水平滚动时，UICollectionView会捕捉到单元格

时间：2015-11-22 14:35:43

标签： ios objective-c uiscrollview uicollectionview

我知道有些人之前已经问过这个问题，但他们都是关于UITableViews或UIScrollViews，我无法获得为我工作的公认解决方案。我想要的是横向滚动UICollectionView时的捕捉效果 - 就像在iOS AppStore中发生的那样。 iOS 9+是我的目标版本，因此请在回答之前先查看UIKit的更改。

感谢。

16 个答案:

答案 0 :(得分：78)

虽然最初我使用的是Objective-C，但是我自从切换到Swift并且原来接受的答案还不够。

我最终创建了一个UICollectionViewLayout子类，它提供了最佳（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函数。

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)
    }
}

对于使用当前布局子类的最原始感觉减速，请确保设置以下内容：

collectionView?.decelerationRate = UIScrollViewDecelerationRateFast

答案 1 :(得分：20)

For what it is worth here is a simple calculation that I use (in swift):

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
        }
    }
}

Call where you need it. I call it in

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

And

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

Where collectionViewFlowLayout could come from:

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()

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

答案 2 :(得分：16)

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()
    }
}

答案 3 :(得分：14)

根据 Mete 的回答和 Chris Chute的评论 ，

这里有一个Swift 4扩展程序，可以完成OP想要的操作。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)
        }
    }
}

您需要为集合视图实现UIScrollViewDelegate协议，然后添加以下两种方法：

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

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

答案 4 :(得分：11)

按照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)
  }
}

答案 5 :(得分：4)

这是我的实现

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)
    }

像这样实施滚动视图委托 

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

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

另外，为了更好的捕捉

self.cvCollectionView.decelerationRate = UIScrollViewDecelerationRateFast

像魅力一样工作

答案 6 :(得分：4)

如果您想要简单的原生行为，而无需自定义：

collectionView.pagingEnabled = YES;

仅当集合视图布局项的大小仅为一个大小且UICollectionViewCell的{​​{1}}属性设置为clipToBounds时，此方法才能正常工作。

答案 7 :(得分：3)

来自SO帖子here和文档here

的回答

首先您可以设置您的收藏视图的滚动视图，通过将您的班级设为滚动视图代表

来委派您的课程 
MyViewController : SuperViewController<... ,UIScrollViewDelegate>

然后将视图控制器设置为代理 

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

或者在界面构建器中通过控制+移动单击您的集合视图然后控制+拖动或右键单击拖动到视图控制器并选择委托来执行此操作。 （你应该知道怎么做）。 那不起作用。 UICollectionView是UIScrollView的子类，因此您现在可以通过控制+转换单击

在界面构建器中查看它

接下来实施委托方法- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView 

MyViewController.m

... 

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

}

文档声明：

  

<强>参数

     

scrollView |正在减速滚动的滚动视图对象   内容视图。

     

讨论滚动视图在滚动时调用此方法   运动停止了。 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滚动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.

}

答案 8 :(得分：2)

您还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];
    }
}

答案 9 :(得分：2)

此解决方案可提供更好，更流畅的动画。

Swift 3

要使第一个和最后一个项目居中添加插入内容：

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

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

然后使用targetContentOffset方法中的scrollViewWillEndDragging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
}

也许在你的情况下，totalContentWidth的计算方式不同，f.e。如果没有minimumInteritemSpacing，请相应调整。 您也可以使用300

中使用的velocity

P.S。确保该类采用UICollectionViewDataSource协议

答案 10 :(得分：2)

我刚刚找到了我认为解决这个问题的最佳解决方案：

首先将目标添加到collectionView已经存在的gestureRecognizer：

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

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

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

然后在此方法中，强制collectionView在平移手势结束时滚动到相应的单元格。 我这样做是通过从集合视图中获取可见项目并根据平移方向确定我想要滚动到哪个项目。

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。我确定这种方法适用于更多项目。

答案 11 :(得分：2)

摘自2012年WWDC视频中的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);
    }

我之所以要问这个问题，是因为它具有本机感，这是我从Mark的可接受答案中得到的...我把它放到collectionView的视图控制器中。

collectionView.decelerationRate = UIScrollViewDecelerationRateFast;

答案 12 :(得分：1)

我一直在通过在uicollectionview的属性检查器上设置“已启用分页”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，就会发生这种情况。

不涉及编码。

答案 13 :(得分：0)

这是一个Swift 3.0版本，根据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)
  }

答案 14 :(得分：0)

迅速4.2。简单。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)
    }

}

答案 15 :(得分：0)

我尝试了@Mark Bourke和@mrcrowley解决方案，但它们给出了相同的结果，但具有不良的粘滞效果。

我设法通过考虑velocity来解决问题。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)
}

}

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