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

BottomNavigationBehavior.kt

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

    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 {
        return axes == ViewCompat.SCROLL_AXIS_VERTICAL
    }

    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))
    }

    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
        }
    }
}

// Apply to FAB anchored to BottomNavigationView
class BottomNavigationFABBehavior(context: Context?, attrs: AttributeSet?) : CoordinatorLayout.Behavior<View>(context, attrs) {

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

    override fun onDependentViewRemoved(parent: CoordinatorLayout, child: View, dependency: View) {
        child.translationY = 0f
    }

    override fun onDependentViewChanged(parent: CoordinatorLayout, child: View, dependency: View): Boolean {
        return updateButton(child, dependency)
    }

    private fun updateButton(child: View, dependency: View): Boolean {
        if (dependency is Snackbar.SnackbarLayout) {
            val oldTranslation = child.translationY
            val height = dependency.height.toFloat()
            val newTranslation = dependency.translationY - height
            child.translationY = newTranslation

            return oldTranslation != newTranslation
        }
        return false
    }
}