Paging UICollectionView by cells not screen

Question

I have UICollectionView with horizontal scrolling and there are always 2 cells side-by-side per the entire screen  I need the scrolling to stop at the begining of a cell  With paging enabled  the collection view scrolls the whole page  which is 2 cells at once  and then it stops   I need to enable scrolling by a single cell  or scrolling by multiple cells with stopping at the edge of the cell   I tried to subclass UICollectionViewFlowLayout and to implement the method targetContentOffsetForProposedContentOffset  but so far I was only able to break my collection view and it stopped scrolling  Is there any easier way to achieve this and how  or do I really need to implement all methods of UICollectionViewFlowLayout subclass  Thanks

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Here is my way to do it by using a UICollectionViewFlowLayout to override the targetContentOffset    Although in the end  I end up not using this and use UIPageViewController instead         A UICollectionViewFlowLayout with     - paged horizontal scrolling  - itemSize is the same as the collectionView bounds size     class PagedFlowLayout  UICollectionViewFlowLayout      override init         super init       self scrollDirection    horizontal     self minimumLineSpacing   8    line spacing is the horizontal spacing in horizontal scrollDirection     self minimumInteritemSpacing   0     if  available iOS 11 0             self sectionInsetReference    fromSafeArea    for iPhone X              required init  coder aDecoder  NSCoder        fatalError  not implemented             Note  Setting  minimumInteritemSpacing  here will be too late  Don t do it here    override func prepare         super prepare       guard let collectionView   collectionView else   return       collectionView decelerationRate   UIScrollViewDecelerationRateFast    mostly you want it fast       let insetedBounds   UIEdgeInsetsInsetRect collectionView bounds  self sectionInset      self itemSize   insetedBounds size           Table  Possible cases of targetContentOffset calculation      -------------------------      start                   near    velocity   end      page               page      -------------------------        0     forward     1        0     still       0        0     backward    0        1     forward     1        1     still       1        1     backward    0      -------------------------   override func targetContentOffset    swiftlint disable this cyclomatic complexity     forProposedContentOffset proposedContentOffset  CGPoint  withScrollingVelocity velocity  CGPoint  - gt  CGPoint        guard let collectionView   collectionView else   return proposedContentOffset        let pageWidth   itemSize width   minimumLineSpacing     let currentPage  CGFloat   collectionView contentOffset x   pageWidth     let nearestPage  CGFloat   round currentPage      let isNearPreviousPage   nearestPage  lt  currentPage      var pageDiff  CGFloat   0     let velocityThreshold  CGFloat   0 5    can customize this threshold     if isNearPreviousPage         if velocity x  gt  velocityThreshold           pageDiff   1               else         if velocity x  lt  -velocityThreshold           pageDiff   -1                    let x    nearestPage   pageDiff    pageWidth     let cappedX   max 0  x     cap to avoid targeting beyond content       print  x    x   velocity    velocity      return CGPoint x  cappedX  y  proposedContentOffset y

User · Answer

Here s the easiest way that i found to do that in Swift 4 2 for horinzontal scroll   I m using the first cell on visibleCells and scrolling to then  if the first visible cell are showing less of the half of it s width i m scrolling to the next one   If your collection scroll vertically  simply change x by y and width by height  func scrollViewWillEndDragging   scrollView  UIScrollView  withVelocity velocity  CGPoint  targetContentOffset  UnsafeMutablePointer lt CGPoint gt         targetContentOffset pointee   scrollView contentOffset     var indexes   self collectionView indexPathsForVisibleItems     indexes sort       var index   indexes first      let cell   self collectionView cellForItem at  index       let position   self collectionView contentOffset x - cell frame origin x     if position  gt  cell frame size width 2         index row   index row 1           self collectionView scrollToItem at  index  at   left  animated  true

