UICollectionView单元格滚动到中心 [英] UICollectionView Cell Scroll to centre

问题描述

我在UIViewController中使用UICollectionView.

I am using UICollectionView in my UIViewController.

我的collectionview属性设置如下.

My collectionview properties are set as below.

现在，我希望滚动后单元格在屏幕上居中！ 选项1:

Now I would like cell to be Centre on screen after scroll! Option 1:

选项2:

要实现选项2，我该怎么做?

What would I have to do achieve option 2?

更新:

最后，我使用以下代码，因为滚动其他答案并不顺利.

In the end I have used following code as scrolling with other answer is not smooth.

  - (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);
}

推荐答案

您可以在UICollectionViewLayout子类中重写targetContentOffsetForProposedContentOffset:withScrollingVelocity:方法，并在其中计算偏移量，如下所示:

You can override targetContentOffsetForProposedContentOffset:withScrollingVelocity: method in your UICollectionViewLayout subclass and calculate your offset there like this:

@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，忘记说您必须先禁用分页:

EDIT: thanks for pointing this out, forgot to say that you have to disable paging first:

self.collectionView.pagingEnabled = NO;

更新:附加Swift 4.2版本

UPDATE: attaching Swift 4.2 version

...
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)
    }
}

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