AppBarLayout 不滚动问题

飞奔的炮台

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);
  }
}

本文来自博客园，作者：徐影魔，转载请注明原文链接：https://www.cnblogs.com/xunevermore/p/16064261.html