a more elegant and non-invasive solution for viewpager in bottomsheet.

BottomSheetBehaviorExtension.kt

import android.view.View
import android.view.ViewGroup
import androidx.core.view.ViewCompat
import androidx.viewpager.widget.ViewPager
import androidx.viewpager.widget.ViewPagerUtils
import com.google.android.material.bottomsheet.BottomSheetBehavior
import java.lang.ref.WeakReference
import java.lang.reflect.Field

private val nestedScrollingChildRef: Field =
    BottomSheetBehavior::class.java.getDeclaredField("nestedScrollingChildRef")

fun BottomSheetBehavior<out ViewGroup>.resetScrollingChild(view: View) {
    // @Nullable WeakReference<View> nestedScrollingChildRef;
    val v = findScrollingChild(view)

    nestedScrollingChildRef.isAccessible = true
    nestedScrollingChildRef.set(this, WeakReference(v))
}

private fun findScrollingChild(view: View?): View? {
    if (view == null || view.visibility != View.VISIBLE) {
        return null
    }
    if (view is ViewPager) {
        //通过ViewPagerUtils找到当前在界面上的页面
        val currentViewPagerChild = ViewPagerUtils.getCurrentView(view)
        val scrollingChild = findScrollingChild(currentViewPagerChild)
        return scrollingChild ?: currentViewPagerChild
    } else if (ViewCompat.isNestedScrollingEnabled(view)) {
        return view
    } else if (view is ViewGroup) {
        var i = 0
        val count = view.childCount
        while (i < count) {
            val scrollingChild = findScrollingChild(view.getChildAt(i))
            if (scrollingChild != null) {
                return scrollingChild
            }
            i++
        }
    }
    return null
}

Sample.kt

viewPager.addOnPageChangeListener(object : ViewPager.OnPageChangeListener {
    override fun onPageScrolled(
        position: Int,
        positionOffset: Float,
        positionOffsetPixels: Int
    ) {
    }

    override fun onPageSelected(position: Int) {
        behavior.resetScrollingChild(viewPager)
    }

    override fun onPageScrollStateChanged(state: Int) {

    }

})

ViewPagerUtils.java

package androidx.viewpager.widget;

import android.view.View;

public class ViewPagerUtils {
    public static View getCurrentView(ViewPager viewPager) {
        final int currentItem = viewPager.getCurrentItem();
        for (int i = 0; i < viewPager.getChildCount(); i++) {
            final View child = viewPager.getChildAt(i);
            final ViewPager.LayoutParams layoutParams = (ViewPager.LayoutParams) child.getLayoutParams();
            if (!layoutParams.isDecor && currentItem == layoutParams.position) {
                return child;
            }
        }
        return null;
    }
}