水平滚动 UICollectionView 时对齐单元格的中心

Question

我知道以前有人问过这个问题，但他们都是关于 UITableViews 或 UIScrollViews 的，而我无法获得适用于我的公认解决方案。我想要的是水平滚动我的UICollectionView 时的捕捉效果——就像在 iOS AppStore 中发生的一样。 iOS 9+ 是我的目标版本，所以在回答这个问题之前请先查看 UIKit 的变化。

谢谢。

Answer 1

虽然我最初使用的是 Objective-C，但后来我改用了 Swift，原来接受的答案还不够。

我最终创建了一个 UICollectionViewLayout 子类，它提供了最好的 (imo) 体验，而不是在用户停止滚动时改变内容偏移或类似内容的其他功能。

class SnappingCollectionViewLayout: UICollectionViewFlowLayout {

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

        var offsetAdjustment = CGFloat.greatestFiniteMagnitude
        let horizontalOffset = proposedContentOffset.x + collectionView.contentInset.left

        let targetRect = CGRect(x: proposedContentOffset.x, y: 0, width: collectionView.bounds.size.width, height: collectionView.bounds.size.height)

        let layoutAttributesArray = super.layoutAttributesForElements(in: targetRect)

        layoutAttributesArray?.forEach({ (layoutAttributes) in
            let itemOffset = layoutAttributes.frame.origin.x
            if fabsf(Float(itemOffset - horizontalOffset)) < fabsf(Float(offsetAdjustment)) {
                offsetAdjustment = itemOffset - horizontalOffset
            }
        })

        return CGPoint(x: proposedContentOffset.x + offsetAdjustment, y: proposedContentOffset.y)
    }
}

对于当前布局子类最原生的感觉减速，请确保设置以下内容：

collectionView?.decelerationRate = UIScrollViewDecelerationRateFast

Answer 2

根据 Mete 的回答和 Chris Chute 的评论，

这是一个 Swift 4 扩展，它可以满足 OP 的需求。它在单行和双行嵌套集合视图上进行了测试，效果很好。

extension UICollectionView {
    func scrollToNearestVisibleCollectionViewCell() {
        self.decelerationRate = UIScrollViewDecelerationRateFast
        let visibleCenterPositionOfScrollView = Float(self.contentOffset.x + (self.bounds.size.width / 2))
        var closestCellIndex = -1
        var closestDistance: Float = .greatestFiniteMagnitude
        for i in 0..<self.visibleCells.count {
            let cell = self.visibleCells[i]
            let cellWidth = cell.bounds.size.width
            let cellCenter = Float(cell.frame.origin.x + cellWidth / 2)

            // Now calculate closest cell
            let distance: Float = fabsf(visibleCenterPositionOfScrollView - cellCenter)
            if distance < closestDistance {
                closestDistance = distance
                closestCellIndex = self.indexPath(for: cell)!.row
            }
        }
        if closestCellIndex != -1 {
            self.scrollToItem(at: IndexPath(row: closestCellIndex, section: 0), at: .centeredHorizontally, animated: true)
        }
    }
}

你需要为你的collection view实现UIScrollViewDelegate协议，然后添加这两个方法：

func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
    self.collectionView.scrollToNearestVisibleCollectionViewCell()
}

func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
    if !decelerate {
        self.collectionView.scrollToNearestVisibleCollectionViewCell()
    }
}

Answer 3

根据滚动速度捕捉到最近的单元格。

没有任何故障。

import UIKit

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

    let itemSpace = itemSize.width + minimumInteritemSpacing
    var currentItemIdx = round(collectionView.contentOffset.x / itemSpace)

    // Skip to the next cell, if there is residual scrolling velocity left.
    // This helps to prevent glitches
    let vX = velocity.x
    if vX > 0 {
      currentItemIdx += 1
    } else if vX < 0 {
      currentItemIdx -= 1
    }

    let nearestPageOffset = currentItemIdx * itemSpace
    return CGPoint(x: nearestPageOffset,
                   y: parent.y)
  }
}

Answer 4

这里值得我用一个简单的计算（快速）：

func snapToNearestCell(_ collectionView: UICollectionView) {
    for i in 0..<collectionView.numberOfItems(inSection: 0) {

        let itemWithSpaceWidth = collectionViewFlowLayout.itemSize.width + collectionViewFlowLayout.minimumLineSpacing
        let itemWidth = collectionViewFlowLayout.itemSize.width

        if collectionView.contentOffset.x <= CGFloat(i) * itemWithSpaceWidth + itemWidth / 2 {                
            let indexPath = IndexPath(item: i, section: 0)
            collectionView.scrollToItem(at: indexPath, at: .centeredHorizontally, animated: true)
            break
        }
    }
}

