Create a Paging UICollectionView with Swift

Question

I'm trying to create a UICollectionView with paging and that each item max width is 250 points, I've managed to create it, but I have 2 problems: The first item start not as it should be, but with more space at start and when I try to swipe, there is always something that wont let me swipe smooth.

This is how it looks:

video link

This is my code:

CenterCellCollectionFlowLayout.swift

class CenterCellCollectionViewFlowLayout: UICollectionViewFlowLayout {

    override func layoutAttributesForElementsInRect(rect: CGRect) -> [AnyObject]? {

        var attributesToReturn:[UICollectionViewLayoutAttributes] = super.layoutAttributesForElementsInRect(rect) as! [UICollectionViewLayoutAttributes]

        for var i = 0 ; i < attributesToReturn.count ; i++
        {
            var currentLayoutAttributes: UICollectionViewLayoutAttributes = attributesToReturn[i]
            var maximumSpacing: CGFloat = 50
            let origin: CGFloat
            if i - 1 >= 0 {
                let previousLayoutAttributes = attributesToReturn[i - 1]
                origin = previousLayoutAttributes.frame.maxX
            } else {
                origin = 0
            }

            if origin + maximumSpacing + currentLayoutAttributes.frame.size.width < self.collectionViewContentSize().width
            {
                var frame: CGRect = currentLayoutAttributes.frame
                frame.origin.x = origin + maximumSpacing
                currentLayoutAttributes.frame = frame
            }
        }

        return attributesToReturn

    }

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

        if let cv = self.collectionView {

            let cvBounds = cv.bounds
            let halfWidth = cvBounds.size.width * 0.5;
            let proposedContentOffsetCenterX = proposedContentOffset.x + halfWidth;

            if let attributesForVisibleCells = self.layoutAttributesForElementsInRect(cvBounds) as? [UICollectionViewLayoutAttributes] {

                var candidateAttributes : UICollectionViewLayoutAttributes?
                for attributes in attributesForVisibleCells {

                    // == Skip comparison with non-cell items (headers and footers) == //
                    if attributes.representedElementCategory != UICollectionElementCategory.Cell {
                        continue
                    }

                    if let candAttrs = candidateAttributes {

                        let a = attributes.center.x - proposedContentOffsetCenterX
                        let b = candAttrs.center.x - proposedContentOffsetCenterX

                        if fabsf(Float(a)) < fabsf(Float(b)) {
                            candidateAttributes = attributes;
                        }

                    }
                    else { // == First time in the loop == //

                        candidateAttributes = attributes;
                        continue;
                    }


                }

                return CGPoint(x : candidateAttributes!.center.x - halfWidth, y : proposedContentOffset.y);

            }

        }

        // Fallback
        return super.targetContentOffsetForProposedContentOffset(proposedContentOffset)
    }
}

MainViewController.swift

class MainViewController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {

    @IBOutlet weak var collectionView: UICollectionView!
    @IBOutlet weak var collectionViewFlowLayout: CenterCellCollectionViewFlowLayout!

    var collectionObjects: NSMutableArray?
    private let reuseIdentifier = "CollectionViewCell"

    override func viewDidLoad() {
        super.viewDidLoad()

        // Do any additional setup after loading the view.

        self.collectionView.registerClass(UICollectionViewCell.self, forCellWithReuseIdentifier: reuseIdentifier)

    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath)
    {
    }

    func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell
    {
        let cell = collectionView.dequeueReusableCellWithReuseIdentifier(reuseIdentifier, forIndexPath: indexPath) as! UICollectionViewCell
        cell.backgroundColor = UIColor.greenColor()
        // Configure the cell
        return cell
    }

    func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int
    {
        return 10
    }

    func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAtIndex section: Int) -> UIEdgeInsets
    {
        return UIEdgeInsetsMake(0, 0, 0, 0)
    }

    func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize
    {
        return CGSizeMake(250, 250)
    }
}

Thanks in advance

Answer 1

A UICollectionView inherits from UIScrollView. It is possible to achieve paging by setting isPagingEnabled to true.

Answer 2

So I figure out how to do it.

First create a custom UICollectionViewFlowLayout and add this override this method:

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

    if let cv = self.collectionView {

        let cvBounds = cv.bounds
        let halfWidth = cvBounds.size.width * 0.5;
        let proposedContentOffsetCenterX = proposedContentOffset.x + halfWidth;

        if let attributesForVisibleCells = self.layoutAttributesForElementsInRect(cvBounds) as? [UICollectionViewLayoutAttributes] {

            var candidateAttributes : UICollectionViewLayoutAttributes?
            for attributes in attributesForVisibleCells {

                // == Skip comparison with non-cell items (headers and footers) == //
                if attributes.representedElementCategory != UICollectionElementCategory.Cell {
                    continue
                }

                if let candAttrs = candidateAttributes {

                    let a = attributes.center.x - proposedContentOffsetCenterX
                    let b = candAttrs.center.x - proposedContentOffsetCenterX

                    if fabsf(Float(a)) < fabsf(Float(b)) {
                        candidateAttributes = attributes;
                    }

                }
                else { // == First time in the loop == //

                    candidateAttributes = attributes;
                    continue;
                }


            }

            return CGPoint(x : candidateAttributes!.center.x - halfWidth, y : proposedContentOffset.y);

        }

    }

    // Fallback
    return super.targetContentOffsetForProposedContentOffset(proposedContentOffset)
}

Then on the class that you implement the UICollectionView do like that:

    let collectionViewLayout: CenterCellCollectionViewFlowLayout = CenterCellCollectionViewFlowLayout()
    collectionViewLayout.itemSize = CGSizeMake(self.itemSize, self.itemSize)
    collectionViewLayout.sectionInset = UIEdgeInsets(top: 0, left: 0, bottom: 0, right: 0)
    collectionViewLayout.minimumInteritemSpacing = 0
    collectionViewLayout.minimumLineSpacing = self.itemSpacing
    collectionViewLayout.scrollDirection = UICollectionViewScrollDirection.Horizontal

    var collectionView: UICollectionView = UICollectionView(frame: self.collectionContainer.bounds, collectionViewLayout: collectionViewLayout)
    collectionView.delegate = self;
    collectionView.dataSource = self;
    collectionView.backgroundColor = UIColor.redColor()

    collectionView.registerClass(LevelsCustomCell.self, forCellWithReuseIdentifier: reuseIdentifier)
    collectionView.registerNib(UINib(nibName: reuseIdentifier, bundle: nil), forCellWithReuseIdentifier: reuseIdentifier)

    self.collectionContainer.addSubview(collectionView)

Thats about it, works like a charm.