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The original answer of                had an issue  so on the last cell collection view was scrolling to the beginning func scrollViewWillEndDragging   scrollView  UIScrollView  withVelocity velocity  CGPoint  targetContentOffset  UnsafeMutablePointer lt CGPoint gt         targetContentOffset pointee   scrollView contentOffset     var indexes   yourCollectionView indexPathsForVisibleItems     indexes sort       var index   indexes first         if velocity x  gt  0  amp  amp   Get the number of items from your data   gt  index row   1       if velocity x  gt  0  amp  amp  yourCollectionView numberOfItems inSection  0   gt  index row   1          index row    1       else if velocity x    0           let cell   yourCollectionView cellForItem at  index           let position   yourCollectionView contentOffset x - cell frame origin x         if position  gt  cell frame size width   2              index row    1                          yourCollectionView scrollToItem at  index  at   centeredHorizontally  animated  true

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Partly based on StevenOjo s answer  I ve tested this using a horizontal scrolling and no Bounce UICollectionView  cellSize is CollectionViewCell size  You can tweak factor to modify scrolling sensitivity   override func scrollViewWillEndDragging   scrollView  UIScrollView  withVelocity velocity  CGPoint  targetContentOffset  UnsafeMutablePointer lt CGPoint gt         targetContentOffset pointee   scrollView contentOffset     var factor  CGFloat   0 5     if velocity x  lt  0           factor   -factor           let indexPath   IndexPath row   scrollView contentOffset x cellSize width   factor  int  section  0      collectionView  scrollToItem at  indexPath  at   left  animated  true

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Approach 1  Collection View  flowLayout is UICollectionViewFlowLayout property  override func scrollViewWillEndDragging scrollView  UIScrollView  withVelocity velocity  CGPoint  targetContentOffset  UnsafeMutablePointer lt CGPoint gt          if let collectionView   collectionView            targetContentOffset memory   scrollView contentOffset         let pageWidth   CGRectGetWidth scrollView frame    flowLayout minimumInteritemSpacing          var assistanceOffset   CGFloat   pageWidth   3 0          if velocity x  lt  0               assistanceOffset   -assistanceOffset                    let assistedScrollPosition    scrollView contentOffset x   assistanceOffset    pageWidth          var targetIndex   Int round assistedScrollPosition             if targetIndex  lt  0               targetIndex   0                   else if targetIndex  gt   collectionView numberOfItemsInSection 0                targetIndex   collectionView numberOfItemsInSection 0  - 1                    print  targetIndex     targetIndex             let indexPath   NSIndexPath forItem  targetIndex  inSection  0           collectionView scrollToItemAtIndexPath indexPath  atScrollPosition   Left  animated  true            Approach 2  Page View Controller  You could use UIPageViewController if it meets your requirements  each page would have a separate view controller

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This is a straight way to do this   The case is simple  but finally quite common   typical thumbnails scroller with fixed cell size and fixed gap between cells    var itemCellSize  CGSize    lt your cell size gt  var itemCellsGap  CGFloat    lt gap in between gt   override func scrollViewWillEndDragging   scrollView  UIScrollView  withVelocity velocity  CGPoint  targetContentOffset  UnsafeMutablePointer lt CGPoint gt         let pageWidth    itemCellSize width   itemCellsGap      let itemIndex    targetContentOffset pointee x    pageWidth     targetContentOffset pointee x   round itemIndex    pageWidth -  itemCellsGap   2        CollectionViewFlowLayoutDelegate  func collectionView   collectionView  UICollectionView  layout collectionViewLayout  UICollectionViewLayout  sizeForItemAt indexPath  IndexPath  - gt  CGSize       return itemCellSize    func collectionView   collectionView  UICollectionView  layout collectionViewLayout  UICollectionViewLayout  minimumLineSpacingForSectionAt section  Int  - gt  CGFloat       return itemCellsGap     Note that there is no reason to call a scrollToOffset or dive into layouts  The native scrolling behaviour already does everything   Cheers All

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OK  so I found the solution here  targetContentOffsetForProposedContentOffset withScrollingVelocity without subclassing UICollectionViewFlowLayout  I should have searched for targetContentOffsetForProposedContentOffset in the begining

