如何避免 CollapsingToolbarLayout 在滚动时不被捕捉或 "wobbly"？
How to avoid CollapsingToolbarLayout not being snapped or being "wobbly" when scrolling?
背景
假设您创建的应用程序与您可以通过 "scrolling activity" 的向导创建的应用程序具有相似的 UI，但您希望滚动标志具有捕捉功能，因为这样：
<android.support.design.widget.CollapsingToolbarLayout ... app:layout_scrollFlags="scroll|exitUntilCollapsed|snap" >
问题
事实证明，在很多情况下它都存在捕捉问题。有时 UI 不会对齐到 top/bottom，使得 CollapsingToolbarLayout 停留在两者之间。
有时它也会尝试捕捉到一个方向，然后决定捕捉到另一个方向。
您可以在附加视频中看到这两个问题 here。
我试过的
我认为这是我在 RecyclerView 中使用 setNestedScrollingEnabled(false) 时遇到的问题之一，所以我询问了它 ，但后来我注意到即使有解决方案也没有解决方案完全使用此命令，甚至在使用简单的 NestedScrollView（由向导创建）时，我仍然可以注意到此行为。
这就是为什么我决定将此作为一个问题进行报告，here。
遗憾的是，我在 Whosebug 上找不到任何解决这些奇怪错误的方法。
问题
它为什么会发生，更重要的是：我如何才能避免这些问题，同时仍然使用它应该具有的行为？
编辑：这是已接受答案的一个很好的改进 Kotlin 版本：
class RecyclerViewEx @JvmOverloads constructor(context: Context, attrs: AttributeSet? = null, defStyle: Int = 0) : RecyclerView(context, attrs, defStyle) {
private var mAppBarTracking: AppBarTracking? = null
private var mView: View? = null
private var mTopPos: Int = 0
private var mLayoutManager: LinearLayoutManager? = null
interface AppBarTracking {
fun isAppBarIdle(): Boolean
fun isAppBarExpanded(): Boolean
}
override fun dispatchNestedPreScroll(dx: Int, dy: Int, consumed: IntArray?, offsetInWindow: IntArray?, type: Int): Boolean {
if (mAppBarTracking == null)
return super.dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow, type)
if (type == ViewCompat.TYPE_NON_TOUCH && mAppBarTracking!!.isAppBarIdle()
&& isNestedScrollingEnabled) {
if (dy > 0) {
if (mAppBarTracking!!.isAppBarExpanded()) {
consumed!![1] = dy
return true
}
} else {
mTopPos = mLayoutManager!!.findFirstVisibleItemPosition()
if (mTopPos == 0) {
mView = mLayoutManager!!.findViewByPosition(mTopPos)
if (-mView!!.top + dy <= 0) {
consumed!![1] = dy - mView!!.top
return true
}
}
}
}
if (dy < 0 && type == ViewCompat.TYPE_TOUCH && mAppBarTracking!!.isAppBarExpanded()) {
consumed!![1] = dy
return true
}
val returnValue = super.dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow, type)
if (offsetInWindow != null && !isNestedScrollingEnabled && offsetInWindow[1] != 0)
offsetInWindow[1] = 0
return returnValue
}
override fun setLayoutManager(layout: RecyclerView.LayoutManager) {
super.setLayoutManager(layout)
mLayoutManager = layoutManager as LinearLayoutManager
}
fun setAppBarTracking(appBarTracking: AppBarTracking) {
mAppBarTracking = appBarTracking
}
fun setAppBarTracking(appBarLayout: AppBarLayout) {
val appBarIdle = AtomicBoolean(true)
val appBarExpanded = AtomicBoolean()
appBarLayout.addOnOffsetChangedListener(object : AppBarLayout.OnOffsetChangedListener {
private var mAppBarOffset = Integer.MIN_VALUE
override fun onOffsetChanged(appBarLayout: AppBarLayout, verticalOffset: Int) {
if (mAppBarOffset == verticalOffset)
return
mAppBarOffset = verticalOffset
appBarExpanded.set(verticalOffset == 0)
appBarIdle.set(mAppBarOffset >= 0 || mAppBarOffset <= -appBarLayout.totalScrollRange)
}
})
setAppBarTracking(object : AppBarTracking {
override fun isAppBarIdle(): Boolean = appBarIdle.get()
override fun isAppBarExpanded(): Boolean = appBarExpanded.get()
})
}
override fun fling(velocityX: Int, inputVelocityY: Int): Boolean {
var velocityY = inputVelocityY
if (mAppBarTracking != null && !mAppBarTracking!!.isAppBarIdle()) {
val vc = ViewConfiguration.get(context)
velocityY = if (velocityY < 0) -vc.scaledMinimumFlingVelocity
else vc.scaledMinimumFlingVelocity
}
return super.fling(velocityX, velocityY)
}
}
编辑 代码已更新，使其更符合已接受答案的代码。这个答案涉及 NestedScrollView 而接受的答案是关于 RecyclerView.