在需要的地方打电话。我叫它

func scrollViewDidEndDragging(scrollView: UIScrollView, willDecelerate decelerate: Bool) {
    snapToNearestCell(scrollView)
}

和

func scrollViewDidEndDecelerating(scrollView: UIScrollView) {
    snapToNearestCell(scrollView)
}

collectionViewFlowLayout 的来源：

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()

    // Set up collection view
    collectionViewFlowLayout = collectionView.collectionViewLayout as! UICollectionViewFlowLayout
}

Answer 5

SWIFT 3 版@Iowa15 回复

func scrollToNearestVisibleCollectionViewCell() {
    let visibleCenterPositionOfScrollView = Float(collectionView.contentOffset.x + (self.collectionView!.bounds.size.width / 2))
    var closestCellIndex = -1
    var closestDistance: Float = .greatestFiniteMagnitude
    for i in 0..<collectionView.visibleCells.count {
        let cell = collectionView.visibleCells[i]
        let cellWidth = cell.bounds.size.width
        let cellCenter = Float(cell.frame.origin.x + cellWidth / 2)

        // Now calculate closest cell
        let distance: Float = fabsf(visibleCenterPositionOfScrollView - cellCenter)
        if distance < closestDistance {
            closestDistance = distance
            closestCellIndex = collectionView.indexPath(for: cell)!.row
        }
    }
    if closestCellIndex != -1 {
        self.collectionView!.scrollToItem(at: IndexPath(row: closestCellIndex, section: 0), at: .centeredHorizontally, animated: true)
    }
}

需要在UIScrollViewDelegate中实现：

func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
    scrollToNearestVisibleCollectionViewCell()
}

func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
    if !decelerate {
        scrollToNearestVisibleCollectionViewCell()
    }
}

Answer 6

这是我的实现

func snapToNearestCell(scrollView: UIScrollView) {
     let middlePoint = Int(scrollView.contentOffset.x + UIScreen.main.bounds.width / 2)
     if let indexPath = self.cvCollectionView.indexPathForItem(at: CGPoint(x: middlePoint, y: 0)) {
          self.cvCollectionView.scrollToItem(at: indexPath, at: .centeredHorizontally, animated: true)
     }
}

像这样实现你的滚动视图代理

func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
    self.snapToNearestCell(scrollView: scrollView)
}

func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
    self.snapToNearestCell(scrollView: scrollView)
}

另外，为了更好地捕捉

self.cvCollectionView.decelerationRate = UIScrollViewDecelerationRateFast

像魅力一样工作

Answer 7

如果您想要简单的原生行为，无需自定义：

collectionView.pagingEnabled = YES;

这只有在集合视图布局项的大小都是一个大小并且UICollectionViewCell 的clipToBounds 属性设置为YES 时才能正常工作。

Answer 8

我尝试了@Mark Bourke 和@mrcrowley 两种解决方案，但它们给出的结果几乎相同，但具有不需要的粘性效果。

考虑到velocity，我设法解决了这个问题。这是完整的代码。

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

    var offsetAdjusment = CGFloat.greatestFiniteMagnitude
    let horizontalCenter = proposedContentOffset.x + (collectionView.bounds.width / 2)

    let targetRect = CGRect(x: proposedContentOffset.x, y: 0, width: collectionView.bounds.size.width, height: collectionView.bounds.size.height)
    let layoutAttributesArray = super.layoutAttributesForElements(in: targetRect)
    layoutAttributesArray?.forEach({ (layoutAttributes) in
        let itemHorizontalCenter = layoutAttributes.center.x

        if abs(itemHorizontalCenter - horizontalCenter) < abs(offsetAdjusment) {
            if abs(velocity.x) < 0.3 { // minimum velocityX to trigger the snapping effect
                offsetAdjusment = itemHorizontalCenter - horizontalCenter
            } else if velocity.x > 0 {
                offsetAdjusment = itemHorizontalCenter - horizontalCenter + layoutAttributes.bounds.width
            } else { // velocity.x < 0
                offsetAdjusment = itemHorizontalCenter - horizontalCenter - layoutAttributes.bounds.width
            }
        }
    })

    return CGPoint(x: proposedContentOffset.x + offsetAdjusment, y: proposedContentOffset.y)
}

}

Answer 9

得到了来自 SO 帖子 here 和文档 here 的答案

首先你可以做的是通过让你的类成为滚动视图委托来设置你的集合视图的滚动视图的委托你的类

MyViewController : SuperViewController<... ,UIScrollViewDelegate>

然后将您的视图控制器设置为委托

UIScrollView *scrollView = (UIScrollView *)super.self.collectionView;
scrollView.delegate = self;

或者在界面构建器中通过 control + shift 单击您的集合视图然后控制 + 拖动或右键单击拖动到您的视图控制器并选择委托。 （你应该知道如何做到这一点）。 这行不通。 UICollectionView 是 UIScrollView 的子类，因此您现在可以通过 control + shift 点击在界面构建器中看到它

接下来实现委托方法- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView

MyViewController.m

... 

