我的应用中有一个非常简单的 collectionView（只有一行方形缩略图）。
我想拦截滚动，以便偏移量始终在左侧留下完整的图像。目前它滚动到任何地方并会留下截断的图像。
无论如何，我知道我需要使用该功能
- (CGPoint)targetContentOffsetForProposedContentOffset:withScrollingVelocity
要做到这一点，但我只是使用标准的UICollectionViewFlowLayout. 我没有继承它。
有没有办法在没有子类化的情况下拦截它UICollectionViewFlowLayout？
谢谢
好的，答案是否定的，没有子类化 UICollectionViewFlowLayout 就没有办法做到这一点。
但是，对于将来阅读本文的任何人来说，对其进行子类化都非常容易。
首先我设置了子类调用MyCollectionViewFlowLayout，然后在界面生成器中我将集合视图布局更改为自定义并选择了我的流布局子类。
因为您这样做是因为您无法在 IB 中指定项目大小等...所以在 MyCollectionViewFlowLayout.m 我有这个...
- (void)awakeFromNib
{
self.itemSize = CGSizeMake(75.0, 75.0);
self.minimumInteritemSpacing = 10.0;
self.minimumLineSpacing = 10.0;
self.scrollDirection = UICollectionViewScrollDirectionHorizontal;
self.sectionInset = UIEdgeInsetsMake(10.0, 10.0, 10.0, 10.0);
}
这为我设置了所有尺寸和滚动方向。
然后 ...
- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{
CGFloat offsetAdjustment = MAXFLOAT;
CGFloat horizontalOffset = proposedContentOffset.x + 5;
CGRect targetRect = CGRectMake(proposedContentOffset.x, 0, self.collectionView.bounds.size.width, self.collectionView.bounds.size.height);
NSArray *array = [super layoutAttributesForElementsInRect:targetRect];
for (UICollectionViewLayoutAttributes *layoutAttributes in array) {
CGFloat itemOffset = layoutAttributes.frame.origin.x;
if (ABS(itemOffset - horizontalOffset) < ABS(offsetAdjustment)) {
offsetAdjustment = itemOffset - horizontalOffset;
}
}
return CGPointMake(proposedContentOffset.x + offsetAdjustment, proposedContentOffset.y);
}
这确保了滚动在左侧边缘以 5.0 的边距结束。
这就是我需要做的。我根本不需要在代码中设置流布局。
丹的解决方案是有缺陷的。它不能很好地处理用户轻弹。用户快速滑动和滚动并没有移动太多的情况下，会出现动画故障。
我提出的替代实现具有与之前提出的相同的分页，但处理用户在页面之间轻弹。
#pragma mark - Pagination
- (CGFloat)pageWidth {
return self.itemSize.width + self.minimumLineSpacing;
}
- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{
CGFloat rawPageValue = self.collectionView.contentOffset.x / self.pageWidth;
CGFloat currentPage = (velocity.x > 0.0) ? floor(rawPageValue) : ceil(rawPageValue);
CGFloat nextPage = (velocity.x > 0.0) ? ceil(rawPageValue) : floor(rawPageValue);
BOOL pannedLessThanAPage = fabs(1 + currentPage - rawPageValue) > 0.5;
BOOL flicked = fabs(velocity.x) > [self flickVelocity];
if (pannedLessThanAPage && flicked) {
proposedContentOffset.x = nextPage * self.pageWidth;
} else {
proposedContentOffset.x = round(rawPageValue) * self.pageWidth;
}
return proposedContentOffset;
}
- (CGFloat)flickVelocity {
return 0.3;
}
已接受答案的 Swift 版本。
override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
var offsetAdjustment = CGFloat.greatestFiniteMagnitude
let horizontalOffset = proposedContentOffset.x
let targetRect = CGRect(origin: CGPoint(x: proposedContentOffset.x, y: 0), size: self.collectionView!.bounds.size)
for layoutAttributes in super.layoutAttributesForElements(in: targetRect)! {
let itemOffset = layoutAttributes.frame.origin.x
if (abs(itemOffset - horizontalOffset) < abs(offsetAdjustment)) {
offsetAdjustment = itemOffset - horizontalOffset
}
}
return CGPoint(x: proposedContentOffset.x + offsetAdjustment, y: proposedContentOffset.y)
}
适用于Swift 5。
这是我在Swift 5中针对基于单元的垂直分页的实现：
override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
guard let collectionView = self.collectionView else {
let latestOffset = super.targetContentOffset(forProposedContentOffset: proposedContentOffset, withScrollingVelocity: velocity)
return latestOffset
}
// Page height used for estimating and calculating paging.
