/// 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 you will be scrolling 
*/

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

namespace UnityEngine.UI.Extensions
{
	[RequireComponent(typeof(ScrollRect))]
	[AddComponentMenu("UI/Extensions/UIScrollToSelection")]
	public class UIScrollToSelection : MonoBehaviour
	{
		#region MEMBERS

		[Header("[ References ]")]
		[SerializeField, Tooltip("View (boundaries/mask) rect transform. Used to check if automatic scroll to selection is required.")]
		private RectTransform viewportRectTransform;
		[SerializeField, Tooltip("Scroll rect used to reach selected element.")]
		private ScrollRect targetScrollRect;

		[Header("[ Scrolling ]")]
		[SerializeField, Tooltip("Allow automatic scrolling only on these axes.")]
		private Axis scrollAxes = Axis.ANY;
		[SerializeField, Tooltip("MOVE_TOWARDS: stiff movement, LERP: smoothed out movement")]
		private ScrollMethod usedScrollMethod = ScrollMethod.MOVE_TOWARDS;
		[SerializeField]
		private float scrollSpeed = 50;

		[Space(5)]
		[SerializeField, Tooltip("Scroll speed used when element to select is out of \"JumpOffsetThreshold\" range")]
		private float endOfListJumpScrollSpeed = 150;
		[SerializeField, Range(0, 1), Tooltip("If next element to scroll to is located over this screen percentage, use \"EndOfListJumpScrollSpeed\" to reach this element faster.")]
		private float jumpOffsetThreshold = 1;

		[Header("[ Input ]")]
		[SerializeField]
		private MouseButton cancelScrollMouseButtons = MouseButton.ANY;
		[SerializeField]
		private KeyCode[] cancelScrollKeys = new KeyCode[0];

		// INTERNAL - MEMBERS ONLY
		private Vector3[] viewRectCorners = new Vector3[4];
		private Vector3[] selectedElementCorners = new Vector3[4];

		#endregion

		#region PROPERTIES

		// REFERENCES
		public RectTransform ViewRectTransform
		{
			get { return viewportRectTransform; }
			set { viewportRectTransform = value; }
		}
		public ScrollRect TargetScrollRect
		{
			get { return targetScrollRect; }
			set { targetScrollRect = value; }
		}

		// SCROLLING
		public Axis ScrollAxes => scrollAxes;
		public ScrollMethod UsedScrollMethod => usedScrollMethod;
		public float ScrollSpeed => scrollSpeed;
		public float EndOfListJumpScrollSpeed => endOfListJumpScrollSpeed;
		public float JumpOffsetThreshold => jumpOffsetThreshold;

		// INPUT
		public MouseButton CancelScrollMouseButtons => cancelScrollMouseButtons;
		public KeyCode[] CancelScrollKeys => cancelScrollKeys;

		// VARIABLES
		private RectTransform scrollRectContentTransform;
		private GameObject lastCheckedSelection;

		// COROUTINES
		private Coroutine animationCoroutine;

		#endregion

		#region FUNCTIONS

		protected void Awake()
		{
			ValidateReferences();
		}

		protected void LateUpdate()
		{
			TryToScrollToSelection();
		}

		protected void Reset()
		{
			TargetScrollRect = gameObject.GetComponentInParent<ScrollRect>() ?? gameObject.GetComponentInChildren<ScrollRect>();
			ViewRectTransform = gameObject.GetComponent<RectTransform>();
		}

		private void ValidateReferences()
		{
            if (!targetScrollRect)
            {
				targetScrollRect = GetComponent<ScrollRect>();
            }

            if (!targetScrollRect)
            {
				Debug.LogError("[UIScrollToSelection] No ScrollRect found. Either attach this script to a ScrollRect or assign on in the 'Target Scroll Rect' property");
				gameObject.SetActive(false);
				return;
			}

			if (ViewRectTransform == null)
			{
				ViewRectTransform = TargetScrollRect.GetComponent<RectTransform>();
			}

			if (TargetScrollRect != null)
			{
				scrollRectContentTransform = TargetScrollRect.content;
			}

			if (EventSystem.current == null)
			{
				Debug.LogError("[UIScrollToSelection] Unity UI EventSystem not found. It is required to check current selected object.");
				gameObject.SetActive(false);
				return;
			}
		}

		private void TryToScrollToSelection()
		{
			// update references if selection changed
			GameObject selection = EventSystem.current.currentSelectedGameObject;

			if (selection == null || selection.activeInHierarchy == false || selection == lastCheckedSelection ||
				selection.transform.IsChildOf(transform) == false)
			{
				return;
			}

			RectTransform selectionRect = selection.GetComponent<RectTransform>();

			ViewRectTransform.GetWorldCorners(viewRectCorners);
			selectionRect.GetWorldCorners(selectedElementCorners);