这是 API 26.0.0-beta2 版本中引入的问题。它不会发生在 beta 1 版本或 API 25 上。正如您所指出的，它也会发生在 API 26.0.0 上。通常，问题似乎与 beta2 中处理 flings 和嵌套滚动的方式有关。对嵌套滚动进行了重大重写（参见 "Carry on Scrolling"），因此出现此类问题也就不足为奇了。
我的想法是 NestedScrollView 中的某个地方没有正确处理多余的滚动条。解决方法是在展开或折叠 AppBar 时安静地消耗某些 "non-touch" 滚动条 (type == ViewCompat.TYPE_NON_TOUCH) 的滚动条。这会停止弹跳，允许快照，并且通常会使 AppBar 表现得更好。
ScrollingActivity 已修改为跟踪 AppBar 的状态以报告其是否展开。一个新的class调用"MyNestedScrollView"覆盖dispatchNestedPreScroll()（新的，见here）来操纵多余滚动的消耗。
下面的代码应该足以阻止 AppBarLayout 摇晃和拒绝捕捉。 （XML 也必须更改以适应 MyNestedSrollView。以下仅适用于支持 lib 26.0.0-beta2 及更高版本。）
AppBarTracking.java
public interface AppBarTracking {
boolean isAppBarIdle();
boolean isAppBarExpanded();
}
ScrollingActivity.java
public class ScrollingActivity extends AppCompatActivity implements AppBarTracking {
private int mAppBarOffset;
private int mAppBarMaxOffset;
private MyNestedScrollView mNestedView;
private boolean mAppBarIdle = true;
@Override
protected void onCreate(Bundle savedInstanceState) {
AppBarLayout appBar;
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_scrolling);
final Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
appBar = findViewById(R.id.app_bar);
mNestedView = findViewById(R.id.nestedScrollView);
mNestedView.setAppBarTracking(this);
appBar.addOnOffsetChangedListener(new AppBarLayout.OnOffsetChangedListener() {
@Override
public void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
mAppBarOffset = verticalOffset;
}
});
appBar.addOnOffsetChangedListener(new AppBarLayout.OnOffsetChangedListener() {
@Override
public final void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
mAppBarOffset = verticalOffset;
// mAppBarOffset = 0 if app bar is expanded; If app bar is collapsed then
// mAppBarOffset = mAppBarMaxOffset
// mAppBarMaxOffset is always <=0 (-AppBarLayout.getTotalScrollRange())
// mAppBarOffset should never be > zero or less than mAppBarMaxOffset
mAppBarIdle = (mAppBarOffset >= 0) || (mAppBarOffset <= mAppBarMaxOffset);
}
});
mNestedView.post(new Runnable() {
@Override
public void run() {
mAppBarMaxOffset = mNestedView.getMaxScrollAmount();
}
});
}
@Override
public boolean isAppBarIdle() {
return mAppBarIdle;
}
@Override
public boolean isAppBarExpanded() {
return mAppBarOffset == 0;
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_scrolling, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
@SuppressWarnings("unused")
private static final String TAG = "ScrollingActivity";
}
MyNestedScrollView.java
public class MyNestedScrollView extends NestedScrollView {
public MyNestedScrollView(Context context) {
this(context, null);
}
public MyNestedScrollView(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public MyNestedScrollView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
setOnScrollChangeListener(new View.OnScrollChangeListener() {
@Override
public void onScrollChange(View view, int x, int y, int oldx, int oldy) {
mScrollPosition = y;
}
});
}
private AppBarTracking mAppBarTracking;
private int mScrollPosition;
@Override
public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow,
int type) {
// App bar latching trouble is only with this type of movement when app bar is expanded
// or collapsed. In touch mode, everything is OK regardless of the open/closed status
// of the app bar.
if (type == ViewCompat.TYPE_NON_TOUCH && mAppBarTracking.isAppBarIdle()
&& isNestedScrollingEnabled()) {
// Make sure the AppBar stays expanded when it should.
if (dy > 0) { // swiped up
if (mAppBarTracking.isAppBarExpanded()) {
// Appbar can only leave its expanded state under the power of touch...
consumed[1] = dy;
return true;
}
} else { // swiped down (or no change)
// Make sure the AppBar stays collapsed when it should.
if (mScrollPosition + dy < 0) {
// Scroll until scroll position = 0 and AppBar is still collapsed.
consumed[1] = dy + mScrollPosition;
return true;
}
}
}
boolean returnValue = super.dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow, type);
// Fix the scrolling problems when scrolling is disabled. This issue existed prior
// to 26.0.0-beta2. (Not sure that this is a problem for 26.0.0-beta2 and later.)
if (offsetInWindow != null && !isNestedScrollingEnabled() && offsetInWindow[1] != 0) {
Log.d(TAG, "<<<<offsetInWindow[1] forced to zero");
offsetInWindow[1] = 0;
}
return returnValue;
}
public void setAppBarTracking(AppBarTracking appBarTracking) {
mAppBarTracking = appBarTracking;
}
@SuppressWarnings("unused")
private static final String TAG = "MyNestedScrollView";
}
更新
我稍微更改了代码以解决剩余的问题 - 至少是我可以重现的问题。关键更新是仅在展开或折叠 AppBar 时处理 dy。在第一次迭代中，dispatchNestedPreScroll() 处理滚动时没有检查 AppBar 的状态是否处于折叠状态。
其他变化较小，属于清理范畴。代码块更新如下。
这个答案解决了关于 RecyclerView 的问题。我给出的另一个答案仍然适用于此。 RecyclerView 与支持库的 26.0.0-beta2 中引入的 NestedScrollView 存在相同的问题。
下面的代码基于 相关问题，但包含对 AppBar 不稳定行为的修复。我删除了修复奇怪滚动的代码，因为它似乎不再需要了。
AppBarTracking.java
public interface AppBarTracking {
boolean isAppBarIdle();
boolean isAppBarExpanded();
}
MyRecyclerView.java
public class MyRecyclerView extends RecyclerView {
public MyRecyclerView(Context context) {
this(context, null);
}
public MyRecyclerView(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public MyRecyclerView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
}
private AppBarTracking mAppBarTracking;
private View mView;
private int mTopPos;
private LinearLayoutManager mLayoutManager;
@Override
public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow,
int type) {
// App bar latching trouble is only with this type of movement when app bar is expanded
// or collapsed. In touch mode, everything is OK regardless of the open/closed status
// of the app bar.
if (type == ViewCompat.TYPE_NON_TOUCH && mAppBarTracking.isAppBarIdle()
&& isNestedScrollingEnabled()) {
// Make sure the AppBar stays expanded when it should.
if (dy > 0) { // swiped up
if (mAppBarTracking.isAppBarExpanded()) {
// Appbar can only leave its expanded state under the power of touch...
consumed[1] = dy;
return true;
}
} else { // swiped down (or no change)
// Make sure the AppBar stays collapsed when it should.
