发布于2022年11月4日3年前 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： 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 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 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); } }
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