com.unity.uiextensions/Scripts/Layout/ScrollSnapBase.cs

using System;
using UnityEngine.Events;
using UnityEngine.EventSystems;

namespace UnityEngine.UI.Extensions
{
    public class ScrollSnapBase : MonoBehaviour, IBeginDragHandler, IDragHandler
	{
		internal RectTransform _screensContainer;
		internal bool _isVertical;

		internal int _screens = 1;

		internal float _scrollStartPosition;
		internal float _childSize;
		private float _childPos, _maskSize;
		internal Vector2 _childAnchorPoint;
		internal ScrollRect _scroll_rect;
		internal Vector3 _lerp_target;
		internal bool _lerp;
		internal bool _pointerDown = false;
		internal bool _settled = true;
		internal Vector3 _startPosition = new Vector3();
		[Tooltip("The currently active page")]
		internal int _currentPage;
		internal int _previousPage;
		internal int _halfNoVisibleItems;
		private int _bottomItem, _topItem;

		[Serializable]
		public class SelectionChangeStartEvent : UnityEvent { }
		[Serializable]
		public class SelectionPageChangedEvent : UnityEvent<int> { }
		[Serializable]
		public class SelectionChangeEndEvent : UnityEvent<int> { }

		[Tooltip("The screen / page to start the control on\n*Note, this is a 0 indexed array")]
		[SerializeField]
		public int StartingScreen = 0;

		[Tooltip("The distance between two pages based on page height, by default pages are next to each other")]
		[SerializeField]
		[Range(0, 8)]
		public float PageStep = 1;

		[Tooltip("The gameobject that contains toggles which suggest pagination. (optional)")]
		public GameObject Pagination;

		[Tooltip("Button to go to the previous page. (optional)")]
		public GameObject PrevButton;

		[Tooltip("Button to go to the next page. (optional)")]
		public GameObject NextButton;

		[Tooltip("Transition speed between pages. (optional)")]
		public float transitionSpeed = 7.5f;

		[Tooltip("Fast Swipe makes swiping page next / previous (optional)")]
		public Boolean UseFastSwipe = false;

		[Tooltip("How far swipe has to travel to initiate a page change (optional)")]
		public int FastSwipeThreshold = 100;

		[Tooltip("Speed at which the ScrollRect will keep scrolling before slowing down and stopping (optional)")]
		public int SwipeVelocityThreshold = 200;

		[Tooltip("The visible bounds area, controls which items are visible/enabled. *Note Should use a RectMask. (optional)")]
		public RectTransform MaskArea;

		[Tooltip("Pixel size to buffer arround Mask Area. (optional)")]
		public float MaskBuffer = 1;

		public int CurrentPage
		{
			get
			{
				return _currentPage;
			}

			internal set
			{
				if ((value != _currentPage && value >= 0 && value < _screensContainer.childCount) || (value == 0 && _screensContainer.childCount == 0))
				{
					_previousPage = _currentPage;
					_currentPage = value;
					if(MaskArea) UpdateVisible();
					if(!_lerp) ScreenChange();
					OnCurrentScreenChange(_currentPage);
				}
			}
		}

        [Tooltip("By default the container will lerp to the start when enabled in the scene, this option overrides this and forces it to simply jump without lerping")]
        public bool JumpOnEnable = false;

        [Tooltip("By default the container will return to the original starting page when enabled, this option overrides this behaviour and stays on the current selection")]
        public bool RestartOnEnable = false;

        [Tooltip("(Experimental)\nBy default, child array objects will use the parent transform\nHowever you can disable this for some interesting effects")]
		public bool UseParentTransform = true;

		[Tooltip("Scroll Snap children. (optional)\nEither place objects in the scene as children OR\nPrefabs in this array, NOT BOTH")]
		public GameObject[] ChildObjects;

