滚动 RecyclerView item 不出屏幕不会触发 addOnLayoutChangeListener 和 getViewTreeObserver().addOnGlobalLayoutListener 事件
滚动 RecyclerView item 出屏幕再回来会触发 addOnLayoutChangeListener 但是不会触发 getViewTreeObserver().addOnGlobalLayoutListener
ScrollView嵌套RecyclerView自动滚动

解决方案一： ScrollView的子viewGroup中增加android:descendantFocusability=”blocksDescendants” 属性
解决方案二： ScrollView的直接子view 上添加focusableInTouchMode=true

reyclerView 嵌套 recyhclerView 自动滚动

ScrollView 嵌套RecyclerView + GridLayoutManager 只显示一行或者不显示，需要滑动RecyclerView

解决方案：将 ScrollView 替换为 NestScrollView

RecyclerView 中EditText singleLine模式下，作为最后一个Item，输入换行崩溃，某些机型（华为）

java.lang.IllegalStateException: focus search returned a view that wasn't able to take focus!
    at android.widget.TextView.onKeyUp(TextView.java:8643)
    at android.widget.EditText.onKeyUp(EditText.java:289)
    at android.view.KeyEvent.dispatch(KeyEvent.java:3094)
    at android.view.View.dispatchKeyEvent(View.java:13537)
    at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1973)
    at com.ooftf.layout.kv.KvLayout.dispatchKeyEvent(KvLayout.kt:423)
    at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1973)
    at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1973)
    at com.android.internal.policy.DecorView.superDispatchKeyEvent(DecorView.java:668)
    at com.android.internal.policy.PhoneWindow.superDispatchKeyEvent(PhoneWindow.java:1993)
    at android.app.Activity.dispatchKeyEvent(Activity.java:4107)
    at androidx.core.app.ComponentActivity.superDispatchKeyEvent(ComponentActivity.java:122)
    at androidx.core.view.KeyEventDispatcher.dispatchKeyEvent(KeyEventDispatcher.java:84)
    at androidx.core.app.ComponentActivity.dispatchKeyEvent(ComponentActivity.java:140)
    at androidx.appcompat.app.AppCompatActivity.dispatchKeyEvent(AppCompatActivity.java:569)
    at androidx.appcompat.view.WindowCallbackWrapper.dispatchKeyEvent(WindowCallbackWrapper.java:59)
    at androidx.appcompat.app.AppCompatDelegateImpl$AppCompatWindowCallback.dispatchKeyEvent(AppCompatDelegateImpl.java
    at com.baidu.mobstat.ak.dispatchKeyEvent(SourceFile:50)
    at com.baidu.mobstat.ak.dispatchKeyEvent(SourceFile:50)
    at com.android.internal.policy.DecorView.dispatchKeyEvent(DecorView.java:516)
    at android.view.ViewRootImpl$ViewPostImeInputStage.processKeyEvent(ViewRootImpl.java:6038)
    at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:5906)
    at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:5350)
    at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:5403)
    at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:5369)
    at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:5527)
    at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:5377)
    at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:5584)
    at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:5350)
    at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:5403)
    at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:5369)
    at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:5377)
    at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:5350)
    at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:8325)
    at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:8245)
    at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:8198)
    at android.view.ViewRootImpl$ViewRootHandler.handleMessage(ViewRootImpl.java:5111)
    at android.os.Handler.dispatchMessage(Handler.java:110)
    at android.os.Looper.loop(Looper.java:219)
    at android.app.ActivityThread.main(ActivityThread.java:8387)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:513)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1055)

方案一

override fun dispatchKeyEvent(event: KeyEvent?): Boolean {
      //解决EditText作为RecyclerView的最后一行时，输入换行，在某些机型（华为）崩溃的问题
      try {
          return super.dispatchKeyEvent(event)
      } catch (e: Exception) {
          e.printStackTrace()
      }
      return true