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Also you can create fake scroll view to handle scrolling   Horizontal or Vertical         Defaults     let bannerSize   CGSize width  280  height  170  let pageWidth  CGFloat   290        paging let insetLeft  CGFloat   20 let insetRight  CGFloat   20                      var pageScrollView  UIScrollView   override func viewDidLoad         super viewDidLoad           Create fake scrollview to properly handle paging     pageScrollView   UIScrollView frame  CGRect origin   zero  size  CGSize width  pageWidth  height  100        pageScrollView isPagingEnabled   true     pageScrollView alwaysBounceHorizontal   true     pageScrollView showsVerticalScrollIndicator   false     pageScrollView showsHorizontalScrollIndicator   false     pageScrollView delegate   self     pageScrollView isHidden   true     view insertSubview pageScrollView  belowSubview  collectionView          Set desired gesture recognizers to the collection view     for gr in pageScrollView gestureRecognizers            collectionView addGestureRecognizer gr           func scrollViewDidScroll   scrollView  UIScrollView        if scrollView    pageScrollView              Return scrolling back to the collection view         collectionView contentOffset x   pageScrollView contentOffset x          func refreshData                  refreshScroll      override func viewDidLayoutSubviews         super viewDidLayoutSubviews        refreshScroll          Refresh fake scrolling view content size if content changes func refreshScroll         let w   collectionView width - bannerSize width - insetLeft - insetRight     pageScrollView contentSize   CGSize width  pageWidth   CGFloat banners count  - w  height  100

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Swift 3 version of Evya s answer    func scrollViewWillEndDragging   scrollView  UIScrollView  withVelocity velocity  CGPoint  targetContentOffset  UnsafeMutablePointer lt CGPoint gt       targetContentOffset pointee   scrollView contentOffset     let pageWidth Float   Float self view bounds width      let minSpace Float   10 0     var cellToSwipe Double   Double Float  scrollView contentOffset x   Float  pageWidth minSpace      Double 0 5      if cellToSwipe  lt  0           cellToSwipe   0       else if cellToSwipe  gt   Double self articles count            cellToSwipe   Double self articles count  - Double 1            let indexPath IndexPath   IndexPath row  Int cellToSwipe   section 0      self collectionView scrollToItem at indexPath  at  UICollectionViewScrollPosition left  animated  true

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Horizontal Paging With Custom Page Width  Swift 4  amp  5   Many solutions presented here result in some weird behaviour that doesn t feel like properly implemented paging     The solution presented in this tutorial  however  doesn t seem to have any issues  It just feels like a perfectly  working paging algorithm  You can implement it in 5 simple steps    Add the following property to your type  private var indexOfCellBeforeDragging   0 Set the collectionView delegate like this  collectionView delegate   self Add conformance to UICollectionViewDelegate via an extension  extension YourType  UICollectionViewDelegate     Add the following method to the extension implementing the UICollectionViewDelegate conformance and set a value for pageWidth   func scrollViewWillBeginDragging   scrollView  UIScrollView        let pageWidth      The width your page should have  plus a possible margin      let proportionalOffset   collectionView contentOffset x   pageWidth     indexOfCellBeforeDragging   Int round proportionalOffset      Add the following method to the extension implementing the UICollectionViewDelegate conformance  set the same value for pageWidth  you may also store this value at a central place  and set a value for collectionViewItemCount   func scrollViewWillEndDragging   scrollView  UIScrollView  withVelocity velocity  CGPoint  targetContentOffset  UnsafeMutablePointer lt CGPoint gt            Stop scrolling     targetContentOffset pointee   scrollView contentOffset         Calculate conditions     let pageWidth      The width your page should have  plus a possible margin      let collectionViewItemCount      The number of items in this section     let proportionalOffset   collectionView contentOffset x   pageWidth     let indexOfMajorCell   Int round proportionalOffset       let swipeVelocityThreshold  CGFloat   0 5     let hasEnoughVelocityToSlideToTheNextCell   indexOfCellBeforeDragging   1  lt  collectionViewItemCount  amp  amp  velocity x  gt  swipeVelocityThreshold     let hasEnoughVelocityToSlideToThePreviousCell   indexOfCellBeforeDragging - 1  gt   0  amp  amp  velocity x  lt  -swipeVelocityThreshold     let majorCellIsTheCellBeforeDragging   indexOfMajorCell    indexOfCellBeforeDragging     let didUseSwipeToSkipCell   majorCellIsTheCellBeforeDragging  amp  amp   hasEnoughVelocityToSlideToTheNextCell    hasEnoughVelocityToSlideToThePreviousCell       if didUseSwipeToSkipCell              Animate so that swipe is just continued         let snapToIndex   indexOfCellBeforeDragging    hasEnoughVelocityToSlideToTheNextCell   1   -1          let toValue   pageWidth   CGFloat snapToIndex          UIView animate              withDuration  0 3              delay  0              usingSpringWithDamping  1              initialSpringVelocity  velocity x              options   allowUserInteraction              animations                    scrollView contentOffset   CGPoint x  toValue  y  0                  scrollView layoutIfNeeded                              completion  nil                 else              Pop back  against velocity          let indexPath   IndexPath row  indexOfMajorCell  section  0          collectionView scrollToItem at  indexPath  at   left  animated  true

