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

/// Credit BinaryX 
/// Sourced from - http://forum.unity3d.com/threads/scripts-useful-4-6-scripts-collection.264161/page-2#post-1945602
/// Updated by simonDarksideJ - removed dependency on a custom ScrollRect script. Now implements drag interfaces and standard Scroll Rect.

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

namespace UnityEngine.UI.Extensions
{
    public class ScrollSnapBase : MonoBehaviour, IBeginDragHandler, IDragHandler, IEndDragHandler, IScrollSnap, IPointerClickHandler
    {
        internal Rect panelDimensions;
        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();
        internal int _currentPage;
        internal int _previousPage;
        internal int _halfNoVisibleItems;
        internal bool _isInfinite; // Is a UI Infinite scroller attached to the control
        internal int _infiniteWindow; // The infinite window the control is in
        internal float _infiniteOffset; // How much to offset a repositioning
        private int _bottomItem, _topItem;
        internal bool _startEventCalled = false;
        internal bool _endEventCalled = false;
        internal bool _suspendEvents = false;

        [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("Hard Swipe forces to swiping to the next / previous page (optional)")]
        public bool UseHardSwipe = false;

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

        [Tooltip("Swipe Delta Threshold looks at the speed of input to decide if a swipe will be initiated (optional)")]
        public bool UseSwipeDeltaThreshold = false;

        [Tooltip("Offset for how far a 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 = 100;

        [Tooltip("Threshold for swipe speed to initiate a swipe, below threshold will return to closest page (optional)")]
        public float SwipeDeltaThreshold = 5.0f;

        [Tooltip("Use time scale instead of unscaled time (optional)")]
        public bool UseTimeScale = true;

        [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 around Mask Area. (optional)")]
        public float MaskBuffer = 1;

        public int CurrentPage
        {
            get
            {
                return _currentPage;
            }

            internal set
            {
                if (_isInfinite)
                {
                    //Work out which infinite window we are in
                    float infWindow = (float)value / (float)_screensContainer.childCount;

                    if (infWindow < 0)
                    {
                        _infiniteWindow = (int)(Math.Floor(infWindow));
                    }
                    else
                    {
                        _infiniteWindow = value / _screensContainer.childCount;
                    }
                    //Invert the value if negative and differentiate from Window 0
                    _infiniteWindow = value < 0 ? (-_infiniteWindow) : _infiniteWindow;

                    //Calculate the page within the child count range
                    value = value % _screensContainer.childCount;
                    if (value < 0)
                    {
                        value = _screensContainer.childCount + value;
                    }
                    else if (value > _screensContainer.childCount - 1)
                    {
                        value = value - _screensContainer.childCount;
                    }
                }
                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; } }

        void Awake()
        {
            if (_scroll_rect == null)
            {
                _scroll_rect = gameObject.GetComponent<ScrollRect>();
            }
            if (_scroll_rect.horizontalScrollbar && _scroll_rect.horizontal)
            {
                var hscroll = _scroll_rect.horizontalScrollbar.gameObject.AddComponent<ScrollSnapScrollbarHelper>();
                hscroll.ss = this;
            }
            if (_scroll_rect.verticalScrollbar && _scroll_rect.vertical)
            {
                var vscroll = _scroll_rect.verticalScrollbar.gameObject.AddComponent<ScrollSnapScrollbarHelper>();
                vscroll.ss = this;
            }
            panelDimensions = gameObject.GetComponent<RectTransform>().rect;

            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(); });

            _isInfinite = GetComponent<UI_InfiniteScroll>() != null;
        }

        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();

                if (GetComponent<UI_InfiniteScroll>() != null)
                {
                    GetComponent<UI_InfiniteScroll>().Init();
                }
            }
            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 || _isInfinite)
            {
                if (!_lerp) StartScreenChange();

                _lerp = true;
                if (_isInfinite)
                {
                    CurrentPage = GetPageforPosition(_screensContainer.anchoredPosition) + 1;
                }
                else
                {
                    CurrentPage = _currentPage + 1;
                }
                GetPositionforPage(_currentPage, ref _lerp_target);
                ScreenChange();
            }

