起因

由于项目App迭代，一个布局发生了改变。因此产生了一个奇怪的问题，按道理，滑动NestedScrollView的时候，AppBarLayout会上移。这是appbar_scrolling_view_behavior和scroll|enterAlwaysCollapsed使用的常规操作嘛。但是拖拽ViewPager里的一个item里的横向RecyclerView时，AppBarLayout没有按照预想中的情况上移。布局如下图：

方法调试

由于AppBarLayout和CoordinatorLayout都是库里的，没办法debug，所以增大了问题的难度。于是尝试了这些办法：

1. 在自己项目下创建一个和CoordinatorLayout同样的包androidx.coordinatorlayout.widget，将其复制进去
2. 由于app模块下代码会覆盖其他模块的代码，所以这样就可以愉快的debug了，但不太好用，debug时间太长会产生ANR
3. 在复制出来的类中添加log，观察异常日志

问题探索

这个问题涉及到嵌套滑动和behavior，所以我查看了相关源代码，大概了解了相关逻辑。
在一次debug中发现CoordinatorLayout的一块代码不对劲：

code0

    public void onNestedPreScroll(View target, int dx, int dy, int[] consumed, int type) {
        int xConsumed = 0;
        int yConsumed = 0;
        boolean accepted = false;

        final int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            final View view = getChildAt(i);
            if (view.getVisibility() == GONE) {
                // If the child is GONE, skip...
                continue;
            }

            final LayoutParams lp = (LayoutParams) view.getLayoutParams();
            Behavior behavior = lp.getBehavior();
            boolean nestedScrollAccepted = lp.isNestedScrollAccepted(type);
            if (!nestedScrollAccepted) {//view为AppBarLayout时这里居然通过了
                continue;
            }

            final Behavior viewBehavior = lp.getBehavior();
            if (viewBehavior != null) {
                mBehaviorConsumed[0] = 0;
                mBehaviorConsumed[1] = 0;
                //正常情况AppBarLayout的viewBehavior会被调用
                viewBehavior.onNestedPreScroll(this, view, target, dx, dy, mBehaviorConsumed, type);

                xConsumed = dx > 0 ? Math.max(xConsumed, mBehaviorConsumed[0])
                        : Math.min(xConsumed, mBehaviorConsumed[0]);
                yConsumed = dy > 0 ? Math.max(yConsumed, mBehaviorConsumed[1])
                        : Math.min(yConsumed, mBehaviorConsumed[1]);

                accepted = true;
            }
        }

        consumed[0] = xConsumed;
        consumed[1] = yConsumed;

        if (accepted) {
            onChildViewsChanged(EVENT_NESTED_SCROLL);
        }
    }

由于CoordinatorLayout本身不处理嵌套滑动，而是交给它子布局的Behavior处理，所以上面这个方法的nestedScrollAccepted一定是true。并且AppBarLayout如果要滑动那么它的onNestedPreScroll方法一定会被调用，HeaderBehavior中相关逻辑如下：

code1

   public void onNestedPreScroll(
        CoordinatorLayout coordinatorLayout,
        @NonNull T child,
        View target,
        int dx,
        int dy,
        int[] consumed,
        int type) {
      if (dy != 0) {
        int min;
        int max;
        if (dy < 0) {
          // We're scrolling down
          min = -child.getTotalScrollRange();
          max = min + child.getDownNestedPreScrollRange();
        } else {
          // We're scrolling up
          min = -child.getUpNestedPreScrollRange();
          max = 0;
        }
        if (min != max) {
          //就是这里AppBarLayout的Behavior消费了嵌套滑动的dy
          consumed[1] = scroll(coordinatorLayout, child, dy, min, max);
        }
      }
      if (child.isLiftOnScroll()) {
        child.setLiftedState(child.shouldLift(target));
      }
    }

通过多次尝试，发现如下可疑日志：

2022-03-27 19:14:57.317 16421-16421/*** I/MyAppBarLayout: onStartNestedScroll: true,nestedScrollAxes:2
2022-03-27 19:14:57.317 16421-16421/*** I/MyAppBarLayout: onNestedScrollAccepted: 2
2022-03-27 19:14:57.318 16421-16421/*** I/MyAppBarLayout: onStartNestedScroll: false,nestedScrollAxes:1

AppBarLayout.Behavior添加的日志

code2

    override fun onStartNestedScroll(
        parent: CoordinatorLayout,
        child: AppBarLayout,
        directTargetChild: View,
        target: View,
        nestedScrollAxes: Int,
        type: Int
    ): Boolean {
        val onStartNestedScroll = super.onStartNestedScroll(
            parent,
            child,
            directTargetChild,
            target,
            nestedScrollAxes,
            type
        )
        Log.i(TAG, "onStartNestedScroll: $onStartNestedScroll,nestedScrollAxes:$nestedScrollAxes")
        return onStartNestedScroll
    }

这是AppBarLayout.Behavior的onStartNestedScroll打印的日志，nestedScrollAxes:2为纵向，1为横向。如果AppBarLayout要滑动，那么onStartNestedScroll一定返回true，因为只有onStartNestedScroll返回true，Behavior后续的onNestedPreScroll才会被回调。可以看到纵向刚返回true,横向就返回false。

