How to center align items in UICollectionView?

Question

I have a CollectionView showing a number of images with horizontal scroll and paging enabled. The first and the last pages are showing perfectly but rest of the pages are not showing properly, they are showing parts of images from the adjacent page due to which they are not aligned properly (screenshots attached). How do I make all pages look same i.e. the images properly center aligned so that adjacent pages don't peek.

Notice the image peeking in Page 2 from the left side due to which the actual image on is getting clipped from the left. How can I fix this so that all pages look like page 1.

Answer 1

As you have replied in the comment, answer can be found here. Here is the Swift solution you have asked for.

Swift 1.2

override func collectionViewContentSize() -> CGSize {
    // Only support single section for now.
    // Only support Horizontal scroll
    let count = collectionView!.dataSource!.collectionView(collectionView!, numberOfItemsInSection: 0)

    let canvasSize = collectionView!.frame.size
    var contentSize = canvasSize

    if (scrollDirection == .Horizontal) {
        let rowCount = (canvasSize.height - itemSize.height) / (itemSize.height + minimumInteritemSpacing) + 1
        let columnCount = (canvasSize.width - itemSize.width) / (itemSize.width + minimumLineSpacing) + 1;
        let page = ceil(CGFloat(count) / (CGFloat)(rowCount * columnCount));
        contentSize.width = page * canvasSize.width;
    }

    return contentSize;
}

func frameForItemAtIndexPath(indexPath: NSIndexPath!) -> CGRect {
    let canvasSize = collectionView!.frame.size

    let rowCount = (canvasSize.height - itemSize.height) / (itemSize.height + minimumInteritemSpacing) + 1
    let columnCount = (canvasSize.width - itemSize.width) / (itemSize.width + minimumLineSpacing) + 1

    let pageMarginX = (canvasSize.width - columnCount * itemSize.width - (columnCount > 1 ? (columnCount - 1) * minimumLineSpacing : 0)) / 2.0
    let pageMarginY = (canvasSize.height - rowCount * itemSize.height - (rowCount > 1 ? (rowCount - 1) * minimumInteritemSpacing : 0)) / 2.0

    let page = CGFloat(indexPath.row) / (rowCount * columnCount)
    let remainder = CGFloat(indexPath.row) - page * (rowCount * columnCount)
    let row = remainder / columnCount
    let column = remainder - row * columnCount

    var cellFrame = CGRect.zeroRect
    cellFrame.origin.x = pageMarginX + column * (itemSize.width + minimumLineSpacing);
    cellFrame.origin.y = pageMarginY + row * (itemSize.height + minimumInteritemSpacing);
    cellFrame.size.width = itemSize.width;
    cellFrame.size.height = itemSize.height;

    if (scrollDirection == .Horizontal) {
        cellFrame.origin.x += page * canvasSize.width;
    }

    return cellFrame;
}

override func layoutAttributesForItemAtIndexPath(indexPath: NSIndexPath) -> UICollectionViewLayoutAttributes! {
    var attributes = super.layoutAttributesForItemAtIndexPath(indexPath)
    attributes.frame = frameForItemAtIndexPath(indexPath)
    return attributes
}

override func layoutAttributesForElementsInRect(rect: CGRect) -> [AnyObject]? {
    let superAttrs = super.layoutAttributesForElementsInRect(rect)
    if let originAttrs = superAttrs as? [UICollectionViewLayoutAttributes] {
        var attrs = [UICollectionViewLayoutAttributes]()

        for (index, var attr) in enumerate(originAttrs) {
            let indexPath = attr.indexPath
            let itemFrame = frameForItemAtIndexPath(indexPath)
            if CGRectIntersectsRect(itemFrame, rect) {
                attr = layoutAttributesForItemAtIndexPath(indexPath)
                attrs.append(attr)
            }
        }

        return attrs
    }

    return superAttrs;
}

Swift 2.0

override func collectionViewContentSize() -> CGSize {
    // Only support single section for now.
    // Only support Horizontal scroll
    let count = collectionView!.dataSource!.collectionView(collectionView!, numberOfItemsInSection: 0)

    let canvasSize = collectionView!.frame.size
    var contentSize = canvasSize

    if (scrollDirection == .Horizontal) {
        let rowCount = (canvasSize.height - itemSize.height) / (itemSize.height + minimumInteritemSpacing) + 1
        let columnCount = (canvasSize.width - itemSize.width) / (itemSize.width + minimumLineSpacing) + 1;
        let page = ceil(CGFloat(count) / (CGFloat)(rowCount * columnCount));
        contentSize.width = page * canvasSize.width;
    }

    return contentSize;
}

func frameForItemAtIndexPath(indexPath: NSIndexPath!) -> CGRect {
    let canvasSize = collectionView!.frame.size

    let rowCount = (canvasSize.height - itemSize.height) / (itemSize.height + minimumInteritemSpacing) + 1
    let columnCount = (canvasSize.width - itemSize.width) / (itemSize.width + minimumLineSpacing) + 1

    let pageMarginX = (canvasSize.width - columnCount * itemSize.width - (columnCount > 1 ? (columnCount - 1) * minimumLineSpacing : 0)) / 2.0
    let pageMarginY = (canvasSize.height - rowCount * itemSize.height - (rowCount > 1 ? (rowCount - 1) * minimumInteritemSpacing : 0)) / 2.0

    let page = CGFloat(indexPath.row) / (rowCount * columnCount)
    let remainder = CGFloat(indexPath.row) - page * (rowCount * columnCount)
    let row = remainder / columnCount
    let column = remainder - row * columnCount

    var cellFrame = CGRect.zeroRect
    cellFrame.origin.x = pageMarginX + column * (itemSize.width + minimumLineSpacing);
    cellFrame.origin.y = pageMarginY + row * (itemSize.height + minimumInteritemSpacing);
    cellFrame.size.width = itemSize.width;
    cellFrame.size.height = itemSize.height;

    if (scrollDirection == .Horizontal) {
        cellFrame.origin.x += page * canvasSize.width;
    }

    return cellFrame;
}

override func layoutAttributesForItemAtIndexPath(indexPath: NSIndexPath) -> UICollectionViewLayoutAttributes? {
    let attributes = super.layoutAttributesForItemAtIndexPath(indexPath)
    attributes!.frame = frameForItemAtIndexPath(indexPath)
    return attributes
}

override func layoutAttributesForElementsInRect(rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
    let superAttrs = super.layoutAttributesForElementsInRect(rect)
    if let originAttrs = superAttrs as [UICollectionViewLayoutAttributes]! {
        var attrs = [UICollectionViewLayoutAttributes]()

        for (_, var attr) in originAttrs.enumerate() {
            let indexPath = attr.indexPath
            let itemFrame = frameForItemAtIndexPath(indexPath)
            if CGRectIntersectsRect(itemFrame, rect) {
                attr = layoutAttributesForItemAtIndexPath(indexPath)!
                attrs.append(attr)
            }
        }

        return attrs
    }

    return superAttrs;
}