// Only dy < 0 will open the AppBar. Stop it from opening by consuming dy if needed.
mTopPos = mLayoutManager.findFirstVisibleItemPosition();
if (mTopPos == 0) {
mView = mLayoutManager.findViewByPosition(mTopPos);
if (-mView.getTop() + dy <= 0) {
// Scroll until scroll position = 0 and AppBar is still collapsed.
consumed[1] = dy - mView.getTop();
return true;
}
}
}
}
boolean returnValue = super.dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow, type);
// Fix the scrolling problems when scrolling is disabled. This issue existed prior
// to 26.0.0-beta2.
if (offsetInWindow != null && !isNestedScrollingEnabled() && offsetInWindow[1] != 0) {
offsetInWindow[1] = 0;
}
return returnValue;
}
@Override
public void setLayoutManager(RecyclerView.LayoutManager layout) {
super.setLayoutManager(layout);
mLayoutManager = (LinearLayoutManager) getLayoutManager();
}
public void setAppBarTracking(AppBarTracking appBarTracking) {
mAppBarTracking = appBarTracking;
}
@SuppressWarnings("unused")
private static final String TAG = "MyRecyclerView";
}
ScrollingActivity.java
public class ScrollingActivity extends AppCompatActivity
implements AppBarTracking {
private MyRecyclerView mNestedView;
private int mAppBarOffset;
private boolean mAppBarIdle = false;
private int mAppBarMaxOffset;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_scrolling);
Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
mNestedView = findViewById(R.id.nestedView);
final AppBarLayout appBar = findViewById(R.id.app_bar);
appBar.addOnOffsetChangedListener(new AppBarLayout.OnOffsetChangedListener() {
@Override
public final void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
mAppBarOffset = verticalOffset;
// mAppBarOffset = 0 if app bar is expanded; If app bar is collapsed then
// mAppBarOffset = mAppBarMaxOffset
// mAppBarMaxOffset is always <=0 (-AppBarLayout.getTotalScrollRange())
// mAppBarOffset should never be > zero or less than mAppBarMaxOffset
mAppBarIdle = (mAppBarOffset >= 0) || (mAppBarOffset <= mAppBarMaxOffset);
}
});
appBar.post(new Runnable() {
@Override
public void run() {
mAppBarMaxOffset = -appBar.getTotalScrollRange();
}
});
findViewById(R.id.disableNestedScrollingButton).setOnClickListener(new OnClickListener() {
@Override
public void onClick(final View v) {
// If the AppBar is fully expanded or fully collapsed (idle), then disable
// expansion and apply the patch; otherwise, set a flag to disable the expansion
// and apply the patch when the AppBar is idle.
setExpandEnabled(false);
}
});
findViewById(R.id.enableNestedScrollingButton).setOnClickListener(new OnClickListener() {
@Override
public void onClick(final View v) {
setExpandEnabled(true);
}
});
mNestedView.setAppBarTracking(this);
mNestedView.setLayoutManager(new LinearLayoutManager(this));
mNestedView.setAdapter(new Adapter() {
@Override
public ViewHolder onCreateViewHolder(final ViewGroup parent, final int viewType) {
return new ViewHolder(LayoutInflater.from(parent.getContext()).inflate(
android.R.layout.simple_list_item_1,
parent,
false)) {
};
}
@SuppressLint("SetTextI18n")
@Override
public void onBindViewHolder(final ViewHolder holder, final int position) {
((TextView) holder.itemView.findViewById(android.R.id.text1)).setText("item " + position);
}
@Override
public int getItemCount() {
return 100;
}
});
}
private void setExpandEnabled(boolean enabled) {
mNestedView.setNestedScrollingEnabled(enabled);
}
@Override
public boolean isAppBarExpanded() {
return mAppBarOffset == 0;
}
@Override
public boolean isAppBarIdle() {
return mAppBarIdle;
}
@SuppressWarnings("unused")
private static final String TAG = "ScrollingActivity";
}
这里发生了什么？
从问题中可以明显看出，当用户的手指不在屏幕上时，布局无法按应有的方式关闭或打开应用栏。拖动时，应用栏会正常运行。
在版本 26.0.0-beta2 中，引入了一些新方法 - 特别是 dispatchNestedPreScroll() with a new type argument. The type argument specifies if the movement specified by dx and dy are due to the user touching the screen ViewCompat.TYPE_TOUCH or not ViewCompat.TYPE_NON_TOUCH。
虽然没有确定导致问题的具体代码，但修复的重点是在需要时通过不让垂直移动传播来终止 dispatchNestedPreScroll() 中的垂直移动（处理 dy） .实际上，应用栏在展开时会锁定到位，并且在通过触摸手势关闭之前不允许开始关闭。应用栏在关闭时也将被锁定，直到 RecyclerView 位于其最顶端并且有足够的 dy 在执行触摸手势时打开应用栏。
因此，这与其说是一种修复，不如说是一种对有问题情况的阻止。
MyRecyclerView 代码的最后一部分处理了在此 中发现的一个问题，即禁用嵌套滚动时不正确的滚动移动。这是在调用 dispatchNestedPreScroll() 的 super 改变 offsetInWindow[1] 的值之后出现的部分。此代码背后的想法与已接受的问题答案中的想法相同。唯一的区别是，由于底层嵌套滚动代码已更改，参数 offsetInWindow 有时为空。幸运的是，它在重要的时候似乎是非空的，所以最后一部分继续工作。
需要注意的是，这个 "fix" 非常针对所问的问题，并不是通用的解决方案。该修复程序的保质期可能会很短，因为我希望这样一个明显的问题很快就会得到解决。
看起来 onStartNestedScroll 和 onStopNestedScroll 调用可以重新排序，这会导致 "wobbly" 崩溃。我在 AppBarLayout.Behavior 里面做了一个小改动。真的不想像其他答案所建议的那样弄乱 activity 中的所有内容。
@SuppressWarnings("unused")
public class ExtAppBarLayoutBehavior extends AppBarLayout.Behavior {
private int mStartedScrollType = -1;
private boolean mSkipNextStop;
public ExtAppBarLayoutBehavior() {
super();
}
public ExtAppBarLayoutBehavior(Context context, AttributeSet attrs) {
super(context, attrs);
}
@Override
public boolean onStartNestedScroll(CoordinatorLayout parent, AppBarLayout child, View directTargetChild, View target, int nestedScrollAxes, int type) {
if (mStartedScrollType != -1) {
onStopNestedScroll(parent, child, target, mStartedScrollType);
mSkipNextStop = true;
}
mStartedScrollType = type;
return super.onStartNestedScroll(parent, child, directTargetChild, target, nestedScrollAxes, type);
}
@Override
public void onStopNestedScroll(CoordinatorLayout coordinatorLayout, AppBarLayout abl, View target, int type) {
if (mSkipNextStop) {
mSkipNextStop = false;
return;
}
if (mStartedScrollType == -1) {
return;
}
mStartedScrollType = -1;
// Always pass TYPE_TOUCH, because want to snap even after fling
super.onStopNestedScroll(coordinatorLayout, abl, target, ViewCompat.TYPE_TOUCH);
}
}
在 XML 布局中的用法：
<android.support.design.widget.CoordinatorLayout>
<android.support.design.widget.AppBarLayout
app:layout_behavior="com.example.ExtAppBarLayoutBehavior">
<!-- Put here everything you usually add to AppBarLayout: CollapsingToolbarLayout, etc... -->
</android.support.design.widget.AppBarLayout>
<!-- Content: recycler for example -->
<android.support.v7.widget.RecyclerView
app:layout_behavior="@string/appbar_scrolling_view_behavior" />
...
