Fix parallel scroll with a NestedScrollView inside a ViewPager2

NestedScrollableHost.kt

/*
 * Copyright 2019 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
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 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
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package androidx.viewpager2.integration.testapp

import android.content.Context
import android.util.AttributeSet
import android.view.MotionEvent
import android.view.View
import android.view.ViewConfiguration
import android.widget.FrameLayout
import androidx.viewpager2.widget.ViewPager2
import androidx.viewpager2.widget.ViewPager2.ORIENTATION_HORIZONTAL
import kotlin.math.absoluteValue
import kotlin.math.sign

/**
 * Layout to wrap a scrollable component inside a ViewPager2. Provided as a solution to the problem
 * where pages of ViewPager2 have nested scrollable elements that scroll in the same direction as
 * ViewPager2. The scrollable element needs to be the immediate and only child of this host layout.
 *
 * This solution has limitations when using multiple levels of nested scrollable elements
 * (e.g. a horizontal RecyclerView in a vertical RecyclerView in a horizontal ViewPager2).
 */
class NestedScrollableHost : FrameLayout {
    constructor(context: Context) : super(context)
    constructor(context: Context, attrs: AttributeSet?) : super(context, attrs)

    private var touchSlop = 0
    private var initialX = 0f
    private var initialY = 0f
    private val parentViewPager: ViewPager2?
        get() {
            var v: View? = parent as? View
            while (v != null && v !is ViewPager2) {
                v = v.parent as? View
            }
            return v as? ViewPager2
        }

    private val child: View? get() = if (childCount > 0) getChildAt(0) else null

    init {
        touchSlop = ViewConfiguration.get(context).scaledTouchSlop
    }

    private fun canChildScroll(orientation: Int, delta: Float): Boolean {
        val direction = -delta.sign.toInt()
        return when (orientation) {
            0 -> child?.canScrollHorizontally(direction) ?: false
            1 -> child?.canScrollVertically(direction) ?: false
            else -> throw IllegalArgumentException()
        }
    }

    override fun onInterceptTouchEvent(e: MotionEvent): Boolean {
        return handleInterceptTouchEvent(e) || super.onInterceptTouchEvent(e)
    }

    /**
     * @return The child does not need to scroll.
     */
    private fun handleInterceptTouchEvent(e: MotionEvent): Boolean {
        val orientation = parentViewPager?.orientation ?: return false

        // Early return if child can't scroll in same direction as parent
        if (!canChildScroll(orientation, -1f) && !canChildScroll(orientation, 1f)) {
            return false
        }

        if (e.action == MotionEvent.ACTION_DOWN) {
            initialX = e.x
            initialY = e.y
            parent.requestDisallowInterceptTouchEvent(true)
        } else if (e.action == MotionEvent.ACTION_MOVE) {
            val dx = e.x - initialX
            val dy = e.y - initialY
            val isVpHorizontal = orientation == ORIENTATION_HORIZONTAL

            if (dx.absoluteValue > touchSlop || dy.absoluteValue > touchSlop) {
                if (isVpHorizontal == (dy.absoluteValue > dx.absoluteValue)) {
                    // Gesture is perpendicular, allow all parents to intercept
                    parent.requestDisallowInterceptTouchEvent(false)

                    // If you are the following case:
                    // ViewPager2(V) -> NestedScrollView(V) -> RecyclerView(H)
                    // return false instead so that the innermost RV(H) can scroll normally.
                    // It is not tested for other use case.
                    return true
                } else {
                    // Gesture is parallel, query child if movement in that direction is possible
                    if (canChildScroll(orientation, if (isVpHorizontal) dx else dy)) {
                        // Child can scroll, disallow all parents to intercept
                        parent.requestDisallowInterceptTouchEvent(true)
                    } else {
                        // Child cannot scroll, allow all parents to intercept
                        parent.requestDisallowInterceptTouchEvent(false)
                        return true
                    }
                }
            }
        }
        return false
    }
}

Fix for the use case: ViewPager2(V) -> NestedScrollView(V) -> RecyclerView(H)
Releated AndroidX issues: 123006042
Original code: views-widgets-samples

Hello i dont know if this is the place to say it but i have used this converted to java version of your code as pointed in : stackoverflow, everything works okay but when i add a swiperefreshlayout as parent of the nestedscrollview the horizontal swiping in the recyclerview stops working, i needed to pointed that, thanks

@eme22 Yes, this code is just for my specific use case which has been mentioned in the comment. The whole scrolling issue is a bit difficult since ViewPager2 team has not solve it. So you may need to refer to my thoughts and change the code to fit your case.