  }

方案二把EditText的android:imeOptions=”actionNone” 即可

RecyclerView clear再添加数据，界面有闪烁

是因为RecyclerView有默认动画， itemAnimator = null 可去除默认动画

LinearSnapHelper、PagerSnapHelper

DiffUtil

DiffUtil 官方指南

   val diffResult = DiffUtil.calculateDiff(object : DiffUtil.Callback() {
            override fun getOldListSize(): Int {
            }

            override fun getNewListSize(): Int {
            }

            override fun areItemsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
                // 数据的ID是否相同，用于判断 item 的 添加删除移动。
            }

            override fun areContentsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
                // 影响界面展示的数据的内容是否相同，用于计算 item 视图更新
            }

        }, true)
        diffResult.dispatchUpdatesTo(recyclerView.adapter)

RecyclerView 源码分析

分析RecyclerView.onLayout

@Override
protected void RecyclerView.onLayout(boolean changed, int l, int t, int r, int b) {
    dispatchLayout();
    mFirstLayoutComplete = true;
}

void RecyclerView.dispatchLayout() {
    boolean needsRemeasureDueToExactSkip = mLastAutoMeasureSkippedDueToExact
                    && (mLastAutoMeasureNonExactMeasuredWidth != getWidth()
                    || mLastAutoMeasureNonExactMeasuredHeight != getHeight());
    mLastAutoMeasureNonExactMeasuredWidth = 0;
    mLastAutoMeasureNonExactMeasuredHeight = 0;
    mLastAutoMeasureSkippedDueToExact = false;
    if (mState.mLayoutStep == State.STEP_START) {
        dispatchLayoutStep1();
        mLayout.setExactMeasureSpecsFrom(this);
        dispatchLayoutStep2();
    } else if (mAdapterHelper.hasUpdates()
            || needsRemeasureDueToExactSkip
            || mLayout.getWidth() != getWidth()
            || mLayout.getHeight() != getHeight()) {
        mLayout.setExactMeasureSpecsFrom(this);
        dispatchLayoutStep2();
    } else {
        // always make sure we sync them (to ensure mode is exact)
        mLayout.setExactMeasureSpecsFrom(this);
    }
    dispatchLayoutStep3();
}

RecyclerView.dispatchLayoutStep1

只有 State.STEP_START 状态才会执行 dispatchLayoutStep1
调用 dispatchLayoutStep1 会将mState.mLayoutStep 置为 State.STEP_LAYOUT;
与 RecyclerView 动画相关
- RunSimpleAnimations 动画只会计算属性，不会进行布局，
- RunPredictiveAnimations 会通过 LayoutManager.onLayoutChildren 进行空间布局

RecyclerView.dispatchLayoutStep3

将 mState.mLayoutStep 重新置为 State.STEP_START
添加动画：mViewInfoStore.addToPostLayout(holder, animationInfo);
执行动画：mViewInfoStore.process(mViewInfoProcessCallback);
mLayout.removeAndRecycleScrapInt(mRecycler); 将 Recycler.mAttachedScrap 中没有复用的 ViewHolder 添加到 RecycledViewPool 中
清除 Recycler.mChangedScrap 中的缓存
mRecycler.updateViewCacheSize(); 将 Recycler.mCachedViews 中的没有复用的 ViewHolder 添加到 RecycledViewPool 中

RecyclerView.dispatchLayoutStep2()

private void dispatchLayoutStep2() {
    mState.mInPreLayout = false;
    mLayout.onLayoutChildren(mRecycler, mState);
    mState.mRunSimpleAnimations = mState.mRunSimpleAnimations &&mItemAnimator != null;
    mState.mLayoutStep = State.STEP_ANIMATIONS;
}

LinearLayoutManager.onLayoutChildren

在每次向RecyclerView填充表项之前都会先清空 LayoutManager 中现存表项，将它们 detach 并同时缓存入 mAttachedScrap列表中。

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

    ...
    detachAndScrapAttachedViews(recycler);
    ...
    fill(recycler, mLayoutState, state, false);
    ...
}

LayoutManager.detachAndScrapAttachedViews  
    LayoutManager.scrapOrRecycleView   
        LayoutManager.detachViewAt(index)   
            LayoutManager.detachViewInternal   
                ChildHelper.detachViewFromParent   
                     mCallback.detachViewFromParent(offset);   
                        RecyclerView.initChildrenHelper()  
                            RecyclerView.this.detachViewFromParent(offset);  
                                ViewGroup.detachViewFromParent  
        Recycler.scrapView 
            mAttachedScrap.add(holder);

