BottomNavigationBehaviourSnap.kt

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

    // rest of the code

    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 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()
    }
}