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Kind of like evya s answer  but a little smoother because it doesn t set the targetContentOffset to zero   -  void scrollViewWillEndDragging  UIScrollView   scrollView withVelocity  CGPoint velocity targetContentOffset  inout CGPoint   targetContentOffset       if   scrollView isKindOfClass  UICollectionView class              UICollectionView  collectionView    UICollectionView  scrollView          if   collectionView collectionViewLayout isKindOfClass  UICollectionViewFlowLayout class                  UICollectionViewFlowLayout  layout    UICollectionViewFlowLayout  collectionView collectionViewLayout               CGFloat pageWidth   layout itemSize width   layout minimumInteritemSpacing              CGFloat usualSideOverhang    scrollView bounds size width - pageWidth  2 0                 k pageWidth - usualSideOverhang   contentOffset for page at index k if k  gt   1  0 if k   0                - gt   contentOffset   usualSideOverhang  pageWidth   k at page stops              NSInteger targetPage   0              CGFloat currentOffsetInPages    scrollView contentOffset x   usualSideOverhang  pageWidth              targetPage   velocity x  lt  0   floor currentOffsetInPages    ceil currentOffsetInPages               targetPage   MAX 0 MIN self projects count - 1 targetPage                  targetContentOffset   CGPointMake MAX targetPage pageWidth - usualSideOverhang 0   0

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Ok so the proposed answers did nt worked for me because I wanted to scroll by sections instead  and thus  have variable width page sizes  I did this  vertical only       var pagesSizes    CGSize       func scrollViewDidScroll   scrollView  UIScrollView            defer               lastOffsetY   scrollView contentOffset y                   if collectionView isDecelerating               var currentPage   0             var currentPageBottom   CGFloat 0              for pagesSize in pagesSizes                   currentPageBottom    pagesSize height                 if currentPageBottom  gt  collectionView  contentOffset y                       break                                   currentPage    1                           if collectionView contentOffset y  gt  currentPageBottom - pagesSizes currentPage  height  collectionView contentOffset y   collectionView frame height  lt  currentPageBottom                   return    100  of view within bounds                           if lastOffsetY  lt  collectionView contentOffset y                   if currentPage   1    pagesSizes count                       collectionView setContentOffset CGPoint x  0  y  currentPageBottom   animated  true                                  else                   collectionView setContentOffset CGPoint x  0  y  currentPageBottom - pagesSizes currentPage  height   animated  true                                  In this case  I calculate each page size beforehand using the section height   header   footer  and store it in the array  That s the pagesSizes member

