启用分页时将水平滚动集合视图的单元格居中，并使其他单元格部分可见

Question

我有一个可水平滚动的集合视图。它上面有几个细胞。为该集合视图启用了分页。

我想让它显示居中单元格左侧和右侧的部分单元格（当它不是第一个或最后一个时），如下所示：

这里是UICollectionViewDelegateFlowLayout的代码：

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
    return CGSize(width: collectionView.frame.width - 50, height: collectionView.frame.height - 16)
}

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {

    return sectionInsets
}


func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {
    return 10
}

和sectionInsets如下：

 private var sectionInsets = UIEdgeInsetsMake(8, 20, 8, 20)

目前，当我向右滚动时，右边的单元格越来越明显（单元格之间的间距线向左边缘移动）

我尝试根据答案使用不同的值 and ，但我无法达到我想要的效果。

如果您对此有任何建议，我将不胜感激。

Answer 1

NSInteger lastOffset, currentIndex;

-(void)scrollViewWillBeginDecelerating:(UIScrollView *)scrollView{
      [self.collectionView scrollToItemAtIndexPath:[NSIndexPath indexPathForItem:currentIndex inSection:0] atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally animated:YES];
 }

-(void)scrollViewWillBeginDragging:(UIScrollView *)scrollView{
      lastOffset = scrollView.contentOffset.x;
 }

-(void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset{

     CGFloat newOffset = scrollView.contentOffset.x;

    if (currentIndex > 0 && lastOffset > 0 && (newOffset < lastOffset - (collectionViewCellwidth / 2.0) || velocity.x < -0.5)) {
         currentIndex -= 1;
    }

   if (currentIndex < (totalNumberOfRows - 1) && lastOffset < ((totalNumberOfRows - 1) * collectionViewCellwidth) && (newOffset > (lastOffset + (collectionViewCellwidth / 2.0)) || velocity.x > 0.5)) {
         currentIndex += 1 ;
   }
   targetContentOffset->x = currentIndex * collectionViewCellwidth;

}

在您的情况下，collectionViewCellwidth 将为 "collectionView.frame.width - 50"

Answer 2

实际上，解决方案并不难。感谢 this 文章展示了实现此功能的正确方法，我终于实现了它。

要了解的主要事情是，在集合视图中构建的分页在这种情况下不起作用，因为单元格的大小小于屏幕的大小。所以，我们需要实现我们的自定义分页技术。

首先，在自定义帮助程序 struct 中，我们定义了一个属性，它将在拖动之前保存单元格的索引：

var indexOfCellBeforeDragging: Int = 0

然后，我们创建一个计算单元格截面插入的方法：

 //Setting the inset
func calculateSectionInset(forCollectionViewLayout collectionViewLayout: UICollectionViewFlowLayout, numberOfCells: Int) -> CGFloat {
    let inset = (collectionViewLayout.collectionView!.frame.width) / CGFloat(numberOfCells)
    return inset
}

现在我们可以使用上面的方法，来设置collection view的items的大小了。

  //Setting the item size of the collection view
func configureCollectionViewLayoutItemSize(forCollectionViewLayout collectionViewLayout: UICollectionViewFlowLayout) {

    let inset: CGFloat = calculateSectionInset(forCollectionViewLayout: collectionViewLayout, numberOfCells: 5)

    collectionViewLayout.sectionInset = UIEdgeInsets(top: 0, left: inset/4, bottom: 0, right: inset/4)

    collectionViewLayout.itemSize = CGSize(width: collectionViewLayout.collectionView!.frame.size.width - inset / 2, height: collectionViewLayout.collectionView!.frame.size.height)
    collectionViewLayout.collectionView?.reloadData()
}

我们需要获取主单元格（当前最大的单元格）的索引。为此，我们在 x 轴上使用有关项目宽度和集合视图的 contentOffset 的信息：

//Getting the index of the major cell
func indexOfMajorCell(in collectionViewLayout: UICollectionViewFlowLayout) -> Int {
    let itemWidth = collectionViewLayout.itemSize.width
    let proportionalLayout = collectionViewLayout.collectionView!.contentOffset.x / itemWidth
    return Int(round(proportionalLayout))
}