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;
    }
    if (viewHolder.isInvalid() && !viewHolder.isRemoved()
            && !mRecyclerView.mAdapter.hasStableIds()) {
        //notifyDataSetChanged 会走这个分支
        // 将viewHolder 缓存到 mCachedViews 和 RecycledViewPool
        removeViewAt(index);
        recycler.recycleViewHolderInternal(viewHolder);
    } else {
        //初次布局
        detachViewAt(index);
        recycler.scrapView(view);
        mRecyclerView.mViewInfoStore.onViewDetached(viewHolder);
    }
}

void Recycler.recycleViewHolderInternal(ViewHolder holder) {
    ...
    //如果已经超过 mCachedViews 最大容量 按找 FIFO 规则将 ViewHodler 转移到 RecycledViewPool
    if (cachedViewSize >= mViewCacheMax && cachedViewSize > 0) {
                   recycleCachedViewAt(0);
                   cachedViewSize--;
               }
    mCachedViews.add(targetCacheIndex, holder);
    ...
}

LinearLayoutManager.fill

LinearLayoutManager.fill 
    LinearLayoutManager.layoutChunk
        LayoutState.next 
            Recycler.getViewForPosition
                Recycler.tryGetViewHolderForPositionByDeadline

 ViewHolder tryGetViewHolderForPositionByDeadline(int position,
            boolean dryRun, long deadlineNs) {
    // 通过 position 从 mChangedScrap 中获取ViewHolder
    holder = getChangedScrapViewForPosition(position);
    // 通过 position 从 mAttachedScrap 或者 mCachedViews 中获取 ViewHolder
    holder = getScrapOrHiddenOrCachedHolderForPosition(position, dryRun)
    // 通过ItemId 从 mAttachedScrap 或者 mCachedViews 中获取 ViewHolder
    holder = getScrapOrCachedViewForId(mAdapter.getItemId(offsetPosition)
    // 通过 position 和 type 从 【用户自定义缓存ViewCacheExtension】中获取ViewHolder
    final View view = mViewCacheExtension
                        .getViewForPositionAndType(this, position, type);
    holder = getChildViewHolder(view);
    // 从 RecycledViewPool.mScrap 中获取 ViewHolder
    holder = getRecycledViewPool().getRecycledView(type);
    holder = mAdapter.createViewHolder(RecyclerView.this, type)
    // 判断是否需要重新调用 adapter.bindViewHolder
    boolean bound = false;
    if (mState.isPreLayout() && holder.isBound()) {
        holder.mPreLayoutPosition = position;
    } else if (!holder.isBound() || holder.needsUpdate() || holderisInvalid()) {
        final int offsetPosition = mAdapterHelper.findPositionOffse(position);
        bound = tryBindViewHolderByDeadline(holder, offsetPosition,position, deadlineNs);
    }
    // 设置 itemView 的 Layoutparams 
    final ViewGroup.LayoutParams lp = holder.itemView.getLayoutParams();
    final LayoutParams rvLayoutParams;
    if (lp == null) {
        rvLayoutParams = (LayoutParams) generateDefaultLayoutParams();
        holder.itemView.setLayoutParams(rvLayoutParams);
    } else if (!checkLayoutParams(lp)) {
        rvLayoutParams = (LayoutParams) generateLayoutParams(lp);
        holder.itemView.setLayoutParams(rvLayoutParams);
    } else {
        rvLayoutParams = (LayoutParams) lp;
    }
}

ViewGroup.removeView 和 ViewGroup.detachViewFromParent 区别

ViewGroup.removeView 和 ViewGroup.detachViewFromParent 功能相似，都会调用ViewGroup.removeFromArray将子控件从父控件的孩子列表中移除
detachViewFromParent 更轻量，仅仅调用 ViewGroup.removeFromArray 将子控件从父控件的孩子列表中移除
ViewGroup.removeView 会关注动画、焦点、触摸等事件，并且会重新布局和重新绘画。最终也会调用ViewGroup.removeFromArray 将子控件从父控件的孩子列表中移除