</android.support.design.widget.CoordinatorLayout>
问题的根本原因很可能在RecyclerView。现在没有机会深入挖掘。
由于截至 2020 年 2 月问题仍未解决（最新的 material 库版本为 1.2.0-alpha5）我想分享我对有问题的 AppBar 动画的解决方案。
想法是通过扩展 AppBarLayout.Behavior（Kotlin 版本）来实现自定义捕捉逻辑：
package com.example
import android.content.Context
import android.os.Handler
import android.util.AttributeSet
import android.view.MotionEvent
import android.view.View
import androidx.coordinatorlayout.widget.CoordinatorLayout
import com.google.android.material.appbar.AppBarLayout
import com.google.android.material.appbar.AppBarLayout.LayoutParams
@Suppress("unused")
class AppBarBehaviorFixed(context: Context?, attrs: AttributeSet?) :
AppBarLayout.Behavior(context, attrs) {
private var view: AppBarLayout? = null
private var snapEnabled = false
private var isUpdating = false
private var isScrolling = false
private var isTouching = false
private var lastOffset = 0
private val handler = Handler()
private val snapAction = Runnable {
val view = view ?: return@Runnable
val offset = -lastOffset
val height = view.run { height - paddingTop - paddingBottom - getChildAt(0).minimumHeight }
if (offset > 1 && offset < height - 1) view.setExpanded(offset < height / 2)
}
private val updateFinishDetector = Runnable {
isUpdating = false
scheduleSnapping()
}
private fun initView(view: AppBarLayout) {
if (this.view != null) return
this.view = view
// Checking "snap" flag existence (applied through child view) and removing it
val child = view.getChildAt(0)
val params = child.layoutParams as LayoutParams
snapEnabled = params.scrollFlags hasFlag LayoutParams.SCROLL_FLAG_SNAP
params.scrollFlags = params.scrollFlags removeFlag LayoutParams.SCROLL_FLAG_SNAP
child.layoutParams = params
// Listening for offset changes
view.addOnOffsetChangedListener(AppBarLayout.OnOffsetChangedListener { _, offset ->
lastOffset = offset
isUpdating = true
scheduleSnapping()
handler.removeCallbacks(updateFinishDetector)
handler.postDelayed(updateFinishDetector, 50L)
})
}
private fun scheduleSnapping() {
handler.removeCallbacks(snapAction)
if (snapEnabled && !isUpdating && !isScrolling && !isTouching) {
handler.postDelayed(snapAction, 50L)
}
}
override fun onLayoutChild(
parent: CoordinatorLayout,
abl: AppBarLayout,
layoutDirection: Int
): Boolean {
initView(abl)
return super.onLayoutChild(parent, abl, layoutDirection)
}
override fun onTouchEvent(
parent: CoordinatorLayout,
child: AppBarLayout,
ev: MotionEvent
): Boolean {
isTouching =
ev.actionMasked != MotionEvent.ACTION_UP && ev.actionMasked != MotionEvent.ACTION_CANCEL
scheduleSnapping()
return super.onTouchEvent(parent, child, ev)
}
override fun onStartNestedScroll(
parent: CoordinatorLayout,
child: AppBarLayout,
directTargetChild: View,
target: View,
nestedScrollAxes: Int,
type: Int
): Boolean {
val started = super.onStartNestedScroll(
parent, child, directTargetChild, target, nestedScrollAxes, type
)
if (started) {
isScrolling = true
scheduleSnapping()
}
return started
}
override fun onStopNestedScroll(
coordinatorLayout: CoordinatorLayout,
abl: AppBarLayout,
target: View,
type: Int
) {
isScrolling = false
scheduleSnapping()
super.onStopNestedScroll(coordinatorLayout, abl, target, type)
}
private infix fun Int.hasFlag(flag: Int) = flag and this == flag
private infix fun Int.removeFlag(flag: Int) = this and flag.inv()
}
现在将此行为应用于 xml 中的 AppBarLayout：
<android.support.design.widget.CoordinatorLayout>
<android.support.design.widget.AppBarLayout
app:layout_behavior="com.example.AppBarBehaviorFixed">
<com.google.android.material.appbar.CollapsingToolbarLayout
app:layout_scrollFlags="scroll|exitUntilCollapsed|snap">
<!-- Toolbar declaration -->
