Here's my implementation in Swift 5 for vertical cell-based paging:

override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {

    // 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 = self.collectionView!.contentOffset.y/pageHeight

    // Determine the current page based on velocity.
    let currentPage = (velocity.y < 0.0) ? floor(approximatePage) : ceil(approximatePage)

    // Create custom flickVelocity.
    let flickVelocity = velocity.y * 0.3

    // Check how many pages the user flicked, if <= 1 then flickedPages should return 0.
    let flickedPages = (abs(round(flickVelocity)) <= 1) ? 0 : round(flickVelocity)

    let newVerticalOffset = ((currentPage + flickedPages) * pageHeight) - self.collectionView!.contentInset.top

    return CGPoint(x: proposedContentOffset.x, y: newVerticalOffset)
}

Some notes:

• Doesn't glitch
• Don't forget to set the default paging to false!
• 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.
• This works best when you set self.collectionView.decelerationRate = UIScollViewDecelerationRateFast.

Here's a horizontal version (haven't tested it thoroughly so please forgive any mistakes):

override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {

    // Page width used for estimating and calculating paging.
    let pageWidth = self.itemSize.width + self.minimumLineSpacing

    // Make an estimation of the current page position.
    let approximatePage = self.collectionView!.contentOffset.x/pageWidth

    // Determine the current page based on velocity.
    let currentPage = (velocity.x < 0.0) ? floor(approximatePage) : ceil(approximatePage)

    // Create custom flickVelocity.
    let flickVelocity = velocity.x * 0.3

    // Check how many pages the user flicked, if <= 1 then flickedPages should return 0.
    let flickedPages = (abs(round(flickVelocity)) <= 1) ? 0 : round(flickVelocity)

    // Calculate newHorizontalOffset.
    let newHorizontalOffset = ((currentPage + flickedPages) * pageWidth) - self.collectionView!.contentInset.left

    return CGPoint(x: newHorizontalOffset, y: proposedContentOffset.y)
}

For anyone looking for a solution that...

• DOES NOT GLITCH when the user performs a short fast scroll (i.e. it considers positive and negative scroll velocities)
• takes the collectionView.contentInset (and safeArea on iPhone X) into consideration
• only considers thoes cells visible at the point of scrolling (for peformance)
• uses well named variables and comments
• is Swift 4

then please see below...

public class CarouselCollectionViewLayout: UICollectionViewFlowLayout {

    override public func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {

        guard let collectionView = collectionView else {
            return super.targetContentOffset(forProposedContentOffset: proposedContentOffset, withScrollingVelocity: velocity)
        }

        // Identify the layoutAttributes of cells in the vicinity of where the scroll view will come to rest
        let targetRect = CGRect(origin: proposedContentOffset, size: collectionView.bounds.size)
        let visibleCellsLayoutAttributes = layoutAttributesForElements(in: targetRect)

        // Translate those cell layoutAttributes into potential (candidate) scrollView offsets
        let candidateOffsets: [CGFloat]? = visibleCellsLayoutAttributes?.map({ cellLayoutAttributes in
            if #available(iOS 11.0, *) {
                return cellLayoutAttributes.frame.origin.x - collectionView.contentInset.left - collectionView.safeAreaInsets.left - sectionInset.left
            } else {
                return cellLayoutAttributes.frame.origin.x - collectionView.contentInset.left - sectionInset.left
            }
        })

        // Now we need to work out which one of the candidate offsets is the best one
        let bestCandidateOffset: CGFloat