我们再来看CoordinatorLayout的这块代码:

code3

    public boolean onStartNestedScroll(View child, View target, int axes, int type) {
        boolean handled = false;
        final int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            final View view = getChildAt(i);
            if (view.getVisibility() == View.GONE) {
                // If it's GONE, don't dispatch
                continue;
            }
            final LayoutParams lp = (LayoutParams) view.getLayoutParams();
            final Behavior viewBehavior = lp.getBehavior();
            if (viewBehavior != null) {
                final boolean accepted = viewBehavior.onStartNestedScroll(this, view, child,
                        target, axes, type);
                handled |= accepted;
                lp.setNestedScrollAccepted(type, accepted);
                        + viewBehavior.getClass().getSimpleName() + "," + accepted + ",type:" + type);
            } else {
                lp.setNestedScrollAccepted(type, false);
            }
        }
        return handled;
    }

CoordinatorLayout的onStartNestedScroll的返回值取决于子view的behavior,如果behavior处理会在子view（AppBarLayout）CoordinatorLayout.LayoutParams中设置accepted为true。问题来了，刚才我们看到日志纵向true后面跟了个横向false,而这个方法并没有区分方向。

code4

        class CoordinatorLayout.LayoutParams
        ...
        void setNestedScrollAccepted(int type, boolean accept) {
            switch (type) {
                case ViewCompat.TYPE_TOUCH:
                    mDidAcceptNestedScrollTouch = accept;
                    break;
                case ViewCompat.TYPE_NON_TOUCH:
                    mDidAcceptNestedScrollNonTouch = accept;
                    break;
            }
        }

于是AppBarLayout.Behavior刚接收了纵向的嵌套滑动返回true，又拒绝了横向嵌套滑动返回false,对应的CoordinatorLayout.LayoutParams#isNestedScrollAccepted被覆盖，返回false。code0处nestedScrollAccepted就为false,所以AppBarLayout.Behavior的没有让AppBarLayout产生滑动。这与操作的现象是符合的，在纵向滑动RecyclerView时，AppBarLayout没有动。

原因分析

RecyclerView也支持嵌套滑动。这里补充一点startNestedScroll是由NestedScrollingChildHelper实现的，它会将嵌套滑动上传，也就是NestedScrollingChild都会将嵌套滑动先交给NestedScrollingParent处理。
看下面代码，发现会将横向纵向的嵌套滑动事件

code5

  class RecyclerView
  ...
     public boolean onInterceptTouchEvent(MotionEvent e) {
        if (mLayoutSuppressed) {
            // When layout is suppressed,  RV does not intercept the motion event.
            // A child view e.g. a button may still get the click.
            return false;
        }

        // Clear the active onInterceptTouchListener.  None should be set at this time, and if one
        // is, it's because some other code didn't follow the standard contract.
        mInterceptingOnItemTouchListener = null;
        if (findInterceptingOnItemTouchListener(e)) {
            cancelScroll();
            return true;
        }

        if (mLayout == null) {
            return false;
        }

        final boolean canScrollHorizontally = mLayout.canScrollHorizontally();
        final boolean canScrollVertically = mLayout.canScrollVertically();

        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        }
        mVelocityTracker.addMovement(e);

        final int action = e.getActionMasked();
        final int actionIndex = e.getActionIndex();

        switch (action) {
            case MotionEvent.ACTION_DOWN:
                if (mIgnoreMotionEventTillDown) {
                    mIgnoreMotionEventTillDown = false;
                }
                mScrollPointerId = e.getPointerId(0);
                mInitialTouchX = mLastTouchX = (int) (e.getX() + 0.5f);
                mInitialTouchY = mLastTouchY = (int) (e.getY() + 0.5f);

                if (mScrollState == SCROLL_STATE_SETTLING) {
                    getParent().requestDisallowInterceptTouchEvent(true);
                    setScrollState(SCROLL_STATE_DRAGGING);
                    stopNestedScroll(TYPE_NON_TOUCH);
                }

                // Clear the nested offsets
                mNestedOffsets[0] = mNestedOffsets[1] = 0;

                int nestedScrollAxis = ViewCompat.SCROLL_AXIS_NONE;
                if (canScrollHorizontally) {
                    nestedScrollAxis |= ViewCompat.SCROLL_AXIS_HORIZONTAL;
                }
                if (canScrollVertically) {
                    nestedScrollAxis |= ViewCompat.SCROLL_AXIS_VERTICAL;
                }
                startNestedScroll(nestedScrollAxis, TYPE_TOUCH);
                break;

                ...
        return mScrollState == SCROLL_STATE_DRAGGING;
    }

这里RecyclerView的mLayout是横向的，最终CoordinatorLayout分发的时候AppBarLayout.Behavior没有处理，而RecyclerView是在NestedScrollViw里的，NestedScrollView的onInterceptTouchEvent会走，于是NestedScrollView先上传纵向的嵌套滑动事件，交给CoordinatorLayout处理，AppBarLayout.Behavior接受了，紧接着走RecyclerView的onInterceptTouchEvent方法，它最终会报告横向滑动事件给CoordinatorLayout（这里会经过NestedScrollView，但是它没处理），而CoordinatorLayout在分发横向嵌套滑动的时候，AppBarLayout.Behavior又拒绝了。一切来得太快，AppBarLayout.Behavior出尔反尔。

解决方法

public class MyCoordinatorLayout extends CoordinatorLayout {
    public MyCoordinatorLayout(@NonNull Context context) {
        super(context);
    }

    public MyCoordinatorLayout(@NonNull Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }

    public MyCoordinatorLayout(@NonNull Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public boolean onStartNestedScroll(View child, View target, int axes, int type) {
        if ((axes & ViewCompat.SCROLL_AXIS_HORIZONTAL) != 0) {
            //不处理横向嵌套滑动事件
            return false;
        }
        return super.onStartNestedScroll(child, target, axes, type);
    }
}