</com.google.android.material.appbar.CollapsingToolbarLayout>
</android.support.design.widget.AppBarLayout>
<!-- Scrolling view (RecyclerView, NestedScrollView) -->
</android.support.design.widget.CoordinatorLayout>
这仍然是一个 hack，但它似乎工作得很好，并且它不需要将脏代码放入您的 activity 或扩展 RecyclerView 和 NestedScrollView 小部件（感谢 @vyndor 的这个想法）。
背景
假设您创建的应用程序与您可以通过 "scrolling activity" 的向导创建的应用程序具有相似的 UI，但您希望滚动标志具有捕捉功能，因为这样：
<android.support.design.widget.CollapsingToolbarLayout ... app:layout_scrollFlags="scroll|exitUntilCollapsed|snap" >
问题
事实证明，在很多情况下它都存在捕捉问题。有时 UI 不会对齐到 top/bottom，使得 CollapsingToolbarLayout 停留在两者之间。
有时它也会尝试捕捉到一个方向，然后决定捕捉到另一个方向。
您可以在附加视频中看到这两个问题 here。
我试过的
我认为这是我在 RecyclerView 中使用 setNestedScrollingEnabled(false) 时遇到的问题之一，所以我询问了它
这就是为什么我决定将此作为一个问题进行报告，here。
遗憾的是，我在 Whosebug 上找不到任何解决这些奇怪错误的方法。
问题
它为什么会发生，更重要的是：我如何才能避免这些问题，同时仍然使用它应该具有的行为？
编辑：这是已接受答案的一个很好的改进 Kotlin 版本：
class RecyclerViewEx @JvmOverloads constructor(context: Context, attrs: AttributeSet? = null, defStyle: Int = 0) : RecyclerView(context, attrs, defStyle) {
private var mAppBarTracking: AppBarTracking? = null
private var mView: View? = null
private var mTopPos: Int = 0
private var mLayoutManager: LinearLayoutManager? = null
interface AppBarTracking {
fun isAppBarIdle(): Boolean
fun isAppBarExpanded(): Boolean
}
override fun dispatchNestedPreScroll(dx: Int, dy: Int, consumed: IntArray?, offsetInWindow: IntArray?, type: Int): Boolean {
if (mAppBarTracking == null)
return super.dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow, type)
if (type == ViewCompat.TYPE_NON_TOUCH && mAppBarTracking!!.isAppBarIdle()
&& isNestedScrollingEnabled) {
if (dy > 0) {
if (mAppBarTracking!!.isAppBarExpanded()) {
consumed!![1] = dy
return true
}
} else {
mTopPos = mLayoutManager!!.findFirstVisibleItemPosition()
if (mTopPos == 0) {
mView = mLayoutManager!!.findViewByPosition(mTopPos)
if (-mView!!.top + dy <= 0) {
consumed!![1] = dy - mView!!.top
return true
}
}
}
}
if (dy < 0 && type == ViewCompat.TYPE_TOUCH && mAppBarTracking!!.isAppBarExpanded()) {
consumed!![1] = dy
return true
}
val returnValue = super.dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow, type)
if (offsetInWindow != null && !isNestedScrollingEnabled && offsetInWindow[1] != 0)
offsetInWindow[1] = 0
return returnValue
}
override fun setLayoutManager(layout: RecyclerView.LayoutManager) {
super.setLayoutManager(layout)
mLayoutManager = layoutManager as LinearLayoutManager
}
fun setAppBarTracking(appBarTracking: AppBarTracking) {
mAppBarTracking = appBarTracking
}
fun setAppBarTracking(appBarLayout: AppBarLayout) {
val appBarIdle = AtomicBoolean(true)
val appBarExpanded = AtomicBoolean()
appBarLayout.addOnOffsetChangedListener(object : AppBarLayout.OnOffsetChangedListener {
private var mAppBarOffset = Integer.MIN_VALUE
override fun onOffsetChanged(appBarLayout: AppBarLayout, verticalOffset: Int) {
if (mAppBarOffset == verticalOffset)
return
mAppBarOffset = verticalOffset
appBarExpanded.set(verticalOffset == 0)
appBarIdle.set(mAppBarOffset >= 0 || mAppBarOffset <= -appBarLayout.totalScrollRange)
}
})
setAppBarTracking(object : AppBarTracking {
override fun isAppBarIdle(): Boolean = appBarIdle.get()
override fun isAppBarExpanded(): Boolean = appBarExpanded.get()
})
}
override fun fling(velocityX: Int, inputVelocityY: Int): Boolean {
var velocityY = inputVelocityY
if (mAppBarTracking != null && !mAppBarTracking!!.isAppBarIdle()) {
val vc = ViewConfiguration.get(context)
velocityY = if (velocityY < 0) -vc.scaledMinimumFlingVelocity
else vc.scaledMinimumFlingVelocity
}
return super.fling(velocityX, velocityY)
}
}
编辑 代码已更新，使其更符合已接受答案的代码。这个答案涉及 NestedScrollView 而接受的答案是关于 RecyclerView.
