Full code of a BottomNavigationBehavior which hides itself on scroll and handles Snackbars, FAB and snapping. https://medium.com/@ValCanBuild/scroll-your-bottom-navigation-view-away-with-10-lines-of-code-346f1ed40e9e

BottomNavigationBehavior.kt

/**
MIT License

Copyright (c) 2018 Valentin Hinov

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/

// Apply to BottomNavigationView
class BottomNavigationBehavior<V : View>(context: Context, attrs: AttributeSet) :
    CoordinatorLayout.Behavior<V>(context, attrs) {

    @NestedScrollType
    private var lastStartedType: Int = 0

    private var offsetAnimator: ValueAnimator? = null

    var isSnappingEnabled = false

    override fun layoutDependsOn(parent: CoordinatorLayout, child: V, dependency: View): Boolean {
        if (dependency is Snackbar.SnackbarLayout) {
            updateSnackbar(child, dependency)
        }
        return super.layoutDependsOn(parent, child, dependency)
    }

    override fun onStartNestedScroll(
        coordinatorLayout: CoordinatorLayout, child: V, directTargetChild: View, target: View, axes: Int, type: Int
    ): Boolean {
        if (axes != ViewCompat.SCROLL_AXIS_VERTICAL)
            return false

        lastStartedType = type

        offsetAnimator?.cancel()

        return true
    }

    override fun onNestedPreScroll(
        coordinatorLayout: CoordinatorLayout, child: V, target: View, dx: Int, dy: Int, consumed: IntArray, type: Int
    ) {
        super.onNestedPreScroll(coordinatorLayout, child, target, dx, dy, consumed, type)
        child.translationY = max(0f, min(child.height.toFloat(), child.translationY + dy))
    }

    override fun onStopNestedScroll(coordinatorLayout: CoordinatorLayout, child: V, target: View, type: Int) {
        if (!isSnappingEnabled)
            return

        // add snap behaviour
        // Logic here borrowed from AppBarLayout onStopNestedScroll code
        if (lastStartedType == ViewCompat.TYPE_TOUCH || type == ViewCompat.TYPE_NON_TOUCH) {
            // find nearest seam
            val currTranslation = child.translationY
            val childHalfHeight = child.height * 0.5f

            // translate down
            if (currTranslation >= childHalfHeight) {
                animateBarVisibility(child, isVisible = false)
            }
            // translate up
            else {
                animateBarVisibility(child, isVisible = true)
            }
        }
    }

    private fun animateBarVisibility(child: View, isVisible: Boolean) {
        if (offsetAnimator == null) {
            offsetAnimator = ValueAnimator().apply {
                interpolator = DecelerateInterpolator()
                duration = 150L
            }

            offsetAnimator?.addUpdateListener {
                child.translationY = it.animatedValue as Float
            }
        } else {
            offsetAnimator?.cancel()
        }

        val targetTranslation = if (isVisible) 0f else child.height.toFloat()
        offsetAnimator?.setFloatValues(child.translationY, targetTranslation)
        offsetAnimator?.start()
    }

    private fun updateSnackbar(child: View, snackbarLayout: Snackbar.SnackbarLayout) {
        if (snackbarLayout.layoutParams is CoordinatorLayout.LayoutParams) {
            val params = snackbarLayout.layoutParams as CoordinatorLayout.LayoutParams

            params.anchorId = child.id
            params.anchorGravity = Gravity.TOP
            params.gravity = Gravity.TOP
            snackbarLayout.layoutParams = params
        }
    }
}

To programmatically do slideUp and slideDown just like with the official HideBottomViewOnScrollBehavior

class BottomNavigationBehavior<V : View>(context: Context, attrs: AttributeSet) :
    CoordinatorLayout.Behavior<V>(context, attrs) {

    @ViewCompat.NestedScrollType
    private var lastStartedType: Int = 0

    private var offsetAnimator: ValueAnimator? = null

    private var isSnappingEnabled = true

    @SuppressLint("RestrictedApi")
    override fun layoutDependsOn(parent: CoordinatorLayout, child: V, dependency: View): Boolean {
        if (dependency is Snackbar.SnackbarLayout) {
            updateSnackbar(child, dependency)
        }
        return super.layoutDependsOn(parent, child, dependency)
    }

    override fun onStartNestedScroll(
        coordinatorLayout: CoordinatorLayout,
        child: V,
        directTargetChild: View,
        target: View,
        axes: Int,
        type: Int
    ): Boolean {
        if (axes != ViewCompat.SCROLL_AXIS_VERTICAL)
            return false

        lastStartedType = type

        offsetAnimator?.cancel()

        return true
    }

    override fun onNestedPreScroll(
        coordinatorLayout: CoordinatorLayout,
        child: V,
        target: View,
        dx: Int,
        dy: Int,
        consumed: IntArray,
        type: Int
    ) {
        super.onNestedPreScroll(coordinatorLayout, child, target, dx, dy, consumed, type)
        child.translationY = max(0f, min(child.height.toFloat(), child.translationY + dy))
    }

    override fun onStopNestedScroll(
        coordinatorLayout: CoordinatorLayout,
        child: V,
        target: View,
        type: Int
    ) {

        if (!isSnappingEnabled) {
            return
        }

        // Add snap behaviour
        // Logic here borrowed from AppBarLayout onStopNestedScroll
        if (lastStartedType == ViewCompat.TYPE_TOUCH || type == ViewCompat.TYPE_NON_TOUCH) {
            // find nearest seam
            val currTranslation = child.translationY
            val childHalfHeight = child.height * 0.5f

            // translate down
            if (currTranslation >= childHalfHeight) {
                animateBarVisibility(child, isVisible = false)
            }
            // translate up
            else {
                animateBarVisibility(child, isVisible = true)
            }
        }

    }

    private fun animateBarVisibility(child: View, isVisible: Boolean) {
        if (offsetAnimator == null) {
            offsetAnimator = ValueAnimator().apply {
                interpolator = DecelerateInterpolator()
                duration = 150L
            }

            offsetAnimator?.addUpdateListener {
                child.translationY = it.animatedValue as Float
            }
        } else {
            offsetAnimator?.cancel()
        }

        val targetTranslation = if (isVisible) 0f else child.height.toFloat()
        offsetAnimator?.setFloatValues(child.translationY, targetTranslation)
        offsetAnimator?.start()
    }

    private fun updateSnackbar(child: View, snackbarLayout: Snackbar.SnackbarLayout) {
        if (snackbarLayout.layoutParams is CoordinatorLayout.LayoutParams) {

            // Remove messy shadow
            snackbarLayout.outlineProvider = null

            val params = snackbarLayout.layoutParams as CoordinatorLayout.LayoutParams
            params.anchorId = child.id
            params.anchorGravity = Gravity.TOP or Gravity.CENTER_HORIZONTAL
            params.gravity = Gravity.TOP or Gravity.CENTER_HORIZONTAL

        }
    }

    fun slideUp(child: BottomNavigationView) {
        child.visibility = View.VISIBLE
        animateBarVisibility(child, isVisible = true)
    }

    fun slideDown(child: BottomNavigationView, isGone: Boolean) {
        animateBarVisibility(child, isVisible = false)
        if (isGone) {
            child.visibility = View.GONE
        }
    }

}