        if velocity.x > 0 {
            // If the scroll velocity was POSITIVE, then only consider cells/offsets to the RIGHT of the proposedContentOffset.x
            // Of the cells/offsets to the right, the NEAREST is the `bestCandidate`
            // If there is no nearestCandidateOffsetToLeft then we default to the RIGHT-MOST (last) of ALL the candidate cells/offsets
            //      (this handles the scenario where the user has scrolled beyond the last cell)
            let candidateOffsetsToRight = candidateOffsets?.toRight(ofProposedOffset: proposedContentOffset.x)
            let nearestCandidateOffsetToRight = candidateOffsetsToRight?.nearest(toProposedOffset: proposedContentOffset.x)
            bestCandidateOffset = nearestCandidateOffsetToRight ?? candidateOffsets?.last ?? proposedContentOffset.x
        }
        else if velocity.x < 0 {
            // If the scroll velocity was NEGATIVE, then only consider cells/offsets to the LEFT of the proposedContentOffset.x
            // Of the cells/offsets to the left, the NEAREST is the `bestCandidate`
            // If there is no nearestCandidateOffsetToLeft then we default to the LEFT-MOST (first) of ALL the candidate cells/offsets
            //      (this handles the scenario where the user has scrolled beyond the first cell)
            let candidateOffsetsToLeft = candidateOffsets?.toLeft(ofProposedOffset: proposedContentOffset.x)
            let nearestCandidateOffsetToLeft = candidateOffsetsToLeft?.nearest(toProposedOffset: proposedContentOffset.x)
            bestCandidateOffset = nearestCandidateOffsetToLeft ?? candidateOffsets?.first ?? proposedContentOffset.x
        }
        else {
            // If the scroll velocity was ZERO we consider all `candidate` cells (regarless of whether they are to the left OR right of the proposedContentOffset.x)
            // The cell/offset that is the NEAREST is the `bestCandidate`
            let nearestCandidateOffset = candidateOffsets?.nearest(toProposedOffset: proposedContentOffset.x)
            bestCandidateOffset = nearestCandidateOffset ??  proposedContentOffset.x
        }

        return CGPoint(x: bestCandidateOffset, y: proposedContentOffset.y)
    }

}

fileprivate extension Sequence where Iterator.Element == CGFloat {

    func toLeft(ofProposedOffset proposedOffset: CGFloat) -> [CGFloat] {

        return filter() { candidateOffset in
            return candidateOffset < proposedOffset
        }
    }

    func toRight(ofProposedOffset proposedOffset: CGFloat) -> [CGFloat] {

        return filter() { candidateOffset in
            return candidateOffset > proposedOffset
        }
    }

    func nearest(toProposedOffset proposedOffset: CGFloat) -> CGFloat? {

        guard let firstCandidateOffset = first(where: { _ in true }) else {
            // If there are no elements in the Sequence, return nil
            return nil
        }

        return reduce(firstCandidateOffset) { (bestCandidateOffset: CGFloat, candidateOffset: CGFloat) -> CGFloat in

            let candidateOffsetDistanceFromProposed = fabs(candidateOffset - proposedOffset)
            let bestCandidateOffsetDistancFromProposed = fabs(bestCandidateOffset - proposedOffset)

            if candidateOffsetDistanceFromProposed < bestCandidateOffsetDistancFromProposed {
                return candidateOffset
            }

            return bestCandidateOffset
        }
    }
}

Swift version of the accepted answer.

override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
    var offsetAdjustment = CGFloat.greatestFiniteMagnitude
    let horizontalOffset = proposedContentOffset.x
    let targetRect = CGRect(origin: CGPoint(x: proposedContentOffset.x, y: 0), size: self.collectionView!.bounds.size)

    for layoutAttributes in super.layoutAttributesForElements(in: targetRect)! {
        let itemOffset = layoutAttributes.frame.origin.x
        if (abs(itemOffset - horizontalOffset) < abs(offsetAdjustment)) {
            offsetAdjustment = itemOffset - horizontalOffset
        }
    }

    return CGPoint(x: proposedContentOffset.x + offsetAdjustment, y: proposedContentOffset.y)
}

Valid for Swift 3.