这是 API 26.0.0-beta2 版本中引入的问题。它不会发生在 beta 1 版本或 API 25 上。正如您所指出的，它也会发生在 API 26.0.0 上。通常，问题似乎与 beta2 中处理 flings 和嵌套滚动的方式有关。对嵌套滚动进行了重大重写（参见 "Carry on Scrolling"），因此出现此类问题也就不足为奇了。
我的想法是 NestedScrollView 中的某个地方没有正确处理多余的滚动条。解决方法是在展开或折叠 AppBar 时安静地消耗某些 "non-touch" 滚动条 (type == ViewCompat.TYPE_NON_TOUCH) 的滚动条。这会停止弹跳，允许快照，并且通常会使 AppBar 表现得更好。
ScrollingActivity 已修改为跟踪 AppBar 的状态以报告其是否展开。一个新的class调用"MyNestedScrollView"覆盖dispatchNestedPreScroll()（新的，见here）来操纵多余滚动的消耗。
下面的代码应该足以阻止 AppBarLayout 摇晃和拒绝捕捉。 （XML 也必须更改以适应 MyNestedSrollView。以下仅适用于支持 lib 26.0.0-beta2 及更高版本。）
AppBarTracking.java
public interface AppBarTracking {
boolean isAppBarIdle();
boolean isAppBarExpanded();
}
ScrollingActivity.java
public class ScrollingActivity extends AppCompatActivity implements AppBarTracking {
private int mAppBarOffset;
private int mAppBarMaxOffset;
private MyNestedScrollView mNestedView;
private boolean mAppBarIdle = true;
@Override
protected void onCreate(Bundle savedInstanceState) {
AppBarLayout appBar;
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_scrolling);
final Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
appBar = findViewById(R.id.app_bar);
mNestedView = findViewById(R.id.nestedScrollView);
mNestedView.setAppBarTracking(this);
appBar.addOnOffsetChangedListener(new AppBarLayout.OnOffsetChangedListener() {
@Override
public void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
mAppBarOffset = verticalOffset;
}
});
appBar.addOnOffsetChangedListener(new AppBarLayout.OnOffsetChangedListener() {
@Override
public final void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
mAppBarOffset = verticalOffset;
// mAppBarOffset = 0 if app bar is expanded; If app bar is collapsed then
// mAppBarOffset = mAppBarMaxOffset
// mAppBarMaxOffset is always <=0 (-AppBarLayout.getTotalScrollRange())
// mAppBarOffset should never be > zero or less than mAppBarMaxOffset
mAppBarIdle = (mAppBarOffset >= 0) || (mAppBarOffset <= mAppBarMaxOffset);
}
});
mNestedView.post(new Runnable() {
@Override
public void run() {
mAppBarMaxOffset = mNestedView.getMaxScrollAmount();
}
});
}
@Override
public boolean isAppBarIdle() {
return mAppBarIdle;
}
@Override
public boolean isAppBarExpanded() {
return mAppBarOffset == 0;
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_scrolling, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
@SuppressWarnings("unused")
private static final String TAG = "ScrollingActivity";
}
MyNestedScrollView.java
public class MyNestedScrollView extends NestedScrollView {
public MyNestedScrollView(Context context) {
this(context, null);
}
public MyNestedScrollView(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public MyNestedScrollView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
setOnScrollChangeListener(new View.OnScrollChangeListener() {
@Override
public void onScrollChange(View view, int x, int y, int oldx, int oldy) {
mScrollPosition = y;
}
});
}
private AppBarTracking mAppBarTracking;
private int mScrollPosition;
@Override
public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow,
int type) {
// App bar latching trouble is only with this type of movement when app bar is expanded
// or collapsed. In touch mode, everything is OK regardless of the open/closed status
// of the app bar.
if (type == ViewCompat.TYPE_NON_TOUCH && mAppBarTracking.isAppBarIdle()
&& isNestedScrollingEnabled()) {
// Make sure the AppBar stays expanded when it should.
if (dy > 0) { // swiped up
if (mAppBarTracking.isAppBarExpanded()) {
// Appbar can only leave its expanded state under the power of touch...
consumed[1] = dy;
return true;
}
} else { // swiped down (or no change)
// Make sure the AppBar stays collapsed when it should.
if (mScrollPosition + dy < 0) {
// Scroll until scroll position = 0 and AppBar is still collapsed.
consumed[1] = dy + mScrollPosition;
return true;
}
}
}
boolean returnValue = super.dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow, type);
// Fix the scrolling problems when scrolling is disabled. This issue existed prior
// to 26.0.0-beta2. (Not sure that this is a problem for 26.0.0-beta2 and later.)
if (offsetInWindow != null && !isNestedScrollingEnabled() && offsetInWindow[1] != 0) {
Log.d(TAG, "<<<<offsetInWindow[1] forced to zero");
offsetInWindow[1] = 0;
}
return returnValue;
}
public void setAppBarTracking(AppBarTracking appBarTracking) {
mAppBarTracking = appBarTracking;
}
@SuppressWarnings("unused")
private static final String TAG = "MyNestedScrollView";
}
更新
我稍微更改了代码以解决剩余的问题 - 至少是我可以重现的问题。关键更新是仅在展开或折叠 AppBar 时处理 dy。在第一次迭代中，dispatchNestedPreScroll() 处理滚动时没有检查 AppBar 的状态是否处于折叠状态。
其他变化较小，属于清理范畴。代码块更新如下。
这个答案解决了关于 RecyclerView 的问题。我给出的另一个答案仍然适用于此。 RecyclerView 与支持库的 26.0.0-beta2 中引入的 NestedScrollView 存在相同的问题。
下面的代码基于
AppBarTracking.java
public interface AppBarTracking {
boolean isAppBarIdle();
boolean isAppBarExpanded();
}
MyRecyclerView.java
public class MyRecyclerView extends RecyclerView {
public MyRecyclerView(Context context) {
this(context, null);
}
public MyRecyclerView(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public MyRecyclerView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
}
private AppBarTracking mAppBarTracking;
private View mView;
private int mTopPos;
private LinearLayoutManager mLayoutManager;
@Override
public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow,
int type) {
// App bar latching trouble is only with this type of movement when app bar is expanded
// or collapsed. In touch mode, everything is OK regardless of the open/closed status
// of the app bar.
if (type == ViewCompat.TYPE_NON_TOUCH && mAppBarTracking.isAppBarIdle()
&& isNestedScrollingEnabled()) {
// Make sure the AppBar stays expanded when it should.
if (dy > 0) { // swiped up
if (mAppBarTracking.isAppBarExpanded()) {
// Appbar can only leave its expanded state under the power of touch...
consumed[1] = dy;
return true;
}
} else { // swiped down (or no change)
// Make sure the AppBar stays collapsed when it should.
