Make an snapped ScrollView

Hi,

So, I m making a menu for an android device.

I would like to make a scrollview using the same behavior that most of the Mobile's menu have. The snapping to the closest element when I scroll.

basically this : https://youtu.be/cd06PzjtIjM?t=6m27s

So I did the following stuff :

     public void OnBeginDrag()
     {
         _scrollStart = _tContainer.localPosition;
     }
 
     public void OnScroll()
     {
         Vector3 totalScrolled = _scrollStart - _tContainer.localPosition;
         if(totalScrolled.y > 80)
         {
             if ((_currIndex + 1) < levels.Count)
             {
                 _currIndex++;
                scrollRect.verticalNormalizedPosition = _currIndex / levels.Count;
             }
         }
         if (totalScrolled.y < -80)
         {
             if (_currIndex-1 > 0)
             {
                 _currIndex--;
                 scrollRect.verticalNormalizedPosition = _currIndex / levels.Count;
             }
         }
     }

Basically, if I drag over 80, I want to set the snap to the next element of my levelsButtonArray. It works fine. But the problem is that I can't find something that work in order to change the snapping position.

http://prntscr.com/banf51

My question is, how do i change the reference 0,0 of the scrollview in order to snap the selected element position?

I searched on the internet that :

 scrollRect.verticalNormalizedPosition = Mathf.Lerp(0, 1, _currIndex / levels.Count

but can't make it work...

Thanks for the help.