水平滚动时，UICollectionView会捕捉到单元格

问题描述：

我知道有些人之前已经问过这个问题，但他们都是关于 UITableViews 或 UIScrollViews 而我无法'获得公认的解决方案，为我工作。我想要的是水平滚动我的 UICollectionView 时的捕捉效果 - 就像在iOS AppStore中发生的那样。 iOS 9+是我的目标版本所以请在回答之前先查看UIKit的更改。

I know some people have asked this question before but they were all about UITableViews or UIScrollViews and I couldn't get the accepted solution to work for me. What I would like is the snapping effect when scrolling through my UICollectionView horizontally - much like what happens in the iOS AppStore. iOS 9+ is my target build so please look at the UIKit changes before answering this.

谢谢。

答

虽然最初我使用的是Objective-C，但我自从切换到Swift并且原来接受的答案还不够。

While originally I was using Objective-C, I since switched so Swift and the original accepted answer did not suffice.

我结束了创建一个 UICollectionViewLayout 子类，它提供了最好的（imo）体验，而不是在用户停止滚动时改变内容偏移或类似内容的其他函数。

I ended up creating a UICollectionViewLayout subclass which provides the best (imo) experience as opposed to the other functions which alter content offset or something similar when the user has stopped scrolling.

class SnappingCollectionViewLayout: UICollectionViewFlowLayout {

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

        var offsetAdjustment = CGFloat.greatestFiniteMagnitude
        let horizontalOffset = proposedContentOffset.x + collectionView.contentInset.left

        let targetRect = CGRect(x: proposedContentOffset.x, y: 0, width: collectionView.bounds.size.width, height: collectionView.bounds.size.height)

        let layoutAttributesArray = super.layoutAttributesForElements(in: targetRect)

        layoutAttributesArray?.forEach({ (layoutAttributes) in
            let itemOffset = layoutAttributes.frame.origin.x
            if fabsf(Float(itemOffset - horizontalOffset)) < fabsf(Float(offsetAdjustment)) {
                offsetAdjustment = itemOffset - horizontalOffset
            }
        })

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

对于使用当前布局子类的最原始感觉减速，请确保设置以下内容：

For the most native feeling deceleration with the current layout subclass, make sure to set the following:

collectionView？.decelerationRate = UIScrollViewDecelerationRateFast

水平滚动时，UICollectionView会捕捉到单元格

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