// Only dy < 0 will open the AppBar. Stop it from opening by consuming dy if needed.
mTopPos = mLayoutManager.findFirstVisibleItemPosition();
if (mTopPos == 0) {
mView = mLayoutManager.findViewByPosition(mTopPos);
if (-mView.getTop() + dy <= 0) {
// Scroll until scroll position = 0 and AppBar is still collapsed.
consumed[1] = dy - mView.getTop();
return true;
}
}
}
}
boolean returnValue = super.dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow, type);
// Fix the scrolling problems when scrolling is disabled. This issue existed prior
// to 26.0.0-beta2.
if (offsetInWindow != null && !isNestedScrollingEnabled() && offsetInWindow[1] != 0) {
offsetInWindow[1] = 0;
}
return returnValue;
}
@Override
public void setLayoutManager(RecyclerView.LayoutManager layout) {
super.setLayoutManager(layout);
mLayoutManager = (LinearLayoutManager) getLayoutManager();
}
public void setAppBarTracking(AppBarTracking appBarTracking) {
mAppBarTracking = appBarTracking;
}
@SuppressWarnings("unused")
private static final String TAG = "MyRecyclerView";
}
ScrollingActivity.java
public class ScrollingActivity extends AppCompatActivity
implements AppBarTracking {
private MyRecyclerView mNestedView;
private int mAppBarOffset;
private boolean mAppBarIdle = false;
private int mAppBarMaxOffset;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_scrolling);
Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
mNestedView = findViewById(R.id.nestedView);
final AppBarLayout appBar = findViewById(R.id.app_bar);
appBar.addOnOffsetChangedListener(new AppBarLayout.OnOffsetChangedListener() {
@Override
public final void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
mAppBarOffset = verticalOffset;
// mAppBarOffset = 0 if app bar is expanded; If app bar is collapsed then
// mAppBarOffset = mAppBarMaxOffset
// mAppBarMaxOffset is always <=0 (-AppBarLayout.getTotalScrollRange())
// mAppBarOffset should never be > zero or less than mAppBarMaxOffset
mAppBarIdle = (mAppBarOffset >= 0) || (mAppBarOffset <= mAppBarMaxOffset);
}
});
appBar.post(new Runnable() {
@Override
public void run() {
mAppBarMaxOffset = -appBar.getTotalScrollRange();
}
});
findViewById(R.id.disableNestedScrollingButton).setOnClickListener(new OnClickListener() {
@Override
public void onClick(final View v) {
// If the AppBar is fully expanded or fully collapsed (idle), then disable
// expansion and apply the patch; otherwise, set a flag to disable the expansion
// and apply the patch when the AppBar is idle.
setExpandEnabled(false);
}
});
findViewById(R.id.enableNestedScrollingButton).setOnClickListener(new OnClickListener() {
@Override
public void onClick(final View v) {
setExpandEnabled(true);
}
});
mNestedView.setAppBarTracking(this);
mNestedView.setLayoutManager(new LinearLayoutManager(this));
mNestedView.setAdapter(new Adapter() {
@Override
public ViewHolder onCreateViewHolder(final ViewGroup parent, final int viewType) {
return new ViewHolder(LayoutInflater.from(parent.getContext()).inflate(
android.R.layout.simple_list_item_1,
parent,
false)) {
};
}
@SuppressLint("SetTextI18n")
@Override
public void onBindViewHolder(final ViewHolder holder, final int position) {
((TextView) holder.itemView.findViewById(android.R.id.text1)).setText("item " + position);
}
@Override
public int getItemCount() {
return 100;
}
});
}
private void setExpandEnabled(boolean enabled) {
mNestedView.setNestedScrollingEnabled(enabled);
}
@Override
public boolean isAppBarExpanded() {
return mAppBarOffset == 0;
}
@Override
public boolean isAppBarIdle() {
return mAppBarIdle;
}
@SuppressWarnings("unused")
private static final String TAG = "ScrollingActivity";
}
这里发生了什么？
从问题中可以明显看出，当用户的手指不在屏幕上时，布局无法按应有的方式关闭或打开应用栏。拖动时，应用栏会正常运行。
在版本 26.0.0-beta2 中，引入了一些新方法 - 特别是 dispatchNestedPreScroll() with a new type argument. The type argument specifies if the movement specified by dx and dy are due to the user touching the screen ViewCompat.TYPE_TOUCH or not ViewCompat.TYPE_NON_TOUCH。
虽然没有确定导致问题的具体代码，但修复的重点是在需要时通过不让垂直移动传播来终止 dispatchNestedPreScroll() 中的垂直移动（处理 dy） .实际上，应用栏在展开时会锁定到位，并且在通过触摸手势关闭之前不允许开始关闭。应用栏在关闭时也将被锁定，直到 RecyclerView 位于其最顶端并且有足够的 dy 在执行触摸手势时打开应用栏。
因此，这与其说是一种修复，不如说是一种对有问题情况的阻止。
MyRecyclerView 代码的最后一部分处理了在此 dispatchNestedPreScroll() 的 super 改变 offsetInWindow[1] 的值之后出现的部分。此代码背后的想法与已接受的问题答案中的想法相同。唯一的区别是，由于底层嵌套滚动代码已更改，参数 offsetInWindow 有时为空。幸运的是，它在重要的时候似乎是非空的，所以最后一部分继续工作。
需要注意的是，这个 "fix" 非常针对所问的问题，并不是通用的解决方案。该修复程序的保质期可能会很短，因为我希望这样一个明显的问题很快就会得到解决。
看起来 onStartNestedScroll 和 onStopNestedScroll 调用可以重新排序，这会导致 "wobbly" 崩溃。我在 AppBarLayout.Behavior 里面做了一个小改动。真的不想像其他答案所建议的那样弄乱 activity 中的所有内容。
@SuppressWarnings("unused")
public class ExtAppBarLayoutBehavior extends AppBarLayout.Behavior {
private int mStartedScrollType = -1;
private boolean mSkipNextStop;
public ExtAppBarLayoutBehavior() {
super();
}
public ExtAppBarLayoutBehavior(Context context, AttributeSet attrs) {
super(context, attrs);
}
@Override
public boolean onStartNestedScroll(CoordinatorLayout parent, AppBarLayout child, View directTargetChild, View target, int nestedScrollAxes, int type) {
if (mStartedScrollType != -1) {
onStopNestedScroll(parent, child, target, mStartedScrollType);
mSkipNextStop = true;
}
mStartedScrollType = type;
return super.onStartNestedScroll(parent, child, directTargetChild, target, nestedScrollAxes, type);
}
@Override
public void onStopNestedScroll(CoordinatorLayout coordinatorLayout, AppBarLayout abl, View target, int type) {
if (mSkipNextStop) {
mSkipNextStop = false;
return;
}
if (mStartedScrollType == -1) {
return;
}
mStartedScrollType = -1;
// Always pass TYPE_TOUCH, because want to snap even after fling
super.onStopNestedScroll(coordinatorLayout, abl, target, ViewCompat.TYPE_TOUCH);
}
}
在 XML 布局中的用法：
<android.support.design.widget.CoordinatorLayout>
<android.support.design.widget.AppBarLayout
app:layout_behavior="com.example.ExtAppBarLayoutBehavior">
<!-- Put here everything you usually add to AppBarLayout: CollapsingToolbarLayout, etc... -->
</android.support.design.widget.AppBarLayout>
<!-- Content: recycler for example -->
<android.support.v7.widget.RecyclerView
app:layout_behavior="@string/appbar_scrolling_view_behavior" />
...
