoutState.next -> recycler.getViewForPosition -> tryGetViewHolderForPositionByDeadline

tryGetViewHolderForPositionByDeadline内缓存复用详情

分几种情况去获取ViewHolder

getChangedScrapViewForPosition -- mChangeScrap与动画相关
getScrapOrHiddenOrCachedHolderForPosition -- 通过位置从mAttachedScrap和mCachedViews中获取ViewHolder
getScrapOrCachedViewForId -- mAttachedScrap、mCachedViews（通过viewType和itemId）
mViewCacheExtension.getViewForPositionAndType -- 自定义的缓存
getRecycledViewPool().getRecycledView(type) -- 从缓存池里面获取

当没有缓存时mAdapter.createViewHolder

创建ViewHolder后绑定 -- tryBindViewHolderByDeadline -> mAdapter.bindViewHolder

# 入口二

onLayout -> dispatchLayout -> dispatchLayoutStep2 -> mLayout.onLayoutChildren->fill

# 四级缓存总结

mChangeScrap与mAttachedScrap

用来缓存还在屏幕内的ViewHolder
mCachedViews(ArrayList 先进先出，最大不超过mViewCacheMax=2)

用来缓存移除屏幕之外的ViewHolder
mViewCacheExtension

开发给用户的自定义拓展缓存，需要用户自己管理View的创建和缓存
RecycledViewPool(SparseArray,key为viewType)

ViewHolder缓存池

static class ScrapData {
  final ArrayList<ViewHolder> mScrapHeap = new ArrayList<>();
  int mMaxScrap = DEFAULT_MAX_SCRAP; //5
  ...
}

# 回收机制流程

onLayout -> dispatchLayout -> dispatchLayoutStep2 -> mLayout.onLayoutChildren ->

# 情况一

fill -> recycleByLayoutState -> recycleViewsFromStart -> recycleChildren -> removeAndRecycleViewAt -> recycler.recycleView -> recycleViewHolderInternal

# 情况二

detachAndScrapAttachedViews -> scrapOrRecycleView

-> recycler.recycleViewHolderInternal //缓存至mCacheViews、RecyclerViewPool

a. -> holder没有变化且mCachedViews的数量大于mViewCacheMax -> recycleCachedViewAt -> addViewHolderToRecycledViewPool,mCachedViews.remove -> mCachedViews.add

先进先出，先判断mCacheViews是否超过2，超过的话把index为0的放入缓存池，再插入新的holder

b. -> addViewHolderToRecycledViewPool -> getRecycledViewPool().putRecycledView

public void putRecycledView(ViewHolder scrap) {
    final int viewType = scrap.getItemViewType();
    final ArrayList<ViewHolder> scrapHeap = getScrapDataForType(viewType).mScrapHeap;
    //超过最大值后直接丢弃
    if (mScrap.get(viewType).mMaxScrap <= scrapHeap.size()) {
        return;
    }
    ...
    scrap.resetInternal(); //viewholder数据清空，只是类型，没有数据
    scrapHeap.add(scrap);
}

-> recycler.scrapView

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);
    }
}

元宇宙的切面