RecyclerView.onMeasuer

protected void onMeasure(int widthSpec, int heightSpec) {
    final int widthMode = MeasureSpec.getMode(widthSpec);
    final int heightMode = MeasureSpec.getMode(heightSpec);
    mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
    mLastAutoMeasureSkippedDueToExact =
            widthMode == MeasureSpec.EXACTLY && heightMode ==MeasureSpecEXACTLY;
    if (mLastAutoMeasureSkippedDueToExact || mAdapter == null) {
        return;
    }
    // 上面代码就是正常View的 onMeasure  当不是精确测量并且Adapter不null的候，开始按照 LayoutManager 的方式测量；如果是精确测量，是不执行下面代码的
    //首先调用 dispatchLayoutStep1 对Children进行布局
    if (mState.mLayoutStep == State.STEP_START) {
        dispatchLayoutStep1();
    }
    mLayout.setMeasureSpecs(widthSpec, heightSpec);
    mState.mIsMeasuring = true;
    dispatchLayoutStep2();
    mLayout.setMeasuredDimensionFromChildren(widthSpec, heightSpec);
    mLastAutoMeasureNonExactMeasuredWidth = getMeasuredWidth();
    mLastAutoMeasureNonExactMeasuredHeight = getMeasuredHeight();
}

LinearLayoutManager.generateDefaultLayoutParams

当ItemView没有设置 LayoutParams 的时候，就会调用这个方法添加 LayoutParams

ViewInfoStore

ViewInfoStore 记录 RecyclerView 动画相关信息，通过 process 执行动画

void addToPreLayout(RecyclerView.ViewHolder holder, RecyclerViewItemAnimator.ItemHolderInfo info) {
    InfoRecord record = mLayoutHolderMap.get(holder);
    if (record == null) {
        record = InfoRecord.obtain();
        mLayoutHolderMap.put(holder, record);
    }
    record.preInfo = info;
    record.flags |= FLAG_PRE;
}
void addToPostLayout(RecyclerView.ViewHolder holder, RecyclerViewItemAnimator.ItemHolderInfo info) {
    InfoRecord record = mLayoutHolderMap.get(holder);
    if (record == null) {
        record = InfoRecord.obtain();
        mLayoutHolderMap.put(holder, record);
    }
    record.postInfo = info;
    record.flags |= FLAG_POST;
}

缓存总结

RecycerView 中用来管理缓存的类是 Recycler ，缓存相关逻辑都在 Recycler 中实现

mChangedScrap
- ArrayList<ViewHolder>
- 缓存屏幕内的ViewHolder
- 位置没有发生变化，数据改变
mAttachedScrap
- ArrayList<ViewHolder>
- 缓存屏幕内的ViewHolder
- 位置可能发生变化
mCachedViews
- ArrayList<ViewHolder>
- 缓存屏幕外的 ViewHolder
- 默认容量为 2，如果达到最大值，会按照 FIFO 规则，调用 Recycler.recycleCachedViewAt 方法将 ViewHodler 转移到 RecycledViewPool
mViewCacheExtension
- 自定义的缓存

RecycledViewPool

数据结构

SparseArray<ScrapData> mScrap = new SparseArray<>();
ScrapData.mScrapHeap  = new ArrayList<ViewHolder> 

添加到RecyclerdViewPool ViewHolder 会清除状态信息

   void resetInternal() {
        mFlags = 0;
        mPosition = NO_POSITION;
        mOldPosition = NO_POSITION;
        mItemId = NO_ID;
        mPreLayoutPosition = NO_POSITION;
        mIsRecyclableCount = 0;
        mShadowedHolder = null;
        mShadowingHolder = null;
        clearPayload();
        mWasImportantForAccessibilityBeforeHidden = ViewCompat.IMPORTANT_FOR_ACCESSIBILITY_AUTO;
        mPendingAccessibilityState = PENDING_ACCESSIBILITY_STATE_NOT_SET;
        clearNestedRecyclerViewIfNotNested(this);
    }