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{

}

文档声明：

参数

滚动视图 |正在减速滚动的滚动视图对象 的内容视图。

讨论 滚动视图在滚动时调用该方法 运动停止。 UIScrollView的减速属性 控制减速。

可用性适用于 iOS 2.0 及更高版本。

然后在该方法内部检查哪个单元格在停止滚动时最接近滚动视图的中心

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
  //NSLog(@"%f", truncf(scrollView.contentOffset.x + (self.pictureCollectionView.bounds.size.width / 2)));

float visibleCenterPositionOfScrollView = scrollView.contentOffset.x + (self.pictureCollectionView.bounds.size.width / 2);

//NSLog(@"%f", truncf(visibleCenterPositionOfScrollView / imageArray.count));


NSInteger closestCellIndex;

for (id item in imageArray) {
    // equation to use to figure out closest cell
    // abs(visibleCenter - cellCenterX) <= (cellWidth + cellSpacing/2)

    // Get cell width (and cell too)
    UICollectionViewCell *cell = (UICollectionViewCell *)[self collectionView:self.pictureCollectionView cellForItemAtIndexPath:[NSIndexPath indexPathWithIndex:[imageArray indexOfObject:item]]];
    float cellWidth = cell.bounds.size.width;

    float cellCenter = cell.frame.origin.x + cellWidth / 2;

    float cellSpacing = [self collectionView:self.pictureCollectionView layout:self.pictureCollectionView.collectionViewLayout minimumInteritemSpacingForSectionAtIndex:[imageArray indexOfObject:item]];

    // Now calculate closest cell

    if (fabsf(visibleCenterPositionOfScrollView - cellCenter) <= (cellWidth + (cellSpacing / 2))) {
        closestCellIndex = [imageArray indexOfObject:item];
        break;
    }
}

if (closestCellIndex != nil) {

[self.pictureCollectionView scrollToItemAtIndexPath:[NSIndexPath indexPathWithIndex:closestCellIndex] atScrollPosition:UICollectionViewScrollPositionCenteredVertically animated:YES];

// This code is untested. Might not work.

}

Answer 10

您也可以尝试修改上述答案：

-(void)scrollToNearestVisibleCollectionViewCell {
    float visibleCenterPositionOfScrollView = _collectionView.contentOffset.x + (self.collectionView.bounds.size.width / 2);

    NSInteger closestCellIndex = -1;
    float closestDistance = FLT_MAX;
    for (int i = 0; i < _collectionView.visibleCells.count; i++) {
        UICollectionViewCell *cell = _collectionView.visibleCells[i];
        float cellWidth = cell.bounds.size.width;

        float cellCenter = cell.frame.origin.x + cellWidth / 2;

        // Now calculate closest cell
        float distance = fabsf(visibleCenterPositionOfScrollView - cellCenter);
        if (distance < closestDistance) {
            closestDistance = distance;
            closestCellIndex = [_collectionView indexPathForCell:cell].row;
        }
    }

    if (closestCellIndex != -1) {
        [self.collectionView scrollToItemAtIndexPath:[NSIndexPath indexPathForRow:closestCellIndex inSection:0] atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally animated:YES];
    }
}

Answer 11

我刚刚找到了我认为是解决这个问题的最佳方法：

首先给collectionView已经存在的gestureRecognizer添加一个target：

[self.collectionView.panGestureRecognizer addTarget:self action:@selector(onPan:)];

让选择器指向一个以 UIPanGestureRecognizer 作为参数的方法：

- (void)onPan:(UIPanGestureRecognizer *)recognizer {};

然后在这个方法中，当平移手势结束时强制collectionView滚动到合适的单元格。 我通过从集合视图中获取可见项目并根据平移的方向确定要滚动到哪个项目来做到这一点。

if (recognizer.state == UIGestureRecognizerStateEnded) {

        // Get the visible items
        NSArray<NSIndexPath *> *indexes = [self.collectionView indexPathsForVisibleItems];
        int index = 0;

        if ([(UIPanGestureRecognizer *)recognizer velocityInView:self.view].x > 0) {
            // Return the smallest index if the user is swiping right
            for (int i = index;i < indexes.count;i++) {
                if (indexes[i].row < indexes[index].row) {
                    index = i;
                }
            }
        } else {
            // Return the biggest index if the user is swiping left
            for (int i = index;i < indexes.count;i++) {
                if (indexes[i].row > indexes[index].row) {
                    index = i;
                }
            }
        }
        // Scroll to the selected item
        [self.collectionView scrollToItemAtIndexPath:indexes[index] atScrollPosition:UICollectionViewScrollPositionLeft animated:YES];
    }