let pageHeight = self.itemSize.height + self.minimumLineSpacing
// Make an estimation of the current page position.
let approximatePage = collectionView.contentOffset.y/pageHeight
// Determine the current page based on velocity.
let currentPage = velocity.y == 0 ? round(approximatePage) : (velocity.y < 0.0 ? floor(approximatePage) : ceil(approximatePage))
// Create custom flickVelocity.
let flickVelocity = velocity.y * 0.3
// Check how many pages the user flicked, if <= 1 then flickedPages should return 0.
let flickedPages = (abs(round(flickVelocity)) <= 1) ? 0 : round(flickVelocity)
let newVerticalOffset = ((currentPage + flickedPages) * pageHeight) - collectionView.contentInset.top
return CGPoint(x: proposedContentOffset.x, y: newVerticalOffset)
}
一些注意事项：
itemSize实际匹配项目的大小，因为这通常是一个问题，尤其是在使用时collectionView(_:layout:sizeForItemAt:)，请改用带有 itemSize 的自定义变量。self.collectionView.decelerationRate = UIScrollView.DecelerationRate.fast。这是一个横向版本（尚未彻底测试，所以请原谅任何错误）：
override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
guard let collectionView = self.collectionView else {
let latestOffset = super.targetContentOffset(forProposedContentOffset: proposedContentOffset, withScrollingVelocity: velocity)
return latestOffset
}
// Page width used for estimating and calculating paging.
let pageWidth = self.itemSize.width + self.minimumInteritemSpacing
// Make an estimation of the current page position.
let approximatePage = collectionView.contentOffset.x/pageWidth
// Determine the current page based on velocity.
let currentPage = velocity.x == 0 ? round(approximatePage) : (velocity.x < 0.0 ? floor(approximatePage) : ceil(approximatePage))
// Create custom flickVelocity.
let flickVelocity = velocity.x * 0.3
// Check how many pages the user flicked, if <= 1 then flickedPages should return 0.
let flickedPages = (abs(round(flickVelocity)) <= 1) ? 0 : round(flickVelocity)
// Calculate newHorizontalOffset.
let newHorizontalOffset = ((currentPage + flickedPages) * pageWidth) - collectionView.contentInset.left
return CGPoint(x: newHorizontalOffset, y: proposedContentOffset.y)
}
此代码基于我在个人项目中使用的代码，您可以通过下载并运行示例目标来查看它。
对于任何寻求解决方案的人...
collectionView.contentInset（和 iPhone X 上的安全区域）那么请看下面...
public class CarouselCollectionViewLayout: UICollectionViewFlowLayout {
override public func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
guard let collectionView = collectionView else {
return super.targetContentOffset(forProposedContentOffset: proposedContentOffset, withScrollingVelocity: velocity)
}
// Identify the layoutAttributes of cells in the vicinity of where the scroll view will come to rest
let targetRect = CGRect(origin: proposedContentOffset, size: collectionView.bounds.size)
let visibleCellsLayoutAttributes = layoutAttributesForElements(in: targetRect)
// Translate those cell layoutAttributes into potential (candidate) scrollView offsets
let candidateOffsets: [CGFloat]? = visibleCellsLayoutAttributes?.map({ cellLayoutAttributes in
if #available(iOS 11.0, *) {
return cellLayoutAttributes.frame.origin.x - collectionView.contentInset.left - collectionView.safeAreaInsets.left - sectionInset.left
} else {
return cellLayoutAttributes.frame.origin.x - collectionView.contentInset.left - sectionInset.left
}
})
// Now we need to work out which one of the candidate offsets is the best one
let bestCandidateOffset: CGFloat
if velocity.x > 0 {
// If the scroll velocity was POSITIVE, then only consider cells/offsets to the RIGHT of the proposedContentOffset.x
// Of the cells/offsets to the right, the NEAREST is the `bestCandidate`
// If there is no nearestCandidateOffsetToLeft then we default to the RIGHT-MOST (last) of ALL the candidate cells/offsets
// (this handles the scenario where the user has scrolled beyond the last cell)