</android.support.design.widget.CoordinatorLayout>
问题的根本原因很可能在RecyclerView。现在没有机会深入挖掘。
由于截至 2020 年 2 月问题仍未解决（最新的 material 库版本为 1.2.0-alpha5）我想分享我对有问题的 AppBar 动画的解决方案。
想法是通过扩展 AppBarLayout.Behavior（Kotlin 版本）来实现自定义捕捉逻辑：
package com.example
import android.content.Context
import android.os.Handler
import android.util.AttributeSet
import android.view.MotionEvent
import android.view.View
import androidx.coordinatorlayout.widget.CoordinatorLayout
import com.google.android.material.appbar.AppBarLayout
import com.google.android.material.appbar.AppBarLayout.LayoutParams
@Suppress("unused")
class AppBarBehaviorFixed(context: Context?, attrs: AttributeSet?) :
AppBarLayout.Behavior(context, attrs) {
private var view: AppBarLayout? = null
private var snapEnabled = false
private var isUpdating = false
private var isScrolling = false
private var isTouching = false
private var lastOffset = 0
private val handler = Handler()
private val snapAction = Runnable {
val view = view ?: return@Runnable
val offset = -lastOffset
val height = view.run { height - paddingTop - paddingBottom - getChildAt(0).minimumHeight }
if (offset > 1 && offset < height - 1) view.setExpanded(offset < height / 2)
}
private val updateFinishDetector = Runnable {
isUpdating = false
scheduleSnapping()
}
private fun initView(view: AppBarLayout) {
if (this.view != null) return
this.view = view
// Checking "snap" flag existence (applied through child view) and removing it
val child = view.getChildAt(0)
val params = child.layoutParams as LayoutParams
snapEnabled = params.scrollFlags hasFlag LayoutParams.SCROLL_FLAG_SNAP
params.scrollFlags = params.scrollFlags removeFlag LayoutParams.SCROLL_FLAG_SNAP
child.layoutParams = params
// Listening for offset changes
view.addOnOffsetChangedListener(AppBarLayout.OnOffsetChangedListener { _, offset ->
lastOffset = offset
isUpdating = true
scheduleSnapping()
handler.removeCallbacks(updateFinishDetector)
handler.postDelayed(updateFinishDetector, 50L)
})
}
private fun scheduleSnapping() {
handler.removeCallbacks(snapAction)
if (snapEnabled && !isUpdating && !isScrolling && !isTouching) {
handler.postDelayed(snapAction, 50L)
}
}
override fun onLayoutChild(
parent: CoordinatorLayout,
abl: AppBarLayout,
layoutDirection: Int
): Boolean {
initView(abl)
return super.onLayoutChild(parent, abl, layoutDirection)
}
override fun onTouchEvent(
parent: CoordinatorLayout,
child: AppBarLayout,
ev: MotionEvent
): Boolean {
isTouching =
ev.actionMasked != MotionEvent.ACTION_UP && ev.actionMasked != MotionEvent.ACTION_CANCEL
scheduleSnapping()
return super.onTouchEvent(parent, child, ev)
}
override fun onStartNestedScroll(
parent: CoordinatorLayout,
child: AppBarLayout,
directTargetChild: View,
target: View,
nestedScrollAxes: Int,
type: Int
): Boolean {
val started = super.onStartNestedScroll(
parent, child, directTargetChild, target, nestedScrollAxes, type
)
if (started) {
isScrolling = true
scheduleSnapping()
}
return started
}
override fun onStopNestedScroll(
coordinatorLayout: CoordinatorLayout,
abl: AppBarLayout,
target: View,
type: Int
) {
isScrolling = false
scheduleSnapping()
super.onStopNestedScroll(coordinatorLayout, abl, target, type)
}
private infix fun Int.hasFlag(flag: Int) = flag and this == flag
private infix fun Int.removeFlag(flag: Int) = this and flag.inv()
}
现在将此行为应用于 xml 中的 AppBarLayout：
<android.support.design.widget.CoordinatorLayout>
<android.support.design.widget.AppBarLayout
app:layout_behavior="com.example.AppBarBehaviorFixed">
<com.google.android.material.appbar.CollapsingToolbarLayout
app:layout_scrollFlags="scroll|exitUntilCollapsed|snap">
<!-- Toolbar declaration -->
</com.google.android.material.appbar.CollapsingToolbarLayout>
</android.support.design.widget.AppBarLayout>
<!-- Scrolling view (RecyclerView, NestedScrollView) -->
</android.support.design.widget.CoordinatorLayout>
这仍然是一个 hack，但它似乎工作得很好，并且它不需要将脏代码放入您的 activity 或扩展 RecyclerView 和 NestedScrollView 小部件（感谢 @vyndor 的这个想法）。