请记住，在我的情况下，一次只能看到两个项目。不过，我确信这种方法可以适用于更多项目。

Answer 12

这来自 2012 年 WWDC 视频，用于 Objective-C 解决方案。我将 UICollectionViewFlowLayout 子类化并添加了以下内容。

-(CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
    {
        CGFloat offsetAdjustment = MAXFLOAT;
        CGFloat horizontalCenter = proposedContentOffset.x + (CGRectGetWidth(self.collectionView.bounds) / 2);

        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);
    }

我提出这个问题的原因是为了以一种原生的感觉捕捉，这是我从 Mark 接受的答案中得到的……我把它放在了 collectionView 的视图控制器中。

collectionView.decelerationRate = UIScrollViewDecelerationRateFast;

Answer 13

此解决方案提供了更好、更流畅的动画效果。

斯威夫特 3

要使第一个和最后一个项目居中添加插图：

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {

    return UIEdgeInsetsMake(0, cellWidth/2, 0, cellWidth/2)
}

然后使用scrollViewWillEndDragging方法中的targetContentOffset来改变结束位置。

func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {

    let numOfItems = collectionView(mainCollectionView, numberOfItemsInSection:0)
    let totalContentWidth = scrollView.contentSize.width + mainCollectionViewFlowLayout.minimumInteritemSpacing - cellWidth
    let stopOver = totalContentWidth / CGFloat(numOfItems)

    var targetX = round((scrollView.contentOffset.x + (velocity.x * 300)) / stopOver) * stopOver
    targetX = max(0, min(targetX, scrollView.contentSize.width - scrollView.frame.width))

    targetContentOffset.pointee.x = targetX
}

也许在您的情况下，totalContentWidth 的计算方式不同，例如没有minimumInteritemSpacing，所以相应地调整它。 您也可以使用velocity 中使用的300

附：确保类采用UICollectionViewDataSource 协议

Answer 14

我一直在通过在 uicollectionview 的属性检查器上设置“启用分页”来解决此问题。

对我来说，当单元格的宽度与 uicollectionview 的宽度相同时会发生这种情况。

不涉及编码。

Answer 15

这是一个 Swift 3.0 版本，根据上面 Mark 的建议，它应该适用于水平和垂直方向：

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

    guard
      let collectionView = collectionView
    else {
      return super.targetContentOffset(
        forProposedContentOffset: proposedContentOffset,
        withScrollingVelocity: velocity
      )
    }

    let realOffset = CGPoint(
      x: proposedContentOffset.x + collectionView.contentInset.left,
      y: proposedContentOffset.y + collectionView.contentInset.top
    )

    let targetRect = CGRect(origin: proposedContentOffset, size: collectionView.bounds.size)

    var offset = (scrollDirection == .horizontal)
      ? CGPoint(x: CGFloat.greatestFiniteMagnitude, y:0.0)
      : CGPoint(x:0.0, y:CGFloat.greatestFiniteMagnitude)

    offset = self.layoutAttributesForElements(in: targetRect)?.reduce(offset) {
      (offset, attr) in
      let itemOffset = attr.frame.origin
      return CGPoint(
        x: abs(itemOffset.x - realOffset.x) < abs(offset.x) ? itemOffset.x - realOffset.x : offset.x,
        y: abs(itemOffset.y - realOffset.y) < abs(offset.y) ? itemOffset.y - realOffset.y : offset.y
      )
    } ?? .zero

    return CGPoint(x: proposedContentOffset.x + offset.x, y: proposedContentOffset.y + offset.y)
  }

Answer 16

斯威夫特 4.2。简单的。对于固定的 itemSize。水平流向。

func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {

    if let layout = collectionView.collectionViewLayout as? UICollectionViewFlowLayout {
        let floatingPage = targetContentOffset.pointee.x/scrollView.bounds.width
        let rule: FloatingPointRoundingRule = velocity.x > 0 ? .up : .down
        let page = CGFloat(Int(floatingPage.rounded(rule)))
        targetContentOffset.pointee.x = page*(layout.itemSize.width + layout.minimumLineSpacing)
    }

}