A shorter solution (assuming you're caching your layout attributes):

override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
    let proposedEndFrame = CGRect(x: proposedContentOffset.x, y: 0, width: collectionView!.bounds.width, height: collectionView!.bounds.height)
    let targetLayoutAttributes = cache.max { $0.frame.intersection(proposedEndFrame).width < $1.frame.intersection(proposedEndFrame).width }!
    return CGPoint(x: targetLayoutAttributes.frame.minX - horizontalPadding, y: 0)
}

To put this in context:

class Layout : UICollectionViewLayout {
    private var cache: [UICollectionViewLayoutAttributes] = []
    private static let horizontalPadding: CGFloat = 16
    private static let interItemSpacing: CGFloat = 8

    override func prepare() {
        let (itemWidth, itemHeight) = (collectionView!.bounds.width - 2 * Layout.horizontalPadding, collectionView!.bounds.height)
        cache.removeAll()
        let count = collectionView!.numberOfItems(inSection: 0)
        var x: CGFloat = Layout.horizontalPadding
        for item in (0..<count) {
            let indexPath = IndexPath(item: item, section: 0)
            let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
            attributes.frame = CGRect(x: x, y: 0, width: itemWidth, height: itemHeight)
            cache.append(attributes)
            x += itemWidth + Layout.interItemSpacing
        }
    }

    override var collectionViewContentSize: CGSize {
        let width: CGFloat
        if let maxX = cache.last?.frame.maxX {
            width = maxX + Layout.horizontalPadding
        } else {
            width = collectionView!.width
        }
        return CGSize(width: width, height: collectionView!.height)
    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        return cache.first { $0.indexPath == indexPath }
    }

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        return cache.filter { $0.frame.intersects(rect) }
    }

    override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
        let proposedEndFrame = CGRect(x: proposedContentOffset.x, y: 0, width: collectionView!.bounds.width, height: collectionView!.bounds.height)
        let targetLayoutAttributes = cache.max { $0.frame.intersection(proposedEndFrame).width < $1.frame.intersection(proposedEndFrame).width }!
        return CGPoint(x: targetLayoutAttributes.frame.minX - Layout.horizontalPadding, y: 0)
    }
}

Swift 4

The easiest solution for collection view with cells of one size (horizontal scroll):

override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
    guard let collectionView = collectionView else { return proposedContentOffset }

    // Calculate width of your page
    let pageWidth = calculatedPageWidth()

    // Calculate proposed page
    let proposedPage = round(proposedContentOffset.x / pageWidth)

    // Adjust necessary offset
    let xOffset = pageWidth * proposedPage - collectionView.contentInset.left

    return CGPoint(x: xOffset, y: 0)
}

func calculatedPageWidth() -> CGFloat {
    return itemSize.width + minimumInteritemSpacing
}

@André Abreu's Code

Swift3 version

class CustomCollectionViewFlowLayout: UICollectionViewFlowLayout {
    override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
        var offsetAdjustment = CGFloat.greatestFiniteMagnitude
        let horizontalOffset = proposedContentOffset.x
        let targetRect = CGRect(x: proposedContentOffset.x, y: 0, width: self.collectionView!.bounds.size.width, height: self.collectionView!.bounds.size.height)
        for layoutAttributes in super.layoutAttributesForElements(in: targetRect)! {
            let itemOffset = layoutAttributes.frame.origin.x
            if abs(itemOffset - horizontalOffset) < abs(offsetAdjustment){
                offsetAdjustment = itemOffset - horizontalOffset
            }
        }
        return CGPoint(x: proposedContentOffset.x + offsetAdjustment, y: proposedContentOffset.y)
    }
}

For those looking for a solution in Swift:

class CustomCollectionViewFlowLayout: UICollectionViewFlowLayout {
    private let collectionViewHeight: CGFloat = 200.0
    private let screenWidth: CGFloat = UIScreen.mainScreen().bounds.width

    override func awakeFromNib() {
        super.awakeFromNib()

        self.itemSize = CGSize(width: [InsertItemWidthHere], height: [InsertItemHeightHere])
        self.minimumInteritemSpacing = [InsertItemSpacingHere]
        self.scrollDirection = .Horizontal
        let inset = (self.screenWidth - CGFloat(self.itemSize.width)) / 2
        self.collectionView?.contentInset = UIEdgeInsets(top: 0,
                                                         left: inset,
                                                         bottom: 0,
                                                         right: inset)
    }

    override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
        var offsetAdjustment = CGFloat.max
        let horizontalOffset = proposedContentOffset.x + ((self.screenWidth - self.itemSize.width) / 2)

        let targetRect = CGRect(x: proposedContentOffset.x, y: 0, width: self.screenWidth, height: self.collectionViewHeight)
        var array = super.layoutAttributesForElementsInRect(targetRect)

        for layoutAttributes in array! {
            let itemOffset = layoutAttributes.frame.origin.x
            if (abs(itemOffset - horizontalOffset) < abs(offsetAdjustment)) {
                offsetAdjustment = itemOffset - horizontalOffset
            }
        }

        return CGPoint(x: proposedContentOffset.x + offsetAdjustment, y: proposedContentOffset.y)
    }
}

