/// sourced from: http://forum.unity3d.com/threads/scripts-useful-4-6-scripts-collection.264161/page-2#post-2011648

/*USAGE:
Simply place the script on the ScrollRect that contains the selectable children we'll be scroling to
and drag'n'drop the RectTransform of the options "container" that we'll be scrolling.*/

using UnityEngine;
using System.Collections;
using UnityEngine.UI;
using UnityEngine.EventSystems;

public class UIScrollToSelection : MonoBehaviour {

/* ### VARIABLES ============================================================== */

	// settings
	public float scrollSpeed = 10f;
	
	[SerializeField]
	private RectTransform layoutListGroup;

	// temporary variables
	private RectTransform targetScrollObject;
	private bool scrollToSelection = true;

	// references
	private RectTransform scrollWindow;
	private RectTransform currentCanvas;
	private ScrollRect targetScrollRect;
	

/* ### MAIN METHODS =========================================================== */
	// Use this for initialization
	private void Start () {
		targetScrollRect = GetComponent<ScrollRect>();
		scrollWindow = targetScrollRect.GetComponent<RectTransform>();
		currentCanvas = transform.root.GetComponent<RectTransform>();
	}
	
	// Update is called once per frame
	private void Update () {
		ScrollRectToLevelSelection();
	}

	private void ScrollRectToLevelSelection (){
		// check main references
		bool referencesAreIncorrect =
			(targetScrollRect == null || layoutListGroup == null || scrollWindow == null);
		if (referencesAreIncorrect == true){
			return;
		}

		// get calculation references
		EventSystem events = EventSystem.current;
		RectTransform selection = events.currentSelectedGameObject != null ?
			events.currentSelectedGameObject.GetComponent<RectTransform>() :
			null;
		RectTransform lastSelection = events.lastSelectedGameObject != null ?
			events.lastSelectedGameObject.GetComponent<RectTransform>() :
			selection;

		if (selection != targetScrollObject)
			scrollToSelection = true;

		// check if scrolling is possible
		bool isScrollDirectionUnknown =
			(selection == null || lastSelection == null || scrollToSelection == false);

		if (isScrollDirectionUnknown == true || selection.transform.parent != layoutListGroup.transform)
			return;

		// move the current scroll rect to correct position
		float selectionPos = -selection.anchoredPosition.y;
		int direction =	(int)Mathf.Sign(selection.anchoredPosition.y - lastSelection.anchoredPosition.y);

		float elementHeight = layoutListGroup.sizeDelta.y / layoutListGroup.transform.childCount;
		float maskHeight = currentCanvas.sizeDelta.y + scrollWindow.sizeDelta.y;
		float listPixelAnchor = layoutListGroup.anchoredPosition.y;

		// get the element offset value depending on the cursor move direction
		float offlimitsValue = 0;
		if (direction > 0 && selectionPos < listPixelAnchor)
		{
			offlimitsValue = listPixelAnchor - selectionPos;
		}
		if (direction < 0 && selectionPos + elementHeight > listPixelAnchor + maskHeight)
		{
			offlimitsValue = (listPixelAnchor + maskHeight) - (selectionPos + elementHeight);
		}
		// move the target scroll rect
		targetScrollRect.verticalNormalizedPosition +=
			(offlimitsValue / layoutListGroup.sizeDelta.y) * Time.deltaTime * scrollSpeed;
		// check if we reached our destination
		if (Mathf.Abs(offlimitsValue) < 2f)
			scrollToSelection = false;
		// save last object we were "heading to" to prevent blocking
		targetScrollObject = selection;
	}

/* ### ENUMS ================================================================== */
	

/* ### CUSTOM TYPES =========================================================== */
	
	
}