        }

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

                _lerp = true;
                if (_isInfinite)
                {
                    CurrentPage = GetPageforPosition(_screensContainer.anchoredPosition) - 1;
                }
                else
                {
                    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>
        /// <param name="pagination">Override the screen movement if driven from a pagination control</param>
        public void GoToScreen(int screenIndex, bool pagination = false)
        {
            if (screenIndex <= _screens - 1 && screenIndex >= 0)
            {
                if (!_lerp || pagination) 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((_scrollStartPosition - pos.y) / _childSize) :
                (int)Math.Round((_scrollStartPosition - pos.x) / _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)
            {
                _infiniteOffset = _screensContainer.anchoredPosition.y < 0 ? -_screensContainer.sizeDelta.y * _infiniteWindow : _screensContainer.sizeDelta.y * _infiniteWindow;
                _infiniteOffset = _infiniteOffset == 0 ? 0 : _infiniteOffset < 0 ? _infiniteOffset - _childSize * _infiniteWindow : _infiniteOffset + _childSize * _infiniteWindow;
                target.y = _childPos + _scrollStartPosition + _infiniteOffset;
            }
            else
            {
                _infiniteOffset = _screensContainer.anchoredPosition.x < 0 ? -_screensContainer.sizeDelta.x * _infiniteWindow : _screensContainer.sizeDelta.x * _infiniteWindow;
                _infiniteOffset = _infiniteOffset == 0 ? 0 : _infiniteOffset < 0 ? _infiniteOffset - _childSize * _infiniteWindow : _infiniteOffset + _childSize * _infiniteWindow;
                target.x = _childPos + _scrollStartPosition + _infiniteOffset;
            }
        }

        /// <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.anchoredPosition);
            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;
                }
            }
        }

        // Make a lock function for pagination, to prevent event leaking

        /// <summary>
        /// disables the page navigation buttons when at the first or last screen
        /// </summary>
        /// <param name="targetScreen"></param>
        private void ToggleNavigationButtons(int targetScreen)
        {
            //If this is using an Infinite Scroll, then don't disable
            if (!_isInfinite)
            {
                if (PrevButton)
                {
                    PrevButton.GetComponent<Button>().interactable = targetScreen > 0;
                }

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

        private void OnValidate()
        {
            if (_scroll_rect == null)
            {
                _scroll_rect = GetComponent<ScrollRect>();
            }
            if (!_scroll_rect.horizontal && !_scroll_rect.vertical)
            {
                Debug.LogError("ScrollRect has to have a direction, please select either Horizontal OR Vertical with the appropriate control.");
            }
            if (_scroll_rect.horizontal && _scroll_rect.vertical)
            {
                Debug.LogError("ScrollRect has to be unidirectional, only use either Horizontal or Vertical on the ScrollRect, NOT both.");
            }
            var ScrollRectContent = gameObject.GetComponent<ScrollRect>().content;
            if (ScrollRectContent != null)
            {
                var children = ScrollRectContent.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;
            }
            var infiniteScroll = GetComponent<UI_InfiniteScroll>();
            if (ChildObjects != null && ChildObjects.Length > 0 && infiniteScroll != null && !infiniteScroll.InitByUser)
            {
                Debug.LogError($"[{gameObject.name}]When using procedural children with a ScrollSnap (Adding Prefab ChildObjects) and the Infinite Scroll component\nYou must set the 'InitByUser' option to true, to enable late initialising");
            }
        }

        /// <summary>
        /// Event fires when the user starts to change the page, either via swipe or button.
        /// </summary>
        public void StartScreenChange()
        {
            if (!_startEventCalled)
            {
                _suspendEvents = true;

                _startEventCalled = true;
                _endEventCalled = false;
                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()
        {
            if (!_endEventCalled)
            {
                _suspendEvents = false;

                _endEventCalled = true;
                _startEventCalled = false;
                _settled = true;
                OnSelectionChangeEndEvent.Invoke(_currentPage);
            }
        }

        /// <summary>
        /// Returns the Transform of the Current page
        /// </summary>
        /// <returns>Currently selected Page Transform</returns>
        public Transform CurrentPageObject()
        {
            return _screensContainer.GetChild(CurrentPage);
        }

        /// <summary>
        /// Returns the Transform of the Current page in an out parameter for performance
        /// </summary>
        /// <param name="returnObject">Currently selected Page Transform</param>
        public void CurrentPageObject(out Transform returnObject)
        {
            returnObject = _screensContainer.GetChild(CurrentPage);
        }

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

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

        public virtual void OnEndDrag(PointerEventData eventData) { }

        #endregion

        #region IScrollSnap Interface

        /// <summary>
        /// Added to provide a uniform interface for the ScrollBarHelper
        /// </summary>
        int IScrollSnap.CurrentPage()
        {
            return CurrentPage = GetPageforPosition(_screensContainer.anchoredPosition);
        }

        /// <summary>
        /// Added to provide a uniform interface for the ScrollBarHelper
        /// </summary>
        public void SetLerp(bool value)
        {
            _lerp = value;
        }

        /// <summary>
        /// Added to provide a uniform interface for the ScrollBarHelper
        /// </summary>
        public void ChangePage(int page)
        {
            GoToScreen(page);
        }

        public void OnPointerClick(PointerEventData eventData)
        {
            var position = _screensContainer.anchoredPosition;
        }

        #endregion
    }
}