adapter.notify

notifyItemChanged 会将发生改变的放入到 mChangedScrap ;没有改变的放入到mAttachedScrap 中
notifyDataSetChanged 会将所有 ViewHolder 通过 recycler.recycleViewHolderInternal 加入到 mCachedViews 和 RecycledViewPool 中
notifyItemRemoved 会将所有 ViewHolder 缓存到 mAttachedScrap 中
查看 RecyclerViewDataObserver 可知事件变化监听都是通过调用 requestLayout 触发重新布局来实现的(也就是会触发回收所有View 重新布局)，不同的是：不同的事件会将不同的 ViewHolder 置为不同的 FLAG 因此会添加到不同的缓存中

nofityDataSetChange

onBindViewHolder: AdapterPosition:0 ,OldPosition:-1 ,LayoutPosition:0 ,lastPosition:6
onBindViewHolder: AdapterPosition:1 ,OldPosition:-1 ,LayoutPosition:1 ,lastPosition:7
onBindViewHolder: AdapterPosition:2 ,OldPosition:-1 ,LayoutPosition:2 ,lastPosition:8
onBindViewHolder: AdapterPosition:3 ,OldPosition:-1 ,LayoutPosition:3 ,lastPosition:9
onBindViewHolder: AdapterPosition:4 ,OldPosition:-1 ,LayoutPosition:4 ,lastPosition:10
onCreateViewHolder: onCreateViewHolder
onBindViewHolder: AdapterPosition:5 ,OldPosition:-1 ,LayoutPosition:5 ,lastPosition:null
onCreateViewHolder: onCreateViewHolder
onBindViewHolder: AdapterPosition:6 ,OldPosition:-1 ,LayoutPosition:6 ,lastPosition:null
onCreateViewHolder: onCreateViewHolder
onBindViewHolder: AdapterPosition:7 ,OldPosition:-1 ,LayoutPosition:7 ,lastPosition:null
onCreateViewHolder: onCreateViewHolder
onBindViewHolder: AdapterPosition:8 ,OldPosition:-1 ,LayoutPosition:8 ,lastPosition:null
onCreateViewHolder: onCreateViewHolder
onBindViewHolder: AdapterPosition:9 ,OldPosition:-1 ,LayoutPosition:9 ,lastPosition:null
onCreateViewHolder: onCreateViewHolder
onBindViewHolder: AdapterPosition:10 ,OldPosition:-1 ,LayoutPosition:10 ,lastPosition:null

从日志可以看出来，只有 5 个 ViewHolder 复用了其他都是重新创建的，而且5个ViewHolder 的位置也发生了变化。有5个服用是因为 RecyclerView.RecycledViewPool.DEFAULT_MAX_SCRAP = 5 如果想改动可以使用 recyclerView.getRecycledViewPool().setMaxRecycledViews() 修改

实战探索

LinearLayoutManager & wrap_content

onMeasure dispatchLayoutStep1 dispatchLayoutStep2 onLayoutChildren
onMeasure dispatchLayoutStep2 onLayoutChildren
onMeasure dispatchLayoutStep2 onLayoutChildren
onMeasure dispatchLayoutStep2 onLayoutChildren
onMeasure dispatchLayoutStep2 onLayoutChildren
onMeasure dispatchLayoutStep2 onLayoutChildren
onLayout dispatchLayout dispatchLayoutStep3

LinearLayoutManager & match_parent

onMeasure
onMeasure
onMeasure
onMeasure
onMeasure
onMeasure
onLayout dispatchLayoutStep1 dispatchLayoutStep2 onLayoutChildren * dispatchLayoutStep3

优化方向

onCreateViewHolder 创建View使用代码创建对象的方式，而不是 xml 省去解析 xml 的时间
如果是多列布局比如 GridLayoutManager 可以设置最大缓存个数避免滑动过程中调用 onCreateViewHolder
- recyclerView.recycledViewPool.setMaxRecycledViews() 设置 RecyclerViewPool 指定ViewType 大小
- recyclerView.setItemViewCacheSize 设置 mCachedViews 大小
使用 DiffUtil 优化更新
使用 DataBinding 可以在不使用 adapter.nofity 的情况下改变 ItemView 的子 View 的内容
RecyclerView 尽可能使用精确大小比或者 match_parent 而不是 wrap_content 可见减少 LayoutManager.onLayoutChildren 的调用次数
Stable Ids
Prefetch 优化; // 默认是开启的 Prefetch

recyclerView.setItemViewCacheSize 设置的是当 item 滚出屏幕后，保留的 item 个数，当有小于 CacheSize item 滚出RecyclerView 再滚回屏幕时，是不会再次调用 onCreateViewHolder 和 onBIndViewHolder 的

recyclerView.recycledViewPool.setMaxRecycledViews() 设置的是当 item 被回收后不被弃用的缓存大小，当值为1是，设置合适值可以减少 onCreateViewHolder 调用次数，但不能减少 onBindViewHolder 的调用次数。

