AppBarLayout 不滚动问题，因此产生了一个奇怪的
AppBarLayout 不滚动问题，因此产生了一个奇怪的
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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