		[SerializeField]
		[Tooltip("Event fires when a user starts to change the selection")]
		private SelectionChangeStartEvent m_OnSelectionChangeStartEvent = new SelectionChangeStartEvent();
		public SelectionChangeStartEvent OnSelectionChangeStartEvent { get { return m_OnSelectionChangeStartEvent; } set { m_OnSelectionChangeStartEvent = value; } }

		[SerializeField]
		[Tooltip("Event fires as the page changes, while dragging or jumping")]
		private SelectionPageChangedEvent m_OnSelectionPageChangedEvent = new SelectionPageChangedEvent();
		public SelectionPageChangedEvent OnSelectionPageChangedEvent { get { return m_OnSelectionPageChangedEvent; } set { m_OnSelectionPageChangedEvent = value; } }

		[SerializeField]
		[Tooltip("Event fires when the page settles after a user has dragged")]
		private SelectionChangeEndEvent m_OnSelectionChangeEndEvent = new SelectionChangeEndEvent();
		public SelectionChangeEndEvent OnSelectionChangeEndEvent { get { return m_OnSelectionChangeEndEvent; } set { m_OnSelectionChangeEndEvent = value; } }

		// Use this for initialization
		void Awake()
		{
			_scroll_rect = gameObject.GetComponent<ScrollRect>();

			if (_scroll_rect.horizontalScrollbar || _scroll_rect.verticalScrollbar)
			{
				Debug.LogWarning("Warning, using scrollbars with the Scroll Snap controls is not advised as it causes unpredictable results");
			}

			if (StartingScreen < 0)
			{
				StartingScreen = 0;
			}

			_screensContainer = _scroll_rect.content;

			InitialiseChildObjects();

			if (NextButton)
				NextButton.GetComponent<Button>().onClick.AddListener(() => { NextScreen(); });

			if (PrevButton)
				PrevButton.GetComponent<Button>().onClick.AddListener(() => { PreviousScreen(); });
			}

        internal void InitialiseChildObjects()
        {
            if (ChildObjects != null && ChildObjects.Length > 0)
            {
                if (_screensContainer.transform.childCount > 0)
                {
                    Debug.LogError("ScrollRect Content has children, this is not supported when using managed Child Objects\n Either remove the ScrollRect Content children or clear the ChildObjects array");
                    return;
                }

                InitialiseChildObjectsFromArray();
            }
            else
            {
                InitialiseChildObjectsFromScene();
            }
        }

        internal void InitialiseChildObjectsFromScene()
		{
			int childCount = _screensContainer.childCount;
			ChildObjects = new GameObject[childCount];
			for (int i = 0; i < childCount; i++)
			{
				ChildObjects[i] = _screensContainer.transform.GetChild(i).gameObject;
				if (MaskArea && ChildObjects[i].activeSelf)
				{
					ChildObjects[i].SetActive(false);
				}
			}
		}

		internal void InitialiseChildObjectsFromArray()
		{
			int childCount = ChildObjects.Length;
			RectTransform childRect;
			GameObject child;
			for (int i = 0; i < childCount; i++)
			{
				child = GameObject.Instantiate(ChildObjects[i]);
				//Optionally, use original GO transform when initialising, by default will use parent RectTransform position/rotation
				if (UseParentTransform)
				{
					childRect = child.GetComponent<RectTransform>();
					childRect.rotation = _screensContainer.rotation;
					childRect.localScale = _screensContainer.localScale;
					childRect.position = _screensContainer.position;
				}

				child.transform.SetParent(_screensContainer.transform);
				ChildObjects[i] = child;
				if (MaskArea && ChildObjects[i].activeSelf)
				{
					ChildObjects[i].SetActive(false);
				}
			}
		}

		internal void UpdateVisible()
		{
			//If there are no objects in the scene or a mask, exit
			if (!MaskArea || ChildObjects == null || ChildObjects.Length < 1 || _screensContainer.childCount < 1)
			{
				return;
			}

