在启用分页时使中心水平可滚动集合视图的单元格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。

我想让它显示中心单元格左右两侧的细胞部分（当它不是第一个或最后一个时），就像这里一样：

以下是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
}

sectionInsets如下：

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

目前，当我向右滚动时，右侧单元格越来越明显（单元格之间的间隔线向左边缘移动）

我尝试根据答案here和here使用不同的值，但我无法达到我想要的效果。

如果您对此有任何建议，我将非常感谢您的帮助。

Answer 1

实际上，解决方案并不是很难。感谢this文章显示了实现此功能的正确方法，我终于实现了它。

要理解的主要是，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。因此，我们需要实现自定义分页技术。

首先，在自定义帮助器struct内部，我们定义一个属性，该属性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的插入部分的方法：

 //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集合视图的项目大小。

  //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()
}

我们需要获取主要单元格的索引（现在最大的单元格）。为此，我们在contentOffset轴上使用有关项目宽度和集合视图x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，我们使用上面的方法设置开始拖动之前的单元格索引：

 //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，我们负责细胞上可能的捕捉手势。这里我们使用一些自定义阈值来滑动速度。

  //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(_:)方法内的视图控制器中，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是辅助结构的实例）：

  func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {

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

最后，我们处理scrollViewWillEndDragging(_:, withVelocity:, targetContentOffset:)方法中主要单元格索引的更改：

 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，这将有助于有类似问题的人。

Answer 2

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;

}

在你的情况下，collectionViewCellwidth将是＆＃34; collectionView.frame.width - 50＆＃34;