com.unity.uiextensions.nosample/Scripts/TabNavigation.cs

/// Credit Melang 
/// Sourced from - http://forum.unity3d.com/members/melang.593409/
/// Updated omatase 10-18-14 - support for Shift + Tab as well
///                         - bug fix to prevent crash if no control selected
///                         - updated to support new semantics for EventSystem in later 4.6 builds
///                        - autoselect "firstSelectedGameObject" since it doesn't seem to work automatically

using UnityEngine;
using UnityEngine.UI;
using UnityEngine.EventSystems;
using System.Collections;
 
public class TabNavigationHelper : MonoBehaviour
{
    private EventSystem _system;
   
    void Start()
    {
        _system = EventSystem.current;
    }
 
    public void Update()
    {
        Selectable next = null;
 
        if (Input.GetKeyDown(KeyCode.Tab) && Input.GetKey(KeyCode.LeftShift))
        {
            if (_system.currentSelectedGameObject != null)
            {
                next = _system.currentSelectedGameObject.GetComponent<Selectable>().FindSelectableOnUp();
            }
            else
            {
                next = _system.firstSelectedGameObject.GetComponent<Selectable>();
            }
        }
        else if (Input.GetKeyDown(KeyCode.Tab))
        {
            if (_system.currentSelectedGameObject != null)
            {
                next = _system.currentSelectedGameObject.GetComponent<Selectable>().FindSelectableOnDown();
            }
            else
            {
                next = _system.firstSelectedGameObject.GetComponent<Selectable>();
            }
        }
        else if (_system.currentSelectedGameObject == null)
        {
            next = _system.firstSelectedGameObject.GetComponent<Selectable>();
        }
 
        selectGameObject(next);
    }
 
    private void selectGameObject(Selectable selectable)
    {
        if (selectable != null)
        {
            InputField inputfield = selectable.GetComponent<InputField>();
            if (inputfield != null) inputfield.OnPointerClick(new PointerEventData(_system));  //if it's an input field, also set the text caret
 
            _system.SetSelectedGameObject(selectable.gameObject, new BaseEventData(_system));
        }
    }
}