let candidateOffsetsToRight = candidateOffsets?.toRight(ofProposedOffset: proposedContentOffset.x)
let nearestCandidateOffsetToRight = candidateOffsetsToRight?.nearest(toProposedOffset: proposedContentOffset.x)
bestCandidateOffset = nearestCandidateOffsetToRight ?? candidateOffsets?.last ?? proposedContentOffset.x
}
else if velocity.x < 0 {
// If the scroll velocity was NEGATIVE, then only consider cells/offsets to the LEFT of the proposedContentOffset.x
// Of the cells/offsets to the left, the NEAREST is the `bestCandidate`
// If there is no nearestCandidateOffsetToLeft then we default to the LEFT-MOST (first) of ALL the candidate cells/offsets
// (this handles the scenario where the user has scrolled beyond the first cell)
let candidateOffsetsToLeft = candidateOffsets?.toLeft(ofProposedOffset: proposedContentOffset.x)
let nearestCandidateOffsetToLeft = candidateOffsetsToLeft?.nearest(toProposedOffset: proposedContentOffset.x)
bestCandidateOffset = nearestCandidateOffsetToLeft ?? candidateOffsets?.first ?? proposedContentOffset.x
}
else {
// If the scroll velocity was ZERO we consider all `candidate` cells (regarless of whether they are to the left OR right of the proposedContentOffset.x)
// The cell/offset that is the NEAREST is the `bestCandidate`
let nearestCandidateOffset = candidateOffsets?.nearest(toProposedOffset: proposedContentOffset.x)
bestCandidateOffset = nearestCandidateOffset ?? proposedContentOffset.x
}
return CGPoint(x: bestCandidateOffset, y: proposedContentOffset.y)
}
}
fileprivate extension Sequence where Iterator.Element == CGFloat {
func toLeft(ofProposedOffset proposedOffset: CGFloat) -> [CGFloat] {
return filter() { candidateOffset in
return candidateOffset < proposedOffset
}
}
func toRight(ofProposedOffset proposedOffset: CGFloat) -> [CGFloat] {
return filter() { candidateOffset in
return candidateOffset > proposedOffset
}
}
func nearest(toProposedOffset proposedOffset: CGFloat) -> CGFloat? {
guard let firstCandidateOffset = first(where: { _ in true }) else {
// If there are no elements in the Sequence, return nil
return nil
}
return reduce(firstCandidateOffset) { (bestCandidateOffset: CGFloat, candidateOffset: CGFloat) -> CGFloat in
let candidateOffsetDistanceFromProposed = fabs(candidateOffset - proposedOffset)
let bestCandidateOffsetDistancFromProposed = fabs(bestCandidateOffset - proposedOffset)
if candidateOffsetDistanceFromProposed < bestCandidateOffsetDistancFromProposed {
return candidateOffset
}
return bestCandidateOffset
}
}
}
虽然这个答案对我有很大帮助，但是当您在一小段距离上快速滑动时会出现明显的闪烁。在设备上重现它要容易得多。
我发现这总是发生在不同的歌声中collectionView.contentOffset.x - proposedContentOffset.x。velocity.x
我的解决方案是确保速度proposedContentOffset大于contentOffset.x正速度，如果速度为负则减少。它在 C# 中，但转换为 Objective C 应该相当简单：
public override PointF TargetContentOffset (PointF proposedContentOffset, PointF scrollingVelocity)
{
/* Determine closest edge */
float offSetAdjustment = float.MaxValue;
float horizontalCenter = (float) (proposedContentOffset.X + (this.CollectionView.Bounds.Size.Width / 2.0));
RectangleF targetRect = new RectangleF (proposedContentOffset.X, 0.0f, this.CollectionView.Bounds.Size.Width, this.CollectionView.Bounds.Size.Height);
var array = base.LayoutAttributesForElementsInRect (targetRect);
foreach (var layoutAttributes in array) {
float itemHorizontalCenter = layoutAttributes.Center.X;
if (Math.Abs (itemHorizontalCenter - horizontalCenter) < Math.Abs (offSetAdjustment)) {
offSetAdjustment = itemHorizontalCenter - horizontalCenter;
}
}
float nextOffset = proposedContentOffset.X + offSetAdjustment;
/*
* ... unless we end up having positive speed
* while moving left or negative speed while moving right.
* This will cause flicker so we resort to finding next page
* in the direction of velocity and use it.
