无论滚动速度如何，如何在每次滑动时将集合视图项捕捉到一个

Question

我在 UICollectionViewFlowLayout 的子类中使用 Apple 演示中的代码：

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{
    CGFloat offsetAdjustment = MAXFLOAT;
    CGFloat horizontalCenter = proposedContentOffset.x + self.collectionView.bounds.size.width / 2.;
    CGRect targetRect = CGRectMake(proposedContentOffset.x, 0., self.collectionView.bounds.size.width, self.collectionView.bounds.size.height);

    UICollectionViewLayoutAttributes *targetAttributes = nil;
    NSArray *attributes = [super layoutAttributesForElementsInRect:targetRect];
    for (UICollectionViewLayoutAttributes *a in attributes) {
        CGFloat itemHorizontalCenter = a.center.x;
        if (ABS(itemHorizontalCenter - horizontalCenter) < ABS(offsetAdjustment)) {
            offsetAdjustment = itemHorizontalCenter - horizontalCenter;
            targetAttributes = a;
        }
    }

    return CGPointMake(proposedContentOffset.x + offsetAdjustment, proposedContentOffset.y);
}

这在某种意义上是有效的，每当我滑动/平移并释放时，一个项目就会卡入到位。但建议的内容偏移量与滑动速度成正比。我想要做的是，无论我滑动多快/多慢，集合视图只会捕捉到当前居中的项目之前或之后的下一个项目。

我试过这样做：

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{
    NSInteger currentPage = (int)(self.collectionView.contentOffset.x / self.collectionView.bounds.size.width);
    if (proposedContentOffset.x > self.collectionView.contentOffset.x) {
        currentPage = (MIN(currentPage + 1, ((int)(self.collectionView.contentSize.width / self.collectionView.bounds.size.width)) - 1));
    }

    proposedContentOffset.x = self.collectionView.bounds.size.width * currentPage;

    CGFloat offsetAdjustment = MAXFLOAT;
    CGFloat horizontalCenter = proposedContentOffset.x + self.collectionView.bounds.size.width / 2.;
    CGRect targetRect = CGRectMake(proposedContentOffset.x, 0., self.collectionView.bounds.size.width, self.collectionView.bounds.size.height);

    UICollectionViewLayoutAttributes *targetAttributes = nil;
    NSArray *attributes = [super layoutAttributesForElementsInRect:targetRect];
    for (UICollectionViewLayoutAttributes *a in attributes) {
        CGFloat itemHorizontalCenter = a.center.x;
        if (ABS(itemHorizontalCenter - horizontalCenter) < ABS(offsetAdjustment)) {
            offsetAdjustment = itemHorizontalCenter - horizontalCenter;
            targetAttributes = a;
        }
    }

    return CGPointMake(proposedContentOffset.x + offsetAdjustment, proposedContentOffset.y);
}

但这并不完全正确。有时它会跳过一个项目并在此之后捕捉到下一个项目（也就是说，它不会捕捉到紧挨我开始滚动的项目旁边的项目，而是紧邻下一个项目的项目）。

有什么想法吗？

Answer 1

好的，所以我想出了如何通过使所有内容都与当前的 contentOffset 相关来使其工作：

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{
    if (proposedContentOffset.x > self.collectionView.contentOffset.x) {
        proposedContentOffset.x = self.collectionView.contentOffset.x + self.collectionView.bounds.size.width / 2.;
    }
    else if (proposedContentOffset.x < self.collectionView.contentOffset.x) {
        proposedContentOffset.x = self.collectionView.contentOffset.x - self.collectionView.bounds.size.width / 2.;
    }

    CGFloat offsetAdjustment = MAXFLOAT;
    CGFloat horizontalCenter = proposedContentOffset.x + self.collectionView.bounds.size.width / 2.;
    CGRect targetRect = CGRectMake(proposedContentOffset.x, 0., self.collectionView.bounds.size.width, self.collectionView.bounds.size.height);

    NSArray *attributes = [super layoutAttributesForElementsInRect:targetRect];
    for (UICollectionViewLayoutAttributes *a in attributes) {
        CGFloat itemHorizontalCenter = a.center.x;
        if (ABS(itemHorizontalCenter - horizontalCenter) < ABS(offsetAdjustment)) {
            offsetAdjustment = itemHorizontalCenter - horizontalCenter;
        }
    }

    return CGPointMake(proposedContentOffset.x + offsetAdjustment, proposedContentOffset.y);
}