			_maskSize = _isVertical ? MaskArea.rect.height : MaskArea.rect.width;
			_halfNoVisibleItems = (int)Math.Round(_maskSize / (_childSize * MaskBuffer), MidpointRounding.AwayFromZero) / 2;
			_bottomItem = _topItem = 0;
			//work out how many items below the current page can be visible
			for (int i = _halfNoVisibleItems + 1; i > 0; i--)
			{
				_bottomItem = _currentPage - i < 0 ? 0 : i;
				if (_bottomItem > 0) break;
			}

			//work out how many items above the current page can be visible
			for (int i = _halfNoVisibleItems + 1; i > 0; i--)
			{
				_topItem = _screensContainer.childCount - _currentPage - i < 0 ? 0 : i;
				if (_topItem > 0) break;
			}

			//Set the active items active
			for (int i = CurrentPage - _bottomItem; i < CurrentPage + _topItem; i++)
			{
				try
				{
					ChildObjects[i].SetActive(true);
				}
				catch
				{
					Debug.Log("Failed to setactive child [" + i + "]");
				}
			}

			//Deactivate items out of visibility at the bottom of the ScrollRect Mask (only on scroll)
			if (_currentPage > _halfNoVisibleItems) ChildObjects[CurrentPage - _bottomItem].SetActive(false);
			//Deactivate items out of visibility at the top of the ScrollRect Mask (only on scroll)
			if (_screensContainer.childCount - _currentPage > _topItem) ChildObjects[CurrentPage + _topItem].SetActive(false);
		}

		//Function for switching screens with buttons
		public void NextScreen()
		{
			if (_currentPage < _screens - 1)
			{
				if (!_lerp) StartScreenChange();

				_lerp = true;
				CurrentPage = _currentPage + 1;
				GetPositionforPage(_currentPage, ref _lerp_target);
				ScreenChange();
			}
		}

		//Function for switching screens with buttons
		public void PreviousScreen()
		{
			if (_currentPage > 0)
			{
				if (!_lerp) StartScreenChange();

				_lerp = true;
				CurrentPage = _currentPage - 1;
				GetPositionforPage(_currentPage, ref _lerp_target);
				ScreenChange();
			}
		}

		/// <summary>
		/// Function for switching to a specific screen
		/// *Note, this is based on a 0 starting index - 0 to x
		/// </summary>
		/// <param name="screenIndex">0 starting index of page to jump to</param>
		public void GoToScreen(int screenIndex)
		{
			if (screenIndex <= _screens - 1 && screenIndex >= 0)
			{
				if (!_lerp) StartScreenChange();

				_lerp = true;
				CurrentPage = screenIndex;
				GetPositionforPage(_currentPage, ref _lerp_target);
				ScreenChange();
			}
		}

		/// <summary>
		/// Gets the closest page for the current Scroll Rect container position
		/// </summary>
		/// <param name="pos">Position to test, normally the Scroll Rect container Local position</param>
		/// <returns>Closest Page number (zero indexed array value)</returns>
		internal int GetPageforPosition(Vector3 pos)
		{
			return _isVertical ?
				-(int)Math.Round((pos.y - _scrollStartPosition) / _childSize) :
				-(int)Math.Round((pos.x - _scrollStartPosition) / _childSize);
		}

		/// <summary>
		/// Validates if the current Scroll Rect container position is within the bounds for a page
		/// </summary>
		/// <param name="pos">Position to test, normally the Scroll Rect container Local position</param>
		/// <returns>True / False, is the position in the bounds of a page</returns>
		internal bool IsRectSettledOnaPage(Vector3 pos)
		{
			return _isVertical ?
				-((pos.y - _scrollStartPosition) / _childSize) == -(int)Math.Round((pos.y - _scrollStartPosition) / _childSize) :
				-((pos.x - _scrollStartPosition) / _childSize) == -(int)Math.Round((pos.x - _scrollStartPosition) / _childSize);
		}

