UICollectionView Cell滚动到中心

Question

我在我的UIViewController中使用UICollectionView.

我的collectionview属性设置如下.

现在我希望滚动后单元格在屏幕上居中!选项1:

选项2:

我需要做什么来实现选项2？

更新:

最后我使用下面的代码作为滚动与其他答案不顺利.

  - (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{    CGFloat offsetAdjustment = MAXFLOAT;
    CGFloat horizontalCenter = proposedContentOffset.x + (CGRectGetWidth(self.collectionView.bounds) / 2.0);

    CGRect targetRect = CGRectMake(proposedContentOffset.x, 0.0, self.collectionView.bounds.size.width, self.collectionView.bounds.size.height);
    NSArray* array = [super layoutAttributesForElementsInRect:targetRect];

    for (UICollectionViewLayoutAttributes* layoutAttributes in array) {
        CGFloat itemHorizontalCenter = layoutAttributes.center.x;

        if (ABS(itemHorizontalCenter - horizontalCenter) < ABS(offsetAdjustment)) {
            offsetAdjustment = itemHorizontalCenter - horizontalCenter;
        }
    }    
    return CGPointMake(proposedContentOffset.x + offsetAdjustment, proposedContentOffset.y);
}

Answer 1

您可以覆盖子类中的targetContentOffsetForProposedContentOffset:withScrollingVelocity:方法UICollectionViewLayout并计算您的偏移量,如下所示:

@property (nonatomic, assign) CGFloat previousOffset;
@property (nonatomic, assign) NSInteger currentPage;

...

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity {
    NSInteger itemsCount = [self.collectionView.dataSource collectionView:self.collectionView numberOfItemsInSection:0];

    // Imitating paging behaviour
    // Check previous offset and scroll direction
    if ((self.previousOffset > self.collectionView.contentOffset.x) && (velocity.x < 0.0f)) {
        self.currentPage = MAX(self.currentPage - 1, 0);
    } else if ((self.previousOffset < self.collectionView.contentOffset.x) && (velocity.x > 0.0f)) {
        self.currentPage = MIN(self.currentPage + 1, itemsCount - 1);
    }

    // Update offset by using item size + spacing
    CGFloat updatedOffset = (self.itemSize.width + self.minimumInteritemSpacing) * self.currentPage;
    self.previousOffset = updatedOffset;

    return CGPointMake(updatedOffset, proposedContentOffset.y);
}

编辑:感谢您指出这一点,忘了说您必须先禁用分页:

self.collectionView.pagingEnabled = NO;

更新:附加Swift 4.2版本

...
collectionView.isPagingEnabled = false
...

class YourCollectionLayoutSubclass: UICollectionViewFlowLayout {

    private var previousOffset: CGFloat = 0
    private var currentPage: Int = 0

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

        let itemsCount = collectionView.numberOfItems(inSection: 0)

        // Imitating paging behaviour
        // Check previous offset and scroll direction
        if previousOffset > collectionView.contentOffset.x && velocity.x < 0 {
            currentPage = max(currentPage - 1, 0)
        } else if previousOffset < collectionView.contentOffset.x && velocity.x > 0 {
            currentPage = min(currentPage + 1, itemsCount - 1)
        }

        // Update offset by using item size + spacing
        let updatedOffset = (itemSize.width + minimumInteritemSpacing) * CGFloat(currentPage)
        previousOffset = updatedOffset

        return CGPoint(x: updatedOffset, y: proposedContentOffset.y)
    }
}

Answer 2

你可以使用代码 self.collectionView.scrollToItem(at: indexPath, at: .centeredHorizontally, animated: true)

Answer 3

这是@ dmitry-zhukov的Swift 3版本(非常感谢btw!)

class PagedCollectionLayout : UICollectionViewFlowLayout {

    var previousOffset : CGFloat = 0
    var currentPage : CGFloat = 0

    override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {

        let sup = super.targetContentOffset(forProposedContentOffset: proposedContentOffset, withScrollingVelocity: velocity)

        guard
            let validCollection = collectionView,
            let dataSource = validCollection.dataSource
            else { return sup }

        let itemsCount = dataSource.collectionView(validCollection, numberOfItemsInSection: 0)

        // Imitating paging behaviour
        // Check previous offset and scroll direction
        if  (previousOffset > validCollection.contentOffset.x) && (velocity.x < 0) {
            currentPage = max(currentPage - 1, 0)
        }
        else if (previousOffset < validCollection.contentOffset.x) && (velocity.x > 0) {
            currentPage = min(currentPage + 1, CGFloat(itemsCount - 1))
        }

        // Update offset by using item size + spacing
        let updatedOffset = ((itemSize.width + minimumInteritemSpacing) * currentPage)
        self.previousOffset = updatedOffset

        let updatedPoint = CGPoint(x: updatedOffset, y: proposedContentOffset.y)

        return updatedPoint
    }
}