水平滚动 UICollectionView 时对齐单元格的中心

问题描述

我知道有些人以前问过这个问题，但他们都是关于 UITableViews 或 UIScrollViews 的，我无法得到可接受的解决方案来为我工作.我想要的是水平滚动我的 UICollectionView 时的捕捉效果——就像在 iOS AppStore 中发生的一样.iOS 9+ 是我的目标版本，所以在回答这个问题之前请先查看 UIKit 的变化.

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.

推荐答案

最初我使用的是 Objective-C，后来我改用 Swift，原来接受的答案不够用.

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

我最终创建了一个 UICollectionViewLayout 子类，它提供了最好的 (imo) 体验，而不是当用户停止滚动时改变内容偏移或类似内容的其他功能.

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)
    }
}

对于当前布局子类最原生的感觉减速，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

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