Here is my Swift solution on a horizontally scrolling collection view. It's simple, sweet and avoids any flickering.

  override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
    guard let collectionView = collectionView else { return proposedContentOffset }

    let currentXOffset = collectionView.contentOffset.x
    let nextXOffset = proposedContentOffset.x
    let maxIndex = ceil(currentXOffset / pageWidth())
    let minIndex = floor(currentXOffset / pageWidth())

    var index: CGFloat = 0

    if nextXOffset > currentXOffset {
      index = maxIndex
    } else {
      index = minIndex
    }

    let xOffset = pageWidth() * index
    let point = CGPointMake(xOffset, 0)

    return point
  }

  func pageWidth() -> CGFloat {
    return itemSize.width + minimumInteritemSpacing
  }

After long testing I found solution to snap to center with custom cell width (each cell has diff. width) which fixes the flickering. Feel free to improve the script.

- (CGPoint) targetContentOffsetForProposedContentOffset: (CGPoint) proposedContentOffset withScrollingVelocity: (CGPoint)velocity
{
    CGFloat offSetAdjustment = MAXFLOAT;
    CGFloat horizontalCenter = (CGFloat) (proposedContentOffset.x + (self.collectionView.bounds.size.width / 2.0));

    //setting fastPaging property to NO allows to stop at page on screen (I have pages lees, than self.collectionView.bounds.size.width)
    CGRect targetRect = CGRectMake(self.fastPaging ? proposedContentOffset.x : self.collectionView.contentOffset.x, 
                                   0.0,
                                   self.collectionView.bounds.size.width,
                                   self.collectionView.bounds.size.height);

    NSArray *attributes = [self layoutAttributesForElementsInRect:targetRect];
    NSPredicate *cellAttributesPredicate = [NSPredicate predicateWithBlock: ^BOOL(UICollectionViewLayoutAttributes * _Nonnull evaluatedObject,
                                                                             NSDictionary<NSString *,id> * _Nullable bindings) 
    {
        return (evaluatedObject.representedElementCategory == UICollectionElementCategoryCell); 
    }];        

    NSArray *cellAttributes = [attributes filteredArrayUsingPredicate: cellAttributesPredicate];

    UICollectionViewLayoutAttributes *currentAttributes;

    for (UICollectionViewLayoutAttributes *layoutAttributes in cellAttributes)
    {
        CGFloat itemHorizontalCenter = layoutAttributes.center.x;
        if (ABS(itemHorizontalCenter - horizontalCenter) < ABS(offSetAdjustment))
        {
            currentAttributes   = layoutAttributes;
            offSetAdjustment    = itemHorizontalCenter - horizontalCenter;
        }
    }

    CGFloat nextOffset          = proposedContentOffset.x + offSetAdjustment;

    proposedContentOffset.x     = nextOffset;
    CGFloat deltaX              = proposedContentOffset.x - self.collectionView.contentOffset.x;
    CGFloat velX                = velocity.x;

    // detection form  gist.github.com/rkeniger/7687301
    // based on http://stackoverflow.com/a/14291208/740949
    if (fabs(deltaX) <= FLT_EPSILON || fabs(velX) <= FLT_EPSILON || (velX > 0.0 && deltaX > 0.0) || (velX < 0.0 && deltaX < 0.0)) 
    {

    } 
    else if (velocity.x > 0.0) 
    {
       // revert the array to get the cells from the right side, fixes not correct center on different size in some usecases
        NSArray *revertedArray = [[array reverseObjectEnumerator] allObjects];

