RecyclerView#Adapter#notifyDataSetChanged方法后盅弛，為何還會新建ViewHolder？

環(huán)境

android sdk版本: 30

依賴:

implementation 'androidx.core:core-ktx:1.3.2'
implementation 'androidx.appcompat:appcompat:1.2.0'
implementation "androidx.recyclerview:recyclerview:1.2.1"

案例分析：

RecyclerView寬高固定铲汪；LayoutManager是LienarLayoutManager熊尉，vertical方向；數(shù)據(jù)20條掌腰，足以鋪滿整個屏幕狰住。

現(xiàn)象：
①先創(chuàng)建Adapter，設置20條數(shù)據(jù)齿梁。
②調(diào)用RecyclerView#Adapter#notifyDataSetChanged方法后催植，當前頁面中只有5個ViewHolder復用，其余的ViewHolder會走Adapter#createViewHolder方法創(chuàng)建新的ViewHolder勺择。

原理：

為了搞清楚原理创南，我們先看一下，剛進入頁面時省核，RecyclerView#Adapter#onCreateViewHolder方法的調(diào)用棧稿辙。

RecyclerView#Adapter#onCreateViewHolder方法的調(diào)用棧

RecyclerView#onLayout(): 4578行
RecyclerView#dispatchLayout(): 4012行
RecyclerView#dispatchLayoutStep2():4309行
LinearLayoutManager#onLayoutChildren(): 668行
LinearLayoutManager#fill(): 1591行
LinearLayoutManager#layoutChunk(): 1631行
LinearLayoutManager#LayoutState#next(): 2330行
RecyclerView#Recycler#getViewForPosition(int position): 6296行
RecyclerView#Recycler#getViewForPosition(position, boolean dryRun): 6300行
RecyclerView#Recycler#tryGetViewHolderForPositionByDeadline(int position, boolean dryRun, long deadlineNs): 6416行
RecyclerView#Adapter#createViewHolder(@NonNull ViewGroup parent, int viewType): 7295行
RecyclerView#Adapter#onCreateViewHolder(@NonNull ViewGroup parent, int viewType)

其核心是RecyclerView#Recycler#tryGetViewHolderForPositionByDeadline方法，它主要有兩個作用气忠，一個是獲取ViewHolder邻储；另一個給ViewHolder綁定數(shù)據(jù)赋咽。

獲取ViewHolder是有順序的，會先嘗試從各級緩存里面去獲取吨娜，會依次從Recycler scrap脓匿、cache、RecycledViewPool中獲取宦赠，如果都獲取不到陪毡，就直接創(chuàng)建一個ViewHolder。

RecyclerView#Recycler#tryGetViewHolderForPositionByDeadline

Attempts to get the ViewHolder for the given position, either from the Recycler scrap, cache, the RecycledViewPool, or creating it directly.
獲取給定位置的ViewHolder勾扭。會依次從Recycler scrap毡琉、cache、RecycledViewPool中獲取尺借，如果都獲取不到绊起，就直接創(chuàng)建一個ViewHolder

核心：獲取viewHolder；給viewHolder綁定數(shù)據(jù)燎斩。

@Nullable
ViewHolder tryGetViewHolderForPositionByDeadline(int position,
        boolean dryRun, long deadlineNs) {
    ...
    ViewHolder holder = null;
    // 0) If there is a changed scrap, try to find from there
    // 如果需要預先布局虱歪，就嘗試從mChangedScrap中去獲取。
    if (mState.isPreLayout()) {
        holder = getChangedScrapViewForPosition(position);
        ...
    }
    // 1) Find by position from scrap/hidden list/cache
    // 嘗試依次從mAttachedScrap栅表、mChildHelper的mHiddenViews笋鄙、mCachedViews中去獲取可復用的ViewHolder
    if (holder == null) {
        holder = getScrapOrHiddenOrCachedHolderForPosition(position, dryRun);
        // 校驗holder是否有效，無效就清除vh
        if (holder != null) {
            if (!validateViewHolderForOffsetPosition(holder)) {
                ...
                holder = null;
            } else {
                fromScrapOrHiddenOrCache = true;
            }
        }
    }
    if (holder == null) {
        final int offsetPosition = mAdapterHelper.findPositionOffset(position);
        ...
        // 獲取這個位置對應的數(shù)據(jù)類型怪瓶，通過重寫的Adapter#getItemViewType方法萧落。
        final int type = mAdapter.getItemViewType(offsetPosition);
        // 2) Find from scrap/cache via stable ids, if exists
        // 如果設置了stable ids，就根據(jù)id依次從mAttachedScrap洗贰、mCachedViews中查找
        if (mAdapter.hasStableIds()) {
            holder = getScrapOrCachedViewForId(mAdapter.getItemId(offsetPosition), type, dryRun);
            ...
        }
        // 嘗試從mViewCacheExtension中獲取VH
        if (holder == null && mViewCacheExtension != null) {
            final View view = mViewCacheExtension
                    .getViewForPositionAndType(this, position, type);
            if (view != null) {
                holder = getChildViewHolder(view);
                ...
            }
        }
        // 嘗試從 RecycledViewPool 中獲取
        if (holder == null) { // fallback to pool
            ...
            holder = getRecycledViewPool().getRecycledView(type);
            ...
        }
        // 調(diào)用RecyclerView#Adapter#onCreateViewHolder生成ViewHolder
        if (holder == null) {
            ...
            holder = mAdapter.createViewHolder(RecyclerView.this, type);
            ...           
        }
    }
    ...
    // 給viewholder綁定數(shù)據(jù)
    boolean bound = false;
    if (mState.isPreLayout() && holder.isBound()) {
        ...
    } else if (!holder.isBound() || holder.needsUpdate() || holder.isInvalid()) {       
        ...
        // 給viewholder綁定數(shù)據(jù)
        bound = tryBindViewHolderByDeadline(holder, offsetPosition, position, deadlineNs);
    }
    // 給holder.itemView設置RecyclerView#LayoutParams找岖。并將其相互綁定。
    ...
    return holder;
}