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Swift 5  I ve found a way to do this without subclassing UICollectionView  just calculating the contentOffset in horizontal  Obviously without isPagingEnabled set true  Here is the code   var offsetScroll1   CGFloat   0 var offsetScroll2   CGFloat   0 let flowLayout   UICollectionViewFlowLayout   let screenSize   CGSize   UIScreen main bounds size var items     1    2    3    4    5    override func viewDidLoad         super viewDidLoad       flowLayout scrollDirection    horizontal     flowLayout minimumLineSpacing   7     let collectionView   UICollectionView frame  CGRect x  0  y  590  width  screenSize width  height  200   collectionViewLayout  flowLayout      collectionView register collectionViewCell1 self  forCellWithReuseIdentifier  cellReuseIdentifier      collectionView delegate   self     collectionView dataSource   self     collectionView backgroundColor   UIColor clear     collectionView showsHorizontalScrollIndicator   false     self view addSubview collectionView     func scrollViewWillBeginDragging   scrollView  UIScrollView        offsetScroll1   offsetScroll2    func scrollViewDidEndDragging   scrollView  UIScrollView  willDecelerate decelerate  Bool        offsetScroll1   offsetScroll2    func scrollViewWillEndDragging   scrollView  UIScrollView  withVelocity velocity  CGPoint  targetContentOffset  UnsafeMutablePointer lt CGPoint gt        let indexOfMajorCell   self desiredIndex       let indexPath   IndexPath row  indexOfMajorCell  section  0      flowLayout collectionView  scrollToItem at  indexPath  at   centeredHorizontally  animated  true      targetContentOffset pointee   scrollView contentOffset    private func desiredIndex   - gt  Int       var integerIndex   0     print flowLayout collectionView  contentOffset x      offsetScroll2   flowLayout collectionView  contentOffset x     if offsetScroll2  gt  offsetScroll1           integerIndex    1         let offset   flowLayout collectionView  contentOffset x   screenSize width         integerIndex   Int round offset           if integerIndex  lt   items count - 1                integerIndex    1                     if offsetScroll2  lt  offsetScroll1           let offset   flowLayout collectionView  contentOffset x   screenSize width         integerIndex   Int offset rounded  towardZero             let targetIndex   integerIndex     return targetIndex

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Here s my implementation in Swift 5 for vertical cell-based paging   override func targetContentOffset forProposedContentOffset proposedContentOffset  CGPoint  withScrollingVelocity velocity  CGPoint  - gt  CGPoint        guard let collectionView   self collectionView else           let latestOffset   super targetContentOffset forProposedContentOffset  proposedContentOffset  withScrollingVelocity  velocity          return latestOffset               Page height used for estimating and calculating paging      let pageHeight   self itemSize height   self minimumLineSpacing         Make an estimation of the current page position      let approximatePage   collectionView contentOffset y pageHeight         Determine the current page based on velocity      let currentPage   velocity y    0   round approximatePage     velocity y  lt  0 0   floor approximatePage    ceil approximatePage           Create custom flickVelocity      let flickVelocity   velocity y   0 3         Check how many pages the user flicked  if  lt   1 then flickedPages should return 0      let flickedPages    abs round flickVelocity    lt   1    0   round flickVelocity       let newVerticalOffset     currentPage   flickedPages    pageHeight  - collectionView contentInset top      return CGPoint x  proposedContentOffset x  y  newVerticalOffset      Some notes    Doesn t glitch SET PAGING TO FALSE   otherwise this won t work  Allows you to set your own flickvelocity easily  If something is still not working after trying this  check if your itemSize actually matches the size of the item as that s often a problem  especially when using collectionView   layout sizeForItemAt    use a custom variable with the itemSize instead  This works best when you set self collectionView decelerationRate   UIScrollView DecelerationRate fast    Here s a horizontal version  haven t tested it thoroughly so please forgive any mistakes    override func targetContentOffset forProposedContentOffset proposedContentOffset  CGPoint  withScrollingVelocity velocity  CGPoint  - gt  CGPoint        guard let collectionView   self collectionView else           let latestOffset   super targetContentOffset forProposedContentOffset  proposedContentOffset  withScrollingVelocity  velocity          return latestOffset               Page width used for estimating and calculating paging      let pageWidth   self itemSize width   self minimumInteritemSpacing         Make an estimation of the current page position      let approximatePage   collectionView contentOffset x pageWidth         Determine the current page based on velocity      let currentPage   velocity x    0   round approximatePage     velocity x  lt  0 0   floor approximatePage    ceil approximatePage           Create custom flickVelocity      let flickVelocity   velocity x   0 3         Check how many pages the user flicked  if  lt   1 then flickedPages should return 0      let flickedPages    abs round flickVelocity    lt   1    0   round flickVelocity          Calculate newHorizontalOffset      let newHorizontalOffset     currentPage   flickedPages    pageWidth  - collectionView contentInset left      return CGPoint x  newHorizontalOffset  y  proposedContentOffset y      This code is based on the code I use in my personal project  you can check it out here by downloading it and running the Example target

