Android ViewPager with RecyclerView works incorrectly inside BottomSheet

I came across the same limitation but were able to solve it.

The reason for the effect you described is that BottomSheetBehavior (as of v24.2.0) only supports one scrolling child which is identified during layout in the following way:

private View findScrollingChild(View view) {
    if (view instanceof NestedScrollingChild) {
        return view;
    }
    if (view instanceof ViewGroup) {
        ViewGroup group = (ViewGroup) view;
        for (int i = 0, count = group.getChildCount(); i < count; i++) {
            View scrollingChild = findScrollingChild(group.getChildAt(i));
            if (scrollingChild != null) {
                return scrollingChild;
            }
        }
    }
    return null;
}

You can see that it essentially finds the first scrolling child using DFS.

I slightly enhanced this implementation and assembled a small library as well as an example app. You can find it here: https://github.com/laenger/ViewPagerBottomSheet

Simply add the maven repo url to your build.gradle:

repositories {
    maven { url "https://raw.github.com/laenger/maven-releases/master/releases" }
}

Add the library to the dependencies:

dependencies {
    compile "biz.laenger.android:vpbs:0.0.2"
}

Use ViewPagerBottomSheetBehavior for your bottom sheet view:

app:layout_behavior="@string/view_pager_bottom_sheet_behavior"

Setup any nested ViewPager inside the bottom sheet:

BottomSheetUtils.setupViewPager(bottomSheetViewPager)

(This also works when the ViewPager is the bottom sheet view and for further nested ViewPagers)

There is another approach that does not require modifying BottomSheetBehavior but instead leverages the fact that the BottomSheetBehavior only recognizes the first NestedScrollView with NestedScrollingEnabled it finds. So instead of altering this logic inside BottomSheetBehavior, enable and disable the appropriate scroll views. I discovered this approach here: https://imnotyourson.com/cannot-scroll-scrollable-content-inside-viewpager-as-bottomsheet-of-coordinatorlayout/

In my case my BottomSheetBehavior was using a TabLayout with a FragmentPagerAdapter so my FragmentPagerAdapter needed the following code:

@Override
public void setPrimaryItem(@NonNull ViewGroup container, int position, @NonNull Object object) {

        super.setPrimaryItem(container, position, object);

        Fragment f = ((Fragment)object);
        String activeFragmentTag = f.getTag();
        View view = f.getView();

        if (view != null) {
            View nestedView = view.findViewWithTag("nested");               
            if ( nestedView != null && nestedView instanceof NestedScrollView) {
                ((NestedScrollView)nestedView).setNestedScrollingEnabled(true);
            }
        }

        FragmentManager fm = f.getFragmentManager();

        for(Fragment frag : fm.getFragments()) {

            if (frag.getTag() != activeFragmentTag) {
                View v = frag.getView();
                if (v!= null) {

                    View nestedView = v.findViewWithTag("nested");

                    if (nestedView!= null && nestedView instanceof NestedScrollView) {
                        ((NestedScrollView)nestedView).setNestedScrollingEnabled(false);
                    }
                }
            }
        }

        container.requestLayout();
    }

Any nested scroll views in your fragments just need to have the "nested" tag.

Here is a sample Fragment layout file:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".ui.MLeftFragment">

    <androidx.core.widget.NestedScrollView
        android:id="@+id/nestedScrollView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:tag="nested"
        android:fillViewport="true">

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical">

            <!-- TODO: Update blank fragment layout -->
            <TextView
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:text="@string/hello_mool_left_fragment" />      

        </LinearLayout>  

    </androidx.core.widget.NestedScrollView>

</androidx.constraintlayout.widget.ConstraintLayout>

This post saved my life: https://medium.com/@hanru.yeh/funny-solution-that-makes-bottomsheetdialog-support-viewpager-with-nestedscrollingchilds-bfdca72235c3

Show my fix for ViewPager inside bottomsheet.

package com.google.android.material.bottomsheet

import android.view.View
import androidx.annotation.VisibleForTesting
import androidx.viewpager.widget.ViewPager
import java.lang.ref.WeakReference


class BottomSheetBehaviorFix<V : View> : BottomSheetBehavior<V>(), ViewPager.OnPageChangeListener {

    override fun onPageScrollStateChanged(state: Int) {}

    override fun onPageScrolled(position: Int, positionOffset: Float, positionOffsetPixels: Int) {}

    override fun onPageSelected(position: Int) {
        val container = viewRef?.get() ?: return
        nestedScrollingChildRef = WeakReference(findScrollingChild(container))
    }

    @VisibleForTesting
    override fun findScrollingChild(view: View): View? {
        return if (view is ViewPager) {
            view.focusedChild?.let { findScrollingChild(it) }
        } else {
            super.findScrollingChild(view)
        }
    }
}