        BOOL found = YES;
        float proposedX = 0.0;

        for (UICollectionViewLayoutAttributes *layoutAttributes in revertedArray)
        {
            if(layoutAttributes.representedElementCategory == UICollectionElementCategoryCell)
            {
                CGFloat itemHorizontalCenter = layoutAttributes.center.x;
                if (itemHorizontalCenter > proposedContentOffset.x) {
                     found = YES;
                     proposedX = nextOffset + (currentAttributes.frame.size.width / 2) + (layoutAttributes.frame.size.width / 2);
                } else {
                     break;
                }
            }
        }

       // dont set on unfound element
        if (found) {
            proposedContentOffset.x = proposedX;
        }
    } 
    else if (velocity.x < 0.0) 
    {
        for (UICollectionViewLayoutAttributes *layoutAttributes in cellAttributes)
        {
            CGFloat itemHorizontalCenter = layoutAttributes.center.x;
            if (itemHorizontalCenter > proposedContentOffset.x) 
            {
                proposedContentOffset.x = nextOffset - ((currentAttributes.frame.size.width / 2) + (layoutAttributes.frame.size.width / 2));
                break;
            }
        }
    }

    proposedContentOffset.y = 0.0;

    return proposedContentOffset;
}

refer to this answer by Dan Abramov here's Swift version

    override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
    var _proposedContentOffset = CGPoint(x: proposedContentOffset.x, y: proposedContentOffset.y)
    var offSetAdjustment: CGFloat = CGFloat.max
    let horizontalCenter: CGFloat = CGFloat(proposedContentOffset.x + (self.collectionView!.bounds.size.width / 2.0))

    let targetRect = CGRect(x: proposedContentOffset.x, y: 0.0, width: self.collectionView!.bounds.size.width, height: self.collectionView!.bounds.size.height)

    let array: [UICollectionViewLayoutAttributes] = self.layoutAttributesForElementsInRect(targetRect)! as [UICollectionViewLayoutAttributes]
    for layoutAttributes: UICollectionViewLayoutAttributes in array {
        if (layoutAttributes.representedElementCategory == UICollectionElementCategory.Cell) {
            let itemHorizontalCenter: CGFloat = layoutAttributes.center.x
            if (abs(itemHorizontalCenter - horizontalCenter) < abs(offSetAdjustment)) {
                offSetAdjustment = itemHorizontalCenter - horizontalCenter
            }
        }
    }

    var nextOffset: CGFloat = proposedContentOffset.x + offSetAdjustment

    repeat {
        _proposedContentOffset.x = nextOffset
        let deltaX = proposedContentOffset.x - self.collectionView!.contentOffset.x
        let velX = velocity.x

        if (deltaX == 0.0 || velX == 0 || (velX > 0.0 && deltaX > 0.0) || (velX < 0.0 && deltaX < 0.0)) {
            break
        }

        if (velocity.x > 0.0) {
            nextOffset = nextOffset + self.snapStep()
        } else if (velocity.x < 0.0) {
            nextOffset = nextOffset - self.snapStep()
        }
    } while self.isValidOffset(nextOffset)

    _proposedContentOffset.y = 0.0

    return _proposedContentOffset
}

func isValidOffset(offset: CGFloat) -> Bool {
    return (offset >= CGFloat(self.minContentOffset()) && offset <= CGFloat(self.maxContentOffset()))
}

func minContentOffset() -> CGFloat {
    return -CGFloat(self.collectionView!.contentInset.left)
}

func maxContentOffset() -> CGFloat {
    return CGFloat(self.minContentOffset() + self.collectionView!.contentSize.width - self.itemSize.width)
}

func snapStep() -> CGFloat {
    return self.itemSize.width + self.minimumLineSpacing;
}

or gist here https://gist.github.com/katopz/8b04c783387f0c345cd9

I prefer to allow user flicking through several pages. So here is my version of targetContentOffsetForProposedContentOffset (which based on DarthMike answer) for vertical layout.