ViewHolder 的 FLAG

有助于理解 ViewHolder 的状态，不是很重要

 /**
 * This ViewHolder has been bound to a position; mPosition, mItemIand mItemViewType
 * are all valid.
 */
static final int FLAG_BOUND = 1 << 0
/**
 * The data this ViewHolder's view reflects is stale and needs to brebound
 * by the adapter. mPosition and mItemId are consistent.
 */
static final int FLAG_UPDATE = 1 << 1
/**
 * This ViewHolder's data is invalid. The identity implied bmPosition and mItemId
 * are not to be trusted and may no longer match the item view type.
 * This ViewHolder must be fully rebound to different data.
 */
static final int FLAG_INVALID = 1 << 2
/**
 * This ViewHolder points at data that represents an item previouslremoved from the
 * data set. Its view may still be used for things like outgoinanimations.
 */
static final int FLAG_REMOVED = 1 << 3
/**
 * This ViewHolder should not be recycled. This flag is set visetIsRecyclable()
 * and is intended to keep views around during animations.
 */
static final int FLAG_NOT_RECYCLABLE = 1 << 4
/**
 * This ViewHolder is returned from scrap which means we arexpecting an addView call
 * for this itemView. When returned from scrap, ViewHolder stays ithe scrap list until
 * the end of the layout pass and then recycled by RecyclerView if iis not added back to
 * the RecyclerView.
 */
static final int FLAG_RETURNED_FROM_SCRAP = 1 << 5
/**
 * This ViewHolder is fully managed by the LayoutManager. We do noscrap, recycle or remove
 * it unless LayoutManager is replaced.
 * It is still fully visible to the LayoutManager.
 */
static final int FLAG_IGNORE = 1 << 7
/**
 * When the View is detached form the parent, we set this flag sthat we can take correct
 * action when we need to remove it or add it back.
 */
static final int FLAG_TMP_DETACHED = 1 << 8
/**
 * Set when we can no longer determine the adapter position of thiViewHolder until it is
 * rebound to a new position. It is different than FLAG_INVALIbecause FLAG_INVALID is
 * set even when the type does not match. AlsoFLAG_ADAPTER_POSITION_UNKNOWN is set as soon
 * as adapter notification arrives vs FLAG_INVALID is set lazilbefore layout is
 * re-calculated.
 */
static final int FLAG_ADAPTER_POSITION_UNKNOWN = 1 << 9
/**
 * Set when a addChangePayload(null) is called
 */
static final int FLAG_ADAPTER_FULLUPDATE = 1 << 10
/**
 * Used by ItemAnimator when a ViewHolder's position changes
 */
static final int FLAG_MOVED = 1 << 11
/**
 * Used by ItemAnimator when a ViewHolder appears in pre-layout
 */
static final int FLAG_APPEARED_IN_PRE_LAYOUT = 1 << 12;

自定义 LayoutManager

瀑布流加载网络图片出现头部空白、item 漂移现象

具体原因和解决办法可以参考我的另一篇文章 StaggeredGridLayoutManager

粘性布局的实现方案

1. 完全手动实现

在布局中添加一份 sticky 的拷贝布局，两个布局应该一样，监听布局的滑动，如果 sticky 还能完全显示。则不显示 sticky-copy，如果 sticky 开始被头部遮挡，那么显示 sticky-copy。