		/// <summary>
		/// Returns the local position for a child page based on the required page number
		/// </summary>
		/// <param name="page">Page that the position is required for (Zero indexed array value)</param>
		/// <param name="target">Outputs the local position for the selected page</param>
		internal void GetPositionforPage(int page, ref Vector3 target)
		{
			_childPos = -_childSize * page;
			if (_isVertical)
			{
				target.y = _childPos + _scrollStartPosition;
			}
			else
			{
				target.x = _childPos + _scrollStartPosition;
			}
		}

		/// <summary>
		/// Updates the _Lerp target to the closest page and updates the pagination bullets.  Each control's update loop will then handle the move.
		/// </summary>
		internal void ScrollToClosestElement()
		{
			_lerp = true;
			CurrentPage = GetPageforPosition(_screensContainer.localPosition);
			GetPositionforPage(_currentPage, ref _lerp_target);
			OnCurrentScreenChange(_currentPage);
		}

		/// <summary>
		/// notifies pagination indicator and navigation buttons of a screen change
		/// </summary>
		internal void OnCurrentScreenChange(int currentScreen)
		{
			ChangeBulletsInfo(currentScreen);
			ToggleNavigationButtons(currentScreen);
		}

		/// <summary>
		/// changes the bullets on the bottom of the page - pagination
		/// </summary>
		/// <param name="targetScreen"></param>
		private void ChangeBulletsInfo(int targetScreen)
		{
			if (Pagination)
				for (int i = 0; i < Pagination.transform.childCount; i++)
				{
					Pagination.transform.GetChild(i).GetComponent<Toggle>().isOn = (targetScreen == i)
			? true
						: false;
		}
	}

	/// <summary>
	/// disables the page navigation buttons when at the first or last screen
	/// </summary>
	/// <param name="targetScreen"></param>
		private void ToggleNavigationButtons(int targetScreen) {
		if (PrevButton) {
			PrevButton.GetComponent<Button>().interactable = targetScreen > 0;
		}

		if (NextButton) {
			NextButton.GetComponent<Button>().interactable = targetScreen < _screensContainer.transform.childCount - 1;
		}
	}

	private void OnValidate()
	{
		var children = gameObject.GetComponent<ScrollRect>().content.childCount;
		if (children != 0 || ChildObjects != null)
		{
			var childCount = ChildObjects == null || ChildObjects.Length == 0 ? children : ChildObjects.Length;
			if (StartingScreen > childCount - 1)
			{
				StartingScreen = childCount - 1;
			}

			if (StartingScreen < 0)
			{
				StartingScreen = 0;
			}
		}

		if (MaskBuffer <= 0)
		{
			MaskBuffer = 1;
		}

		if (PageStep < 0)
		{
			PageStep = 0;
		}

		if (PageStep > 8)
		{
			PageStep = 9;
		}
	}

	/// <summary>
	/// Event fires when the user starts to change the page, either via swipe or button.
	/// </summary>
		internal void StartScreenChange()
	{
		OnSelectionChangeStartEvent.Invoke();
	}

	/// <summary>
	/// Event fires when the currently viewed page changes, also updates while the scroll is moving
	/// </summary>
		internal void ScreenChange()
	{
		OnSelectionPageChangedEvent.Invoke(_currentPage);
	}

	/// <summary>
	/// Event fires when control settles on a page, outputs the new page number
	/// </summary>
		internal void EndScreenChange()
	{
		OnSelectionChangeEndEvent.Invoke(_currentPage);
		_settled = true;
	}

	#region Interfaces
	/// <summary>
	/// Touch screen to start swiping
	/// </summary>
	/// <param name="eventData"></param>
		public void OnBeginDrag(PointerEventData eventData)
	{
		_pointerDown = true;
		_settled = false;
		StartScreenChange();
		_startPosition = _screensContainer.localPosition;
	}

	/// <summary>
	/// While dragging do
	/// </summary>
	/// <param name="eventData"></param>
		public void OnDrag(PointerEventData eventData)
	{
		_lerp = false;
	}

	#endregion
}
}