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity {
    CGFloat approximatePage = self.collectionView.contentOffset.y / self.pageHeight;
    CGFloat currentPage = (velocity.y < 0.0) ? floor(approximatePage) : ceil(approximatePage);

    NSInteger flickedPages = ceil(velocity.y / self.flickVelocity);

    if (flickedPages) {
        proposedContentOffset.y = (currentPage + flickedPages) * self.pageHeight;
    } else {
        proposedContentOffset.y = currentPage * self.pageHeight;
    }

    return proposedContentOffset;
}

- (CGFloat)pageHeight {
    return self.itemSize.height + self.minimumLineSpacing;
}

- (CGFloat)flickVelocity {
    return 1.2;
}

Dan's solution is flawed. It does not handle user flicking well. The cases when user flicks fast and scroll did not move so much, have animation glitches.

My proposed alternative implementation has the same pagination as proposed before, but handles user flicking between pages.

 #pragma mark - Pagination
 - (CGFloat)pageWidth {
     return self.itemSize.width + self.minimumLineSpacing;
 }

 - (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
 {           
        CGFloat rawPageValue = self.collectionView.contentOffset.x / self.pageWidth;
        CGFloat currentPage = (velocity.x > 0.0) ? floor(rawPageValue) : ceil(rawPageValue);
        CGFloat nextPage = (velocity.x > 0.0) ? ceil(rawPageValue) : floor(rawPageValue);

        BOOL pannedLessThanAPage = fabs(1 + currentPage - rawPageValue) > 0.5;
        BOOL flicked = fabs(velocity.x) > [self flickVelocity];
        if (pannedLessThanAPage && flicked) {
            proposedContentOffset.x = nextPage * self.pageWidth;
        } else {
            proposedContentOffset.x = round(rawPageValue) * self.pageWidth;
        }

        return proposedContentOffset;
 }

 - (CGFloat)flickVelocity {
     return 0.3;
 }

a small issue I encountered while using targetContentOffsetForProposedContentOffset is a problem with the last cell not adjusting according to the new point I returned.
I found out that the CGPoint I returned had a Y value bigger then allowed so i used the following code at the end of my targetContentOffsetForProposedContentOffset implementation:

// if the calculated y is bigger then the maximum possible y we adjust accordingly
CGFloat contentHeight = self.collectionViewContentSize.height;
CGFloat collectionViewHeight = self.collectionView.bounds.size.height;
CGFloat maxY = contentHeight - collectionViewHeight;
if (newY > maxY)
{
    newY = maxY;
}

return CGPointMake(0, newY);

just to make it clearer this is my full layout implementation which just imitates vertical paging behavior:

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{
    return [self targetContentOffsetForProposedContentOffset:proposedContentOffset];
}

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset
{
    CGFloat heightOfPage = self.itemSize.height;
    CGFloat heightOfSpacing = self.minimumLineSpacing;

    CGFloat numOfPage = lround(proposedContentOffset.y / (heightOfPage + heightOfSpacing));
    CGFloat newY = numOfPage * (heightOfPage + heightOfSpacing);

    // if the calculated y is bigger then the maximum possible y we adjust accordingly
    CGFloat contentHeight = self.collectionViewContentSize.height;
    CGFloat collectionViewHeight = self.collectionView.bounds.size.height;
    CGFloat maxY = contentHeight - collectionViewHeight;
    if (newY > maxY)
    {
        newY = maxY;
    }

    return CGPointMake(0, newY);
}

hopefully this will save someone some time and a headache

Fogmeisters answer worked for me unless I scrolled to the end of the row. My cells don't fit neatly on the screen so it would scroll to the end and jump back with a jerk so that the last cell always overlapped the right edge of the screen.

To prevent this add the following line of code at the start of the targetcontentoffset method

if(proposedContentOffset.x>self.collectionViewContentSize.width-320-self.sectionInset.right)
    return proposedContentOffset;

While this answer has been a great help to me, there is a noticeable flicker when you swipe fast on a small distance. It's much easier to reproduce it on the device.

I found that this always happens when collectionView.contentOffset.x - proposedContentOffset.x and velocity.x have different sings.