*/
do {
proposedContentOffset.X = nextOffset;
float deltaX = proposedContentOffset.X - CollectionView.ContentOffset.X;
float velX = scrollingVelocity.X;
// If their signs are same, or if either is zero, go ahead
if (Math.Sign (deltaX) * Math.Sign (velX) != -1)
break;
// Otherwise, look for the closest page in the right direction
nextOffset += Math.Sign (scrollingVelocity.X) * SnapStep;
} while (IsValidOffset (nextOffset));
return proposedContentOffset;
}
bool IsValidOffset (float offset)
{
return (offset >= MinContentOffset && offset <= MaxContentOffset);
}
此代码正在使用MinContentOffset，您可以轻松定义它MaxContentOffset。SnapStep就我而言，他们原来是
float MinContentOffset {
get { return -CollectionView.ContentInset.Left; }
}
float MaxContentOffset {
get { return MinContentOffset + CollectionView.ContentSize.Width - ItemSize.Width; }
}
float SnapStep {
get { return ItemSize.Width + MinimumLineSpacing; }
}
经过长时间的测试，我找到了使用自定义单元格宽度（每个单元格都有不同的宽度）捕捉到中心的解决方案，从而修复了闪烁。随意改进脚本。
- (CGPoint) targetContentOffsetForProposedContentOffset: (CGPoint) proposedContentOffset withScrollingVelocity: (CGPoint)velocity
{
CGFloat offSetAdjustment = MAXFLOAT;
CGFloat horizontalCenter = (CGFloat) (proposedContentOffset.x + (self.collectionView.bounds.size.width / 2.0));
//setting fastPaging property to NO allows to stop at page on screen (I have pages lees, than self.collectionView.bounds.size.width)
CGRect targetRect = CGRectMake(self.fastPaging ? proposedContentOffset.x : self.collectionView.contentOffset.x,
0.0,
self.collectionView.bounds.size.width,
self.collectionView.bounds.size.height);
NSArray *attributes = [self layoutAttributesForElementsInRect:targetRect];
NSPredicate *cellAttributesPredicate = [NSPredicate predicateWithBlock: ^BOOL(UICollectionViewLayoutAttributes * _Nonnull evaluatedObject,
NSDictionary<NSString *,id> * _Nullable bindings)
{
return (evaluatedObject.representedElementCategory == UICollectionElementCategoryCell);
}];
NSArray *cellAttributes = [attributes filteredArrayUsingPredicate: cellAttributesPredicate];
UICollectionViewLayoutAttributes *currentAttributes;
for (UICollectionViewLayoutAttributes *layoutAttributes in cellAttributes)
{
CGFloat itemHorizontalCenter = layoutAttributes.center.x;
if (ABS(itemHorizontalCenter - horizontalCenter) < ABS(offSetAdjustment))
{
currentAttributes = layoutAttributes;
offSetAdjustment = itemHorizontalCenter - horizontalCenter;
}
}
CGFloat nextOffset = proposedContentOffset.x + offSetAdjustment;
proposedContentOffset.x = nextOffset;
CGFloat deltaX = proposedContentOffset.x - self.collectionView.contentOffset.x;
CGFloat velX = velocity.x;
// detection form gist.github.com/rkeniger/7687301
// based on http://stackoverflow.com/a/14291208/740949
if (fabs(deltaX) <= FLT_EPSILON || fabs(velX) <= FLT_EPSILON || (velX > 0.0 && deltaX > 0.0) || (velX < 0.0 && deltaX < 0.0))
{
}
else if (velocity.x > 0.0)
{
// revert the array to get the cells from the right side, fixes not correct center on different size in some usecases
NSArray *revertedArray = [[array reverseObjectEnumerator] allObjects];
BOOL found = YES;
float proposedX = 0.0;
for (UICollectionViewLayoutAttributes *layoutAttributes in revertedArray)
{
if(layoutAttributes.representedElementCategory == UICollectionElementCategoryCell)
{
CGFloat itemHorizontalCenter = layoutAttributes.center.x;
if (itemHorizontalCenter > proposedContentOffset.x) {
found = YES;
proposedX = nextOffset + (currentAttributes.frame.size.width / 2) + (layoutAttributes.frame.size.width / 2);
} else {
break;
}
}
}
// dont set on unfound element
if (found) {
proposedContentOffset.x = proposedX;
}
}
else if (velocity.x < 0.0)
{
for (UICollectionViewLayoutAttributes *layoutAttributes in cellAttributes)
{
CGFloat itemHorizontalCenter = layoutAttributes.center.x;