			ScrollToSelection(selection);

			lastCheckedSelection = selection;
		}

		private void ScrollToSelection(GameObject selection)
		{
			// initial check if we can scroll at all
			if (selection == null)
			{
				return;
			}

			// this is just to make names shorter a bit
			Vector3[] corners = viewRectCorners;
			Vector3[] selectionCorners = selectedElementCorners;

			// calculate scroll offset
			Vector2 offsetToSelection = Vector2.zero;

			offsetToSelection.x =
				(selectionCorners[0].x < corners[0].x ? selectionCorners[0].x - corners[0].x : 0) +
				(selectionCorners[2].x > corners[2].x ? selectionCorners[2].x - corners[2].x : 0);
			offsetToSelection.y =
				(selectionCorners[0].y < corners[0].y ? selectionCorners[0].y - corners[0].y : 0) +
				(selectionCorners[1].y > corners[1].y ? selectionCorners[1].y - corners[1].y : 0);

			// calculate final scroll speed
			float finalScrollSpeed = ScrollSpeed;

			if (Math.Abs(offsetToSelection.x) / Screen.width >= JumpOffsetThreshold || Math.Abs(offsetToSelection.y) / Screen.height >= JumpOffsetThreshold)
			{
				finalScrollSpeed = EndOfListJumpScrollSpeed;
			}

			// initiate animation coroutine
			Vector2 targetPosition = (Vector2)scrollRectContentTransform.localPosition - offsetToSelection;

			if (animationCoroutine != null)
			{
				StopCoroutine(animationCoroutine);
			}

			animationCoroutine = StartCoroutine(ScrollToPosition(targetPosition, finalScrollSpeed));
		}

		private IEnumerator ScrollToPosition(Vector2 targetPosition, float speed)
		{
			Vector3 startPosition = scrollRectContentTransform.localPosition;

			// cancel movement on axes not specified in ScrollAxes mask
			targetPosition.x = ((ScrollAxes | Axis.HORIZONTAL) == ScrollAxes) ? targetPosition.x : startPosition.x;
			targetPosition.y = ((ScrollAxes | Axis.VERTICAL) == ScrollAxes) ? targetPosition.y : startPosition.y;

			// move to target position
			Vector2 currentPosition2D = startPosition;
			float horizontalSpeed = (Screen.width / Screen.dpi) * speed;
			float verticalSpeed = (Screen.height / Screen.dpi) * speed;

			while (currentPosition2D != targetPosition && CheckIfScrollInterrupted() == false)
			{
				currentPosition2D.x = MoveTowardsValue(currentPosition2D.x, targetPosition.x, horizontalSpeed, UsedScrollMethod);
				currentPosition2D.y = MoveTowardsValue(currentPosition2D.y, targetPosition.y, verticalSpeed, UsedScrollMethod);

				scrollRectContentTransform.localPosition = currentPosition2D;

				yield return null;
			}

			scrollRectContentTransform.localPosition = currentPosition2D;
		}

		private bool CheckIfScrollInterrupted()
		{
			bool mouseButtonClicked = false;

			// check mouse buttons
			switch (CancelScrollMouseButtons)
			{
				case MouseButton.LEFT:
					mouseButtonClicked |= Input.GetMouseButtonDown(0);
					break;
				case MouseButton.RIGHT:
					mouseButtonClicked |= Input.GetMouseButtonDown(1);
					break;
				case MouseButton.MIDDLE:
					mouseButtonClicked |= Input.GetMouseButtonDown(2);
					break;
			}

			if (mouseButtonClicked == true)
			{
				return true;
			}

			// check keyboard buttons
			for (int i = 0; i < CancelScrollKeys.Length; i++)
			{
				if (Input.GetKeyDown(CancelScrollKeys[i]) == true)
				{
					return true;
				}
			}

			return false;
		}

		private float MoveTowardsValue(float from, float to, float delta, ScrollMethod method)
		{
			switch (method)
			{
				case ScrollMethod.MOVE_TOWARDS:
					return Mathf.MoveTowards(from, to, delta * Time.unscaledDeltaTime);
				case ScrollMethod.LERP:
					return Mathf.Lerp(from, to, delta * Time.unscaledDeltaTime);
				default:
					return from;
			}
		}

		#endregion

		#region CLASS_ENUMS

		[Flags]
		public enum Axis
		{
			NONE = 0x00000000,
			HORIZONTAL = 0x00000001,
			VERTICAL = 0x00000010,
			ANY = 0x00000011
		}

		[Flags]
		public enum MouseButton
		{
			NONE = 0x00000000,
			LEFT = 0x00000001,
			RIGHT = 0x00000010,
			MIDDLE = 0x00000100,
			ANY = 0x00000111
		}

		public enum ScrollMethod
		{
			MOVE_TOWARDS,
			LERP
		}

		#endregion
	}
}