My solution was to ensure that proposedContentOffset is more than contentOffset.x if velocity is positive, and less if it is negative. It's in C# but should be fairly simple to translate to Objective C:

public override PointF TargetContentOffset (PointF proposedContentOffset, PointF scrollingVelocity)
{
    /* Determine closest edge */

    float offSetAdjustment = float.MaxValue;
    float horizontalCenter = (float) (proposedContentOffset.X + (this.CollectionView.Bounds.Size.Width / 2.0));

    RectangleF targetRect = new RectangleF (proposedContentOffset.X, 0.0f, this.CollectionView.Bounds.Size.Width, this.CollectionView.Bounds.Size.Height);
    var array = base.LayoutAttributesForElementsInRect (targetRect);

    foreach (var layoutAttributes in array) {
        float itemHorizontalCenter = layoutAttributes.Center.X;
        if (Math.Abs (itemHorizontalCenter - horizontalCenter) < Math.Abs (offSetAdjustment)) {
            offSetAdjustment = itemHorizontalCenter - horizontalCenter;
        }
    }

    float nextOffset = proposedContentOffset.X + offSetAdjustment;

    /*
     * ... unless we end up having positive speed
     * while moving left or negative speed while moving right.
     * This will cause flicker so we resort to finding next page
     * in the direction of velocity and use it.
     */

    do {
        proposedContentOffset.X = nextOffset;

        float deltaX = proposedContentOffset.X - CollectionView.ContentOffset.X;
        float velX = scrollingVelocity.X;

        // If their signs are same, or if either is zero, go ahead
        if (Math.Sign (deltaX) * Math.Sign (velX) != -1)
            break;

        // Otherwise, look for the closest page in the right direction
        nextOffset += Math.Sign (scrollingVelocity.X) * SnapStep;
    } while (IsValidOffset (nextOffset));

    return proposedContentOffset;
}

bool IsValidOffset (float offset)
{
    return (offset >= MinContentOffset && offset <= MaxContentOffset);
}

This code is using MinContentOffset, MaxContentOffset and SnapStep which should be trivial for you to define. In my case they turned out to be

float MinContentOffset {
    get { return -CollectionView.ContentInset.Left; }
}

float MaxContentOffset {
    get { return MinContentOffset + CollectionView.ContentSize.Width - ItemSize.Width; }
}

float SnapStep {
    get { return ItemSize.Width + MinimumLineSpacing; }
}

OK, answer is no, there is no way to do this without subclassing UICollectionViewFlowLayout.

However, subclassing it is incredibly easy for anyone who is reading this in the future.

First I set up the subclass call MyCollectionViewFlowLayout and then in interface builder I changed the collection view layout to Custom and selected my flow layout subclass.

Because you're doing it this way you can't specify items sizes, etc... in IB so in MyCollectionViewFlowLayout.m I have this...

- (void)awakeFromNib
{
    self.itemSize = CGSizeMake(75.0, 75.0);
    self.minimumInteritemSpacing = 10.0;
    self.minimumLineSpacing = 10.0;
    self.scrollDirection = UICollectionViewScrollDirectionHorizontal;
    self.sectionInset = UIEdgeInsetsMake(10.0, 10.0, 10.0, 10.0);
}

This sets up all the sizes for me and the scroll direction.

Then ...

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{
    CGFloat offsetAdjustment = MAXFLOAT;
    CGFloat horizontalOffset = proposedContentOffset.x + 5;

    CGRect targetRect = CGRectMake(proposedContentOffset.x, 0, self.collectionView.bounds.size.width, self.collectionView.bounds.size.height);

    NSArray *array = [super layoutAttributesForElementsInRect:targetRect];

    for (UICollectionViewLayoutAttributes *layoutAttributes in array) {
        CGFloat itemOffset = layoutAttributes.frame.origin.x;
        if (ABS(itemOffset - horizontalOffset) < ABS(offsetAdjustment)) {
            offsetAdjustment = itemOffset - horizontalOffset;
        }
    }

    return CGPointMake(proposedContentOffset.x + offsetAdjustment, proposedContentOffset.y);
}

This ensures that the scrolling ends with a margin of 5.0 on the left hand edge.

That's all I needed to do. I didn't need to set the flow layout in code at all.