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i created a custom collection view layout here that supports    paging one cell at a time paging 2  cells at a time depending on swipe velocity horizontal or vertical directions   it s as easy as   let layout   PagingCollectionViewLayout    layout itemSize    layout minimumLineSpacing    layout scrollDirection      you can just add PagingCollectionViewLayout swift to your project  or  add pod  PagingCollectionViewLayout  to your podfile

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Here is my version of it in Swift 3  Calculate the offset after scrolling ended and adjust the offset with animation   collectionLayout is a  UICollectionViewFlowLayout    func scrollViewDidEndDecelerating   scrollView  UIScrollView        let index   scrollView contentOffset x   collectionLayout itemSize width     let fracPart   index truncatingRemainder dividingBy  1      let item  Int fracPart  gt   0 5   ceil index    floor index        let indexPath   IndexPath item  item  section  0      collectionView scrollToItem at  indexPath  at   left  animated  true

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You can use the following library  https   github com ink-spot UPCarouselFlowLayout  It s very simple and ofc you do not need to think about details like other answers contain

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modify Romulo BM answer for velocity listening  func scrollViewWillEndDragging        scrollView  UIScrollView      withVelocity velocity  CGPoint      targetContentOffset  UnsafeMutablePointer lt CGPoint gt          targetContentOffset pointee   scrollView contentOffset     var indexes   collection indexPathsForVisibleItems     indexes sort       var index   indexes first      if velocity x  gt  0          index row    1       else if velocity x    0           let cell   self collection cellForItem at  index           let position   self collection contentOffset x - cell frame origin x         if position  gt  cell frame size width   2              index row    1                      self collection scrollToItem at  index  at   centeredHorizontally  animated  true

User · Answer

just override the method   -  void scrollViewWillEndDragging  UIScrollView   scrollView withVelocity  CGPoint velocity targetContentOffset  inout CGPoint   targetContentOffset        targetContentOffset   scrollView contentOffset     set acceleration to 0 0     float pageWidth    float self articlesCollectionView bounds size width      int minSpace   10       int cellToSwipe    scrollView contentOffset x   pageWidth   minSpace    0 5     cell width   min spacing for lines     if  cellToSwipe  lt  0            cellToSwipe   0        else if  cellToSwipe  gt   self articles count            cellToSwipe   self articles count - 1             self articlesCollectionView scrollToItemAtIndexPath  NSIndexPath indexPathForRow cellToSwipe inSection 0  atScrollPosition UICollectionViewScrollPositionLeft animated YES