2. 使用协调布局

这个只要熟悉协调布局的性质就可以了。利用 AppbarLayout 的 ScrollFlag 实现粘性布局

3. 借助 RecyclerView 的 ItemDecoration 实现

需要熟悉自定义 ItemDecoration 的流程。有相关的库可以使用,这种实现是将 sticky 进行“截图”挂在 ItemDecoration

4. 自定 RecyclerView 的 LayoutManager ，比如 vLayout

实现起来可能会比较复杂，可以直接使用 vLayout。

5. 添加悬浮层

在 recyclerView 上面覆盖一层 layout ，RecyclerView 中添加的是一个空白 View 真正的 View 添加到覆盖层，将两个 View 大小设置相同，通过监听 RecyclerView 的滚动，获取空白 View 的位置，然后设置真正 View 的位置；

混排布局实现

所谓的混排布局就是：有的每行有两个 item ，有的每行有一个 item ，总之就是每行 item 个数不同

如果 item 高度相同，可以使用 GridLayoutManager.setSpanSizeLookup 动态设置 item 所占 span 的大小
如果 item 高度不同，也是瀑布流的方式，可以使用 StaggeredGridLayoutManager 然后重写 adapter 的 onViewAttachedToWindow

    override fun onViewAttachedToWindow(holder: RecyclerView.ViewHolder) {
        super.onViewAttachedToWindow(holder)
        val lp = holder.itemView.layoutParams
        if (lp is StaggeredGridLayoutManager.LayoutParams) {
            lp.isFullSpan = true // 每行一个 item  false：每行多个 item
        }
    }

这种方式只能设置每行，1 个或者 n 个，并不能设置中间值。

RecyelrView 嵌套 RecyclerView ，内层 RecyclerView 不可滑动问题

可以使用 NestScroll 机制解决这个滑动冲突问题，具体解决方案可以参考我的项目github 项目中的 demo3

RecyclerView 边界判断

//RecyclerView 是否滑动到底部
!recyclerView.canScrollVertically(1)  

//RecyclerView 是否滑动到顶部 
!recyclerView.canScrollVertically(-1)

滚动相关的方法

computeVerticalScrollOffset()：

已经向下滚动的距离，为0时表示已处于顶部。

computeVerticalScrollRange()：

整体的高度，注意是整体，包括在显示区域之外的。

computeVerticalScrollExtent()：

显示区域的高度。

scrollBy 方法可以滚动 RecyclerView ，scrollTo 不能滚动 RecyclerView

fling 滚动可以直接调用 RecyclerView.fling 方法；

监听 item 可见和不可见

如果已经确定 RecyclerView 是全部可见的，可以通过监听 Adapter 的 onViewAttachedToWindow 和 onViewDetachedFromWindow 来监听 View 的可见和不可见。因为，及时 ReyclerView 会预加载 item ，但是也仅仅只是调用 onBindViewHolder 预创建，但是attched 到RecyclerView 上面；

但是如果 RecyclerView 外面还嵌套的一层滑动控件，也就是说 RecyclerView 本身有可能只是部分可见，那么上述方法，拿到的是 RecyclewrView 全部可见情况下的结果，具体解决方案可以参考 github 项目中的 demo3

ViewHolder.setIsRecyclable

如果设置为 false 在滑动出屏幕时不会参与与原来位置不同的复用：例如 ViewHolder = 3，当滚动出屏幕后，当position = 4 需要 ViewHolder 时，原来 ViewHolder = 3 的这个不会参与复用，
item 滚出屏幕再回到屏幕都会重新创建ViewHolder，因为一半不要设置成 false
如果设置为 false 但是当 adapter.notifyDataSetChanged 的时候，这个ViewHolder 也不会参数复用，而是创建一个新的

recyclerView.smoothScrollToPosition(0)

如果 item 的高度大于 RecyclerView，那么调用 recyclerView.smoothScrollToPosition(0) 不一定是回到顶部，分两种情况

已经在顶部：会滚动到 recyclerView底部和 item0 底部对齐的位置；
不在顶部：回滚动到顶部

可以使用 canScrollVertically 线判断是否一定到顶部，在调用 smoothScrollToPosition

recyclerView.getChildCount() layoutManager.getChildCount()

这两个数值并不一定是想等的，比如在添加删除动化期间就有可能不同

动画

原理详解 https://www.jianshu.com/p/65523b2ce15b
holder.setIsRecyclable(false) 再调用 notifyItemRemoved 会导致 item 重叠

ScrollView嵌套RecyclerView自动滚动