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);
}
你可以在你的targetContentOffsetForProposedContentOffset:withScrollingVelocity:子类中覆盖UICollectionViewLayout方法，并像这样计算你的偏移量：
@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)
}
}
你可以使用代码self.collectionView.scrollToItem(at: indexPath, at: .centeredHorizontally, animated: true)
这是@ dmitry-zhukov的Swift 3版本（谢谢顺便说一句！）
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
}
}
我找到了很多信息和解决方案。
现在，我用这个。
在UICollectionViewFlowLayout覆盖：
override public func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
if display != .inline {
return super.targetContentOffset(forProposedContentOffset: proposedContentOffset, withScrollingVelocity: velocity)
}
guard let collectionView = collectionView else {
return super.targetContentOffset(forProposedContentOffset: proposedContentOffset, withScrollingVelocity: velocity)
}
let willtoNextX: CGFloat
if proposedContentOffset.x <= 0 || collectionView.contentOffset == proposedContentOffset {
willtoNextX = proposedContentOffset.x
} else {
let width = collectionView.bounds.size.width
willtoNextX = collectionView.contentOffset.x + (velocity.x > 0 ? width : -width)
}
let targetRect = CGRect(x: willtoNextX, y: 0, width: collectionView.bounds.size.width, height: collectionView.bounds.size.height)
var offsetAdjustCoefficient = CGFloat.greatestFiniteMagnitude
let horizontalOffset = proposedContentOffset.x + collectionView.contentInset.left
let layoutAttributesArray = super.layoutAttributesForElements(in: targetRect)
layoutAttributesArray?.forEach({ (layoutAttributes) in
let itemOffset = layoutAttributes.frame.origin.x
if fabsf(Float(itemOffset - horizontalOffset)) < fabsf(Float(offsetAdjustCoefficient)) {
offsetAdjustCoefficient = itemOffset - horizontalOffset
}
})
return CGPoint(x: proposedContentOffset.x + offsetAdjustCoefficient, y: proposedContentOffset.y)
}
并在UICollectionViewController上：
collectionView.decelerationRate = .fast
collectionView.isPagingEnabled = false
collectionView.contentInset = UIEdgeInsets.init(top: 0, left: 16, bottom: 0, right: 16)
现在，细胞在中心!!
我不知道为什么每个人的答案都是如此复杂，我只需在Interface Builder中打开Paging enabled就可以了。
设置适当的itemSize，left和right插入后，我更喜欢这样做而不是子类化布局
//Setting decelerationRate to fast gives a nice experience
collectionView.decelerationRate = .fast
//Add this to your view anywhere
func centerCell () {
let centerPoint = CGPoint(x: collectionView.contentOffset.x + collectionView.frame.midX, y: 100)
if let path = collectionView.indexPathForItem(at: centerPoint) {
collectionView.scrollToItem(at: path, at: .centeredHorizontally, animated: true)
}
}
//Set collectionView.delegate = self then add below funcs
func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
centerCell()
}
func scrollViewDidEndScrollingAnimation(_ scrollView: UIScrollView) {
centerCell()
}
func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
if !decelerate {
centerCell()
}
}