if (itemHorizontalCenter > proposedContentOffset.x)
{
proposedContentOffset.x = nextOffset - ((currentAttributes.frame.size.width / 2) + (layoutAttributes.frame.size.width / 2));
break;
}
}
}
proposedContentOffset.y = 0.0;
return proposedContentOffset;
}
请参阅Dan Abramov 的这个答案，这里是 Swift 版本
override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
var _proposedContentOffset = CGPoint(x: proposedContentOffset.x, y: proposedContentOffset.y)
var offSetAdjustment: CGFloat = CGFloat.max
let horizontalCenter: CGFloat = CGFloat(proposedContentOffset.x + (self.collectionView!.bounds.size.width / 2.0))
let targetRect = CGRect(x: proposedContentOffset.x, y: 0.0, width: self.collectionView!.bounds.size.width, height: self.collectionView!.bounds.size.height)
let array: [UICollectionViewLayoutAttributes] = self.layoutAttributesForElementsInRect(targetRect)! as [UICollectionViewLayoutAttributes]
for layoutAttributes: UICollectionViewLayoutAttributes in array {
if (layoutAttributes.representedElementCategory == UICollectionElementCategory.Cell) {
let itemHorizontalCenter: CGFloat = layoutAttributes.center.x
if (abs(itemHorizontalCenter - horizontalCenter) < abs(offSetAdjustment)) {
offSetAdjustment = itemHorizontalCenter - horizontalCenter
}
}
}
var nextOffset: CGFloat = proposedContentOffset.x + offSetAdjustment
repeat {
_proposedContentOffset.x = nextOffset
let deltaX = proposedContentOffset.x - self.collectionView!.contentOffset.x
let velX = velocity.x
if (deltaX == 0.0 || velX == 0 || (velX > 0.0 && deltaX > 0.0) || (velX < 0.0 && deltaX < 0.0)) {
break
}
if (velocity.x > 0.0) {
nextOffset = nextOffset + self.snapStep()
} else if (velocity.x < 0.0) {
nextOffset = nextOffset - self.snapStep()
}
} while self.isValidOffset(nextOffset)
_proposedContentOffset.y = 0.0
return _proposedContentOffset
}
func isValidOffset(offset: CGFloat) -> Bool {
return (offset >= CGFloat(self.minContentOffset()) && offset <= CGFloat(self.maxContentOffset()))
}
func minContentOffset() -> CGFloat {
return -CGFloat(self.collectionView!.contentInset.left)
}
func maxContentOffset() -> CGFloat {
return CGFloat(self.minContentOffset() + self.collectionView!.contentSize.width - self.itemSize.width)
}
func snapStep() -> CGFloat {
return self.itemSize.width + self.minimumLineSpacing;
}
这是我在水平滚动集合视图上的 Swift 解决方案。它简单，甜美，避免任何闪烁。
override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
guard let collectionView = collectionView else { return proposedContentOffset }
let currentXOffset = collectionView.contentOffset.x
let nextXOffset = proposedContentOffset.x
let maxIndex = ceil(currentXOffset / pageWidth())
let minIndex = floor(currentXOffset / pageWidth())
var index: CGFloat = 0
if nextXOffset > currentXOffset {
index = maxIndex
} else {
index = minIndex
}
let xOffset = pageWidth() * index
let point = CGPointMake(xOffset, 0)
return point
}
func pageWidth() -> CGFloat {
return itemSize.width + minimumInteritemSpacing
}
我在使用 targetContentOffsetForProposedContentOffset 时遇到的一个小问题是最后一个单元格没有根据我返回的新点进行调整的问题。
我发现我返回的 CGPoint 的 Y 值大于允许的值，因此我在 targetContentOffsetForProposedContentOffset 实现的末尾使用了以下代码：
// if the calculated y is bigger then the maximum possible y we adjust accordingly
CGFloat contentHeight = self.collectionViewContentSize.height;
CGFloat collectionViewHeight = self.collectionView.bounds.size.height;
CGFloat maxY = contentHeight - collectionViewHeight;
if (newY > maxY)
{
newY = maxY;
}
return CGPointMake(0, newY);
只是为了更清楚，这是我的完整布局实现，它只是模仿垂直分页行为：
- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{
return [self targetContentOffsetForProposedContentOffset:proposedContentOffset];
}
- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset
{
CGFloat heightOfPage = self.itemSize.height;
