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背景

ViewPager2内嵌套横向滑动的RecyclerView，会有滑动冲突的情况，引入官方提供的NestedScrollableHost类可以解决冲突问题，但是有一些瑕疵，滑动横向RecyclerView到顶部，按住它不放手继续往左拖再往右拖，这时候会发现外层ViewPager2滑动了，而不是横向RecyclerView滑动，于是参考NestedScrollableHost进行逻辑完善

完整代码

• 主要是增加判断外层ViewPager2是否可滚动来设置是否允许父View拦截事件

open class NestRecyclerView @JvmOverloads constructor(context: Context,attrs: AttributeSet? = null
): RecyclerView(context, attrs) {private var initialX = 0fprivate var initialY = 0fprivate val parentViewPager: ViewPager2?get() {var v: View? = parent as? Viewwhile (v != null && v !is ViewPager2) {v = v.parent as? View}return v as? ViewPager2}private fun canViewScroll(target: View?, orientation: Int, delta: Float): Boolean {val direction = -delta.sign.toInt()return when (orientation) {0 -> target?.canScrollHorizontally(direction) ?: false1 -> target?.canScrollVertically(direction) ?: falseelse -> throw IllegalArgumentException()}}override fun onInterceptTouchEvent(event: MotionEvent?): Boolean {val orientation = parentViewPager?.orientation ?: return super.onInterceptTouchEvent(event)if (!canViewScroll(this, orientation, -1f) && !canViewScroll(this, orientation, 1f)) {return super.onInterceptTouchEvent(event)}when (event?.action) {MotionEvent.ACTION_DOWN -> {initialX = event.xinitialY = event.yparent.requestDisallowInterceptTouchEvent(true)}MotionEvent.ACTION_MOVE -> {val dx = event.x - initialXval dy = event.y - initialYval isVpHorizontal = orientation == ViewPager2.ORIENTATION_HORIZONTALif (isVpHorizontal == dy.absoluteValue > dx.absoluteValue) {parent.requestDisallowInterceptTouchEvent(false)} else {if (canViewScroll(this, orientation, if (isVpHorizontal) dx else dy)) {parent.requestDisallowInterceptTouchEvent(true)} else {if (canViewScroll(parentViewPager, orientation, if (isVpHorizontal) dx else dy)) {parent.requestDisallowInterceptTouchEvent(false)} else {parent.requestDisallowInterceptTouchEvent(true)}}}}}return super.onInterceptTouchEvent(event)}
}

向上滑动AppBarLayout不联动问题

如果布局CoordinatorLayout + AppBarLayout + ViewPager2内嵌套横向滑动的RecyclerView，这时拖拽横向滑动的RecyclerView向上移，AppBarLayout不会跟着向上移

原因分析

• 拖拽横向滑动的RecyclerView向上移时，CoordinatorLayout.onNestedPreScroll内的lp.isNestedScrollAccepted(type)返回false，造成AppBarLayout没有执行scroll
  

• lp.isNestedScrollAccepted(type)被赋值的地方，会根据AppBarLayout$Behavior.onStartNestedScroll返回的accepted进行赋值
  

• AppBarLayout$Behavior.onStartNestedScroll内，会判断nestedScrollAxes的值不是2就返回false
  

• RecyclerView也支持嵌套滑动。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。

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 offsetsmNestedOffsets[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是横向的，所以nestedScrollAxis会被赋值为1，RecyclerView内调用startNestedScroll会向上层view传递，直到交给CoordinatorLayout处理，而CoordinatorLayout在调用onStartNestedScroll的时候，AppBarLayout$Behavior.onStartNestedScroll又返回false了，造成CoordinatorLayout回调onNestedPreScroll（由RecyclerView在ACTION_MOVE时调用dispatchNestedPreScroll触发）时无法调用AppBarLayout的滚动。

解决方法

在CoordinatorLayout调用onStartNestedScroll的时候不处理横向的情况，就不会导致lp.isNestedScrollAccepted(type)被赋值

class NestedCoordinatorLayout @JvmOverloads constructor(context: Context,attrs: AttributeSet? = null
): CoordinatorLayout(context, attrs) {override fun onStartNestedScroll(child: View, target: View, axes: Int, type: Int): Boolean {return if (axes and ViewCompat.SCROLL_AXIS_HORIZONTAL != 0) {false} else super.onStartNestedScroll(child, target, axes, type)}
}