最新項(xiàng)目遇到一個(gè)問題，就是RecycleView的itemview會頻繁拉取圖片，同一時(shí)間多次拉取同一張照片。
初探，是因?yàn)樵搱鼍皀otifyDataSetChanged()過于頻繁，一秒鐘會調(diào)用5次左右，
導(dǎo)致ViewHolder沒有復(fù)用，也不是沒有復(fù)用而是復(fù)用的并沒有像理想中的樣式。

notify

notifyDataSetChanged

mObservable.notifyChanged()

mObservers.get(i).onChanged();

setDataSetChangedAfterLayout();
4.1.1 markKnownViewsInvalid
//1 Mark all known views as invalid，僅標(biāo)記可見的布局為ViewHolder.FLAG_UPDATE | ViewHolder.FLAG_INVALID
//2 mAdapter.hasStableIds() true -> mCachedViews 標(biāo)記 ViewHolder.FLAG_UPDATE | ViewHolder.FLAG_INVALID
//3 mAdapter.hasStableIds() false -> addViewHolderToRecycledViewPool ->getRecycledViewPool().putRecycledView(holder); AND mCachedViews.clear();
// we cannot re-use cached views in this case. Recycle them all
4.1.2 markItemDecorInsetsDirty
//child.getLayoutParams()).mInsetsDirty = true;
//mCachedViews layoutParams.mInsetsDirty = true;

requestLayout();

回收邏輯

void onLayout(boolean changed, int l, int t, int r, int b)

dispatchLayout()

void dispatchLayoutStep2()

void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state)

detachAndScrapAttachedViews(recycler); //先回收，在fill，也就是后去查找可以復(fù)用的holder。僅處理children holder

scrapOrRecycleView(recycler, i, v);
6.1 viewHolder.isInvalid() && !viewHolder.isRemoved() && !mRecyclerView.mAdapter.hasStableIds() -->
6.1.1 true recycler.recycleViewHolderInternal(viewHolder);
addViewHolderToRecycledViewPool(holder, true); //最終放入getRecycledViewPool()
6.1.2 false scrapView(View view)
holder.hasAnyOfTheFlags(ViewHolder.FLAG_REMOVED | ViewHolder.FLAG_INVALID) || !holder.isUpdated() || canReuseUpdatedViewHolder(holder) -->
6.1.2.1 true mAttachedScrap.add(holder);
6.1.2.2 false mChangedScrap.add(holder);
總結(jié)：如果Holder無效最終放入getRecycledViewPool，否則mAttachedScrap

回收另外一個(gè)分支

onLayout

dispatchLayout

dispatchLayoutStep3

removeAndRecycleScrapInt //僅處理mAttachedScrap所緩存Holder
vh.setIsRecyclable(true);

recycler.quickRecycleScrapView(scrap);

recycleViewHolderInternal(ViewHolder holder)
forceRecycle || holder.isRecyclable() 成立
(mViewCacheMax > 0 && !holder.hasAnyOfTheFlags(ViewHolder.FLAG_INVALID | ViewHolder.FLAG_REMOVED | ViewHolder.FLAG_UPDATE | ViewHolder.FLAG_ADAPTER_POSITION_UNKNOWN)) 不成立，包含ViewHolder.FLAG_INVALID

addViewHolderToRecycledViewPool(ViewHolder holder, boolean dispatchRecycled)

getRecycledViewPool().putRecycledView(holder); 最終放到了getRecycledViewPool中。

查找可復(fù)用Holder過程

void onLayout(boolean changed, int l, int t, int r, int b)

dispatchLayout()

void dispatchLayoutStep2()

void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state)

fill(RecyclerView.Recycler recycler, LayoutState layoutState,
RecyclerView.State state, boolean stopOnFocusable)

layoutChunk(RecyclerView.Recycler recycler, RecyclerView.State state,
LayoutState layoutState, LayoutChunkResult result)

LinerLayout.next

getViewForPosition

tryGetViewHolderForPositionByDeadline
9.1 isPreLayout() --> getChangedScrapViewForPosition(int position) //如果是pre-layout，會從mChangedScrap獲取復(fù)用
9.2 getScrapOrHiddenOrCachedHolderForPosition(position, dryRun)
9.2.1 mAttachedScrap 首先從該處獲取，
9.2.2 mChildHelper.findHiddenNonRemovedView(position) 拿到隱藏的ViewHolder
9.2.3 mCachedViews //Search in our first-level recycled view cache. 最后從一級緩存獲取
// invalid view holders may be in cache if adapter has stable ids as they can be
// retrieved via getScrapOrCachedViewForId
// 通過原廠注釋可以知道，has stable才可以使用mCachedViews緩存。在回收處也可以看出。
9.2.4 mAdapter.hasStableIds() --> getScrapOrCachedViewForId() //仍然需要hasStableIds
// 首先mAttachedScrap
// 之后mCachedViews
9.3 mViewCacheExtension //外部擴(kuò)展
9.4 getRecycledViewPool().getRecycledView(type) //此處查到holder被resetInternal，即需要重新bind
9.5 mAdapter.createViewHolder(RecyclerView.this, type); //最后，create

bind過程

tryGetViewHolderForPositionByDeadline(int position,
boolean dryRun, long deadlineNs)

tryBindViewHolderByDeadline

mAdapter.bindViewHolder(holder, offsetPosition);

至此RecyclerView的復(fù)用機(jī)制已經(jīng)差不多了。至此也看到了我的問題也出現(xiàn)了答案的線索。
因?yàn)槭莕otifyDataSetChanged出發(fā)的刷新，會將所有ViewHolder標(biāo)記為FLAG_INVALID。
在回收ViewHolder時(shí)，會放入getRecycledViewPool中。
RecycledViewPool官方注釋

RecycledViewPool lets you share Views between multiple RecyclerViews.
If you want to recycle views across RecyclerViews, create an instance of RecycledViewPool
and use {@link RecyclerView#setRecycledViewPool(RecycledViewPool)}.
RecyclerView automatically creates a pool for itself if you don’t provide one.

作用為多個(gè)RecyclerViews之間復(fù)用ViewHolder。
可以通過setRecycledViewPool進(jìn)行設(shè)置。
SparseArray mScrap = new SparseArray<>(); 用來存儲不同類型的ViewHolder

static class ScrapData {ArrayList<ViewHolder> mScrapHeap = new ArrayList<>();int mMaxScrap = DEFAULT_MAX_SCRAP;long mCreateRunningAverageNs = 0;long mBindRunningAverageNs = 0; }

默認(rèn)mMaxScrap大小為5個(gè)

private static final int DEFAULT_MAX_SCRAP = 5;

也就是做多存儲5個(gè)。我的項(xiàng)目正好有6個(gè)Item顯示，就導(dǎo)致只能存儲 1,2,3,4,5 而0沒有進(jìn)行存儲
onBind的時(shí)候就會出現(xiàn)0 - 綁定 1,1 綁定2… 的問題。

解決辦法為通過
new一個(gè)RecyclerView.RecycledViewPool， 并設(shè)置recycledViewPool.setMaxRecycledViews(0, 6);
第一個(gè)參數(shù)viewType，我這里僅有一種type，所以就寫0
最終通過mRecycleList.setRecycledViewPool(recycledViewPool); 設(shè)置RecycledViewPool。
至此大工告成。

RecyclerView源碼有1w多行。剛看的時(shí)候無從下手，向其他博主所說的見森林而不見樹木。
我就是從notifyDataSetChanged 和 onBindViewHolder入手，查找上下邏輯從而追蹤定位問題。

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