接下来，我们在开始拖动之前设置cell的索引，使用上面的方法：

 //Setting the index of cell before starting dragging
mutating func setIndexOfCellBeforeStartingDragging(indexOfMajorCell: Int) {
    indexOfCellBeforeDragging = indexOfMajorCell
}

现在我们应该处理拖动的结束。此外，我们还处理了单元格上可能的捕捉手势。这里我们为滑动速度使用一些自定义阈值。

  //Handling dragging end of a scroll view
func handleDraggingWillEndForScrollView(_ scrollView: UIScrollView, inside collectionViewLayout: UICollectionViewFlowLayout, withVelocity velocity: CGPoint, usingIndexOfMajorCell indexOfMajorCell: Int) {

    //Calculating where scroll view should snap
    let indexOfMajorCell = indexOfMajorCell

    let swipeVelocityThreshold: CGFloat = 0.5

    let majorCellIsTheCellBeforeDragging = indexOfMajorCell == indexOfCellBeforeDragging
    let hasEnoughVelocityToSlideToTheNextCell = indexOfCellBeforeDragging + 1 < 5 && velocity.x > swipeVelocityThreshold
    let hasEnoughVelocityToSlideToThePreviousCell = ((indexOfCellBeforeDragging - 1) >= 0) && (velocity.x < -swipeVelocityThreshold)

    let didUseSwipeToSkipCell = majorCellIsTheCellBeforeDragging && (hasEnoughVelocityToSlideToTheNextCell || hasEnoughVelocityToSlideToThePreviousCell)

    if didUseSwipeToSkipCell {

        let snapToIndex = indexOfCellBeforeDragging + (hasEnoughVelocityToSlideToTheNextCell ? 1 : -1)
        let toValue = collectionViewLayout.itemSize.width * CGFloat(snapToIndex)

        // Damping equal 1 => no oscillations => decay animation
        UIView.animate(withDuration: 0.3, delay: 0, usingSpringWithDamping: 1, initialSpringVelocity: velocity.x, options: .allowUserInteraction, animations: {
            scrollView.contentOffset = CGPoint(x: toValue, y: 0)
            scrollView.layoutIfNeeded()
        }, completion: nil)

    } else {
        let indexPath = IndexPath(row: indexOfMajorCell, section: 0)
        collectionViewLayout.collectionView!.scrollToItem(at: indexPath, at: .centeredHorizontally, animated: true)
    }
}

现在，在 scrollViewWillBeginDragging(_:) 方法中的视图控制器中，我们需要获取主要单元格的索引并在开始拖动之前将其设置为索引（commentsPagingHelper 是辅助结构的一个实例):

  func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {

    let indexOfMajorCell = commentsPagingHelper.indexOfMajorCell(in: collectionViewLayout)
    commentsPagingHelper.setIndexOfCellBeforeStartingDragging(indexOfMajorCell: indexOfMajorCell)
}

最后，我们在scrollViewWillEndDragging(_:, withVelocity:, targetContentOffset:)方法中处理主单元格索引的变化：

 func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {

    //Stop scrollView sliding:
    targetContentOffset.pointee = scrollView.contentOffset


    let indexOfMajorCell = commentsPagingHelper.indexOfMajorCell(in: collectionViewLayout)

    commentsPagingHelper.handleDraggingWillEndForScrollView(scrollView, inside: collectionViewLayout, withVelocity: velocity, usingIndexOfMajorCell: indexOfMajorCell)
}

就是这样。希望，这会帮助有类似问题的人。

启用分页时将水平滚动集合视图的单元格居中，并使其他单元格部分可见

Centring a cell of a horizontally scrollable collection view when paging is enabled and making other cells partly visible, too

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swift