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Here is the optimised solution in Swift5  including handling the wrong indexPath  - Michael Lin Liu  Step1  Get the indexPath of the current cell  Step2  Detect the velocity when scroll  Step3  Increase the indexPath s row when the velocity is increased  Step4  Tell the collection view to scroll to the next item      func scrollViewWillEndDragging   scrollView  UIScrollView  withVelocity velocity  CGPoint  targetContentOffset  UnsafeMutablePointer lt CGPoint gt                      targetContentOffset pointee   scrollView contentOffset                    M  Get the first visiable item s indexPath from visibaleItems          var indexPaths    YOURCOLLECTIONVIEW  indexPathsForVisibleItems         indexPaths sort           var indexPath   indexPaths first             M  Use the velocity to detect the paging control movement            M  If the movement is forward  then increase the indexPath          if velocity x  gt  0              indexPath row    1                            M  If the movement is in the next section  which means the indexPath s row is out range  We set the indexPath to the first row of the next section              if indexPath row     YOURCOLLECTIONVIEW  numberOfItems inSection  indexPath section                   indexPath row   0                 indexPath section    1                                 else                M  If the movement is backward  the indexPath will be automatically changed to the first visiable item which is indexPath row - 1  So there is no need to write the logic                               M  Tell the collection view to scroll to the next item           YOURCOLLECTIONVIEW  scrollToItem at  indexPath  at   left  animated  true

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This is my solution  in Swift 4 2  I wish it could help you   class SomeViewController  UIViewController      private lazy var flowLayout  UICollectionViewFlowLayout         let layout   UICollectionViewFlowLayout       layout itemSize   CGSize width     width     height     height         layout minimumLineSpacing      margin     layout minimumInteritemSpacing   0 0     layout sectionInset   UIEdgeInsets top  0 0  left     margin     bottom  0 0  right     margin         layout scrollDirection    horizontal     return layout          private lazy var collectionView  UICollectionView         let collectionView   UICollectionView frame   zero  collectionViewLayout  flowLayout      collectionView showsHorizontalScrollIndicator   false     collectionView dataSource   self     collectionView delegate   self        collectionView register SomeCell self      return collectionView          private var currentIndex  Int   0       MARK  - UIScrollViewDelegate  extension SomeViewController     func scrollViewWillBeginDragging   scrollView  UIScrollView        guard scrollView    collectionView else   return        let pageWidth   flowLayout itemSize width   flowLayout minimumLineSpacing     currentIndex   Int scrollView contentOffset x   pageWidth         func scrollViewWillEndDragging   scrollView  UIScrollView  withVelocity velocity  CGPoint  targetContentOffset  UnsafeMutablePointer lt CGPoint gt         guard scrollView    collectionView else   return        let pageWidth   flowLayout itemSize width   flowLayout minimumLineSpacing     var targetIndex   Int roundf Float targetContentOffset pointee x   pageWidth        if targetIndex  gt  currentIndex         targetIndex   currentIndex   1       else if targetIndex  lt  currentIndex         targetIndex   currentIndex - 1           let count   collectionView numberOfItems inSection  0      targetIndex   max min targetIndex  count - 1   0      print  targetIndex    targetIndex         targetContentOffset pointee   scrollView contentOffset     var offsetX  CGFloat   0 0     if targetIndex  lt  count - 1         offsetX   pageWidth   CGFloat targetIndex        else         offsetX   scrollView contentSize width - scrollView width           collectionView setContentOffset CGPoint x  offsetX  y  0 0   animated  true

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final class PagingFlowLayout  UICollectionViewFlowLayout       private var currentIndex   0      override func targetContentOffset forProposedContentOffset proposedContentOffset  CGPoint  withScrollingVelocity velocity  CGPoint  - gt  CGPoint           let count   collectionView  numberOfItems inSection  0          let currentAttribute   layoutAttributesForItem              at  IndexPath item  currentIndex  section  0                   UICollectionViewLayoutAttributes            let direction   proposedContentOffset x  gt  currentAttribute frame minX         if collectionView  contentOffset x   collectionView  bounds width  lt  collectionView  contentSize width    currentIndex  lt  count - 1               currentIndex    direction   1   -1             currentIndex   max min currentIndex  count - 1   0                     let indexPath   IndexPath item  currentIndex  section  0          let closestAttribute   layoutAttributesForItem at  indexPath     UICollectionViewLayoutAttributes            let centerOffset   collectionView  bounds size width   2         return CGPoint x  closestAttribute center x - centerOffset  y  0

[ios] Paging UICollectionView by cells, not screen

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