起因
由於項目App迭代，一個布局發生了改變。因此產生了一個奇怪的問題，按道理，滑動NestedScrollView的時候，AppBarLayout會上移。這是appbar_scrolling_view_behavior和scroll|enterAlwaysCollapsed使用的常規操作嘛。但是拖拽ViewPager里的一個item里的橫向RecyclerView時，AppBarLayout沒有按照預想中的情況上移。布局如下圖：
方法調試
由於AppBarLayout和CoordinatorLayout都是庫里的，沒辦法debug，所以增大了問題的難度。於是嘗試了這些辦法：
- 在自己項目下創建一個和
CoordinatorLayout同樣的包androidx.coordinatorlayout.widget，將其復制進去 - 由於app模塊下代碼會覆蓋其他模塊的代碼，所以這樣就可以愉快的debug了，但不太好用，debug時間太長會產生ANR
- 在復制出來的類中添加log，觀察異常日志
問題探索
這個問題涉及到嵌套滑動和behavior，所以我查看了相關源代碼，大概了解了相關邏輯。
在一次debug中發現CoordinatorLayout的一塊代碼不對勁：
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為AppBarLayout時這里居然通過了
continue;
}
final Behavior viewBehavior = lp.getBehavior();
if (viewBehavior != null) {
mBehaviorConsumed[0] = 0;
mBehaviorConsumed[1] = 0;
//正常情況AppBarLayout的viewBehavior會被調用
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);
}
}
由於CoordinatorLayout本身不處理嵌套滑動，而是交給它子布局的Behavior處理，所以上面這個方法的nestedScrollAccepted一定是true。並且AppBarLayout如果要滑動那么它的onNestedPreScroll方法一定會被調用，HeaderBehavior中相關邏輯如下：
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) {
//就是這里AppBarLayout的Behavior消費了嵌套滑動的dy
consumed[1] = scroll(coordinatorLayout, child, dy, min, max);
}
}
if (child.isLiftOnScroll()) {
child.setLiftedState(child.shouldLift(target));
}
}
通過多次嘗試，發現如下可疑日志：
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
}
這是AppBarLayout.Behavior的onStartNestedScroll打印的日志，nestedScrollAxes:2為縱向，1為橫向。如果AppBarLayout要滑動，那么onStartNestedScroll一定返回true，因為只有onStartNestedScroll返回true，Behavior后續的onNestedPreScroll才會被回調。可以看到縱向剛返回true,橫向就返回false。
我們再來看CoordinatorLayout的這塊代碼:
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的返回值取決於子view的behavior,如果behavior處理會在子view（AppBarLayout）CoordinatorLayout.LayoutParams中設置accepted為true。問題來了，剛才我們看到日志縱向true后面跟了個橫向false,而這個方法並沒有區分方向。
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;
}
}
於是AppBarLayout.Behavior剛接收了縱向的嵌套滑動返回true，又拒絕了橫向嵌套滑動返回false,對應的CoordinatorLayout.LayoutParams#isNestedScrollAccepted被覆蓋，返回false。code0處nestedScrollAccepted就為false,所以AppBarLayout.Behavior的沒有讓AppBarLayout產生滑動。這與操作的現象是符合的，在縱向滑動RecyclerView時，AppBarLayout沒有動。
原因分析
RecyclerView也支持嵌套滑動。這里補充一點startNestedScroll是由NestedScrollingChildHelper實現的，它會將嵌套滑動上傳，也就是NestedScrollingChild都會將嵌套滑動先交給NestedScrollingParent處理。
看下面代碼，發現會將橫向縱向的嵌套滑動事件
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;
}
這里RecyclerView的mLayout是橫向的，最終CoordinatorLayout分發的時候AppBarLayout.Behavior沒有處理，而RecyclerView是在NestedScrollViw里的，NestedScrollView的onInterceptTouchEvent會走，於是NestedScrollView先上傳縱向的嵌套滑動事件，交給CoordinatorLayout處理，AppBarLayout.Behavior接受了，緊接着走RecyclerView的onInterceptTouchEvent方法，它最終會報告橫向滑動事件給CoordinatorLayout（這里會經過NestedScrollView，但是它沒處理），而CoordinatorLayout在分發橫向嵌套滑動的時候，AppBarLayout.Behavior又拒絕了。一切來得太快，AppBarLayout.Behavior出爾反爾。
解決方法
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) {
//不處理橫向嵌套滑動事件
return false;
}
return super.onStartNestedScroll(child, target, axes, type);
}
}