實例分析敛滋。

我們這里調(diào)用RecyclerView#Adapter#notifyDataSetChanged方法后许布，既有復用的ViewHolder，也有新建的ViewHolder绎晃。復用的ViewHolder來自于哪里蜜唾？為什么是5個？為什么還要新建ViewHolder庶艾？

帶著這些問題袁余，我們debug下我們的場景，看下ViewHolder的來源咱揍。

核心在于調(diào)用RecyclerView#Recycler#tryGetViewHolderForPositionByDeadline方法颖榜，關鍵在于下面這段代碼：

holder = getRecycledViewPool().getRecycledView(type);

我們知道，RecycledViewPool中是以viewType來存放不同的ViewHolder的，每個type最多存放五個朱转。

所以我們在RecyclerView#Recycler#tryGetViewHolderForPositionByDeadline中蟹地，從RecycledViewPool中最多能找到五個可復用的ViewHolder积暖，其余的只能走新建ViewHolder流程了藤为。

RecycledViewPool

先來看下RecycledViewPool的說明：

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

大意是RecycledViewPool可以讓你在多個recyclerview之間共享視圖。
如果你想在RecyclerViews中回收視圖夺刑，可以創(chuàng)建一個RecycledViewPool的實例并使用setRecycledViewPool(RecyclerView.RecycledViewPool)缅疟。
如果你不提供一個RecycledViewPool實例，那么RecyclerView會自動為自己創(chuàng)建一個遍愿。

我們看下RecyclerView#Recycler#getRecycledViewPool方法：確實是自動創(chuàng)建了一個存淫。

RecycledViewPool getRecycledViewPool() {
    if (mRecyclerPool == null) {
        mRecyclerPool = new RecycledViewPool();
    }
    return mRecyclerPool;
}

再看下RecycledViewPool的結構：

public static class RecycledViewPool {
    private static final int DEFAULT_MAX_SCRAP = 5;

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

    SparseArray<ScrapData> mScrap = new SparseArray<>();
    ...
}

如上所示，里面有一個SparseArray<ScrapData>類型的變量mScrap沼填，用來存儲不同類型的ViewHolder桅咆。ScrapData數(shù)據(jù)中包含ArrayList<ViewHolder>類型變量mScrapHeap，用來存放具體的ViewHolder坞笙，它的最大容量是5(DEFAULT_MAX_SCRAP)岩饼。

RecycledViewPool中的數(shù)據(jù)何時添加的

本例中RecycledViewPool中的數(shù)據(jù)是從哪里添加的呢？

本例中薛夜，向ScrapData#mScrapHeap添加ViewHolder數(shù)據(jù)的調(diào)用鏈如下：

RecyclerView#onLayout(): 4578行
RecyclerView#dispatchLayout(): 4012行
RecyclerView#dispatchLayoutStep2():4309行
LinearLayoutManager#onLayoutChildren(): 633行
RecyclerView#LayoutManager#detachAndScrapAttachedViews(): 9493行
RecyclerView#LayoutManager#scrapOrRecycleView(): 9508行
RecyclerView#Recycler#recycleViewHolderInternal(): 6671行
RecyclerView#Recycler#addViewHolderToRecycledViewPool(): 6723行
RecyclerView#RecyclerViewPool#putRecycledView(ViewHolder scrap): 5931行
scrapHeap.add(scrap);

核心是LinearLayoutManager#onLayoutChildren()方法中籍茧，如下的這段代碼：

detachAndScrapAttachedViews(recycler);

也就是說，在調(diào)用RecyclerView#Adapter#notifyDataSetChanged方法后梯澜，會觸發(fā)繪制流程寞冯。在Linearlayout#layoutChildren方法中，會先對ViewHolder進行緩存晚伙，然后會對ViewHolder進行復用吮龄。

RecyclerView#Adapter#notifyDataSetChanged方法后，為何還會新建ViewHolder？