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

/// Credit zero3growlithe
/// 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.*/
 [RequireComponent(typeof(ScrollRect))]
public class UIScrollToSelectionXY : MonoBehaviour
{

    #region Variables

    // settings
    public float scrollSpeed = 10f;

    [SerializeField]
    private RectTransform layoutListGroup = null;

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

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

    // Use this for initialization
    private void Start()
    {
        targetScrollRect = GetComponent<ScrollRect>();
        scrollWindow = targetScrollRect.GetComponent<RectTransform>();
    }

    // Update is called once per frame
    private void Update()
    {
        ScrollRectToLevelSelection();
    }

    private void ScrollRectToLevelSelection()
    {
        // FIX: if you dont do that here events can have null value
        var events = EventSystem.current;

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

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

        if (selection != targetScrollObject)
        {
            scrollToSelection = true;
        }

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

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

        bool finishedX = false, finishedY = false;

        if (targetScrollRect.vertical)
        {
            // move the current scroll rect to correct position
            float selectionPos = -selection.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;

            offlimitsValue = listPixelAnchor - selectionPos;
            // move the target scroll rect
            targetScrollRect.verticalNormalizedPosition += (offlimitsValue / layoutListGroup.sizeDelta.y) * Time.deltaTime * scrollSpeed;

            finishedY = Mathf.Abs(offlimitsValue) < 2f;
        }

        if (targetScrollRect.horizontal)
        {
            // move the current scroll rect to correct position
            float selectionPos = -selection.anchoredPosition.x;

            //float elementWidth = layoutListGroup.sizeDelta.x / layoutListGroup.transform.childCount;
            //float maskWidth = currentCanvas.sizeDelta.y + scrollWindow.sizeDelta.y;
            float listPixelAnchor = layoutListGroup.anchoredPosition.x;

            // get the element offset value depending on the cursor move direction
            float offlimitsValue = 0;

            offlimitsValue = listPixelAnchor - selectionPos;
            // move the target scroll rect
            targetScrollRect.horizontalNormalizedPosition += (offlimitsValue / layoutListGroup.sizeDelta.x) * Time.deltaTime * scrollSpeed;

            finishedX = Mathf.Abs(offlimitsValue) < 2f;
        }
        // check if we reached our destination
        if (finishedX && finishedY)
        {
            scrollToSelection = false;
        }
        // save last object we were "heading to" to prevent blocking
        targetScrollObject = selection;
    }
}