CGFloat heightOfSpacing = self.minimumLineSpacing;
CGFloat numOfPage = lround(proposedContentOffset.y / (heightOfPage + heightOfSpacing));
CGFloat newY = numOfPage * (heightOfPage + heightOfSpacing);
// if the calculated y is bigger then the maximum possible y we adjust accordingly
CGFloat contentHeight = self.collectionViewContentSize.height;
CGFloat collectionViewHeight = self.collectionView.bounds.size.height;
CGFloat maxY = contentHeight - collectionViewHeight;
if (newY > maxY)
{
newY = maxY;
}
return CGPointMake(0, newY);
}
希望这可以节省一些时间和头痛
我更喜欢允许用户浏览多个页面。所以这是我的垂直布局版本targetContentOffsetForProposedContentOffset（基于 DarthMike 的回答） 。
- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity {
CGFloat approximatePage = self.collectionView.contentOffset.y / self.pageHeight;
CGFloat currentPage = (velocity.y < 0.0) ? floor(approximatePage) : ceil(approximatePage);
NSInteger flickedPages = ceil(velocity.y / self.flickVelocity);
if (flickedPages) {
proposedContentOffset.y = (currentPage + flickedPages) * self.pageHeight;
} else {
proposedContentOffset.y = currentPage * self.pageHeight;
}
return proposedContentOffset;
}
- (CGFloat)pageHeight {
return self.itemSize.height + self.minimumLineSpacing;
}
- (CGFloat)flickVelocity {
return 1.2;
}
除非我滚动到行尾，否则 Fogmeisters 的回答对我有用。我的单元格不能整齐地放在屏幕上，所以它会滚动到最后并突然跳回来，这样最后一个单元格总是与屏幕的右边缘重叠。
为了防止这种情况，在 targetcontentoffset 方法的开头添加以下代码行
if(proposedContentOffset.x>self.collectionViewContentSize.width-320-self.sectionInset.right)
return proposedContentOffset;
@André Abreu的代码
Swift3 版本
class CustomCollectionViewFlowLayout: UICollectionViewFlowLayout {
override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
var offsetAdjustment = CGFloat.greatestFiniteMagnitude
let horizontalOffset = proposedContentOffset.x
let targetRect = CGRect(x: proposedContentOffset.x, y: 0, width: self.collectionView!.bounds.size.width, height: self.collectionView!.bounds.size.height)
for layoutAttributes in super.layoutAttributesForElements(in: targetRect)! {
let itemOffset = layoutAttributes.frame.origin.x
if abs(itemOffset - horizontalOffset) < abs(offsetAdjustment){
offsetAdjustment = itemOffset - horizontalOffset
}
}
return CGPoint(x: proposedContentOffset.x + offsetAdjustment, y: proposedContentOffset.y)
}
}
斯威夫特 4
具有一种尺寸的单元格（水平滚动）的集合视图的最简单解决方案：
override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
guard let collectionView = collectionView else { return proposedContentOffset }
// Calculate width of your page
let pageWidth = calculatedPageWidth()
// Calculate proposed page
let proposedPage = round(proposedContentOffset.x / pageWidth)
// Adjust necessary offset
let xOffset = pageWidth * proposedPage - collectionView.contentInset.left
return CGPoint(x: xOffset, y: 0)
}
func calculatedPageWidth() -> CGFloat {
return itemSize.width + minimumInteritemSpacing
}
一个更短的解决方案（假设您正在缓存布局属性）：
override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
let proposedEndFrame = CGRect(x: proposedContentOffset.x, y: 0, width: collectionView!.bounds.width, height: collectionView!.bounds.height)
let targetLayoutAttributes = cache.max { $0.frame.intersection(proposedEndFrame).width < $1.frame.intersection(proposedEndFrame).width }!
return CGPoint(x: targetLayoutAttributes.frame.minX - horizontalPadding, y: 0)
}
把它放在上下文中：
class Layout : UICollectionViewLayout {
private var cache: [UICollectionViewLayoutAttributes] = []
private static let horizontalPadding: CGFloat = 16
private static let interItemSpacing: CGFloat = 8
override func prepare() {
let (itemWidth, itemHeight) = (collectionView!.bounds.width - 2 * Layout.horizontalPadding, collectionView!.bounds.height)
cache.removeAll()
let count = collectionView!.numberOfItems(inSection: 0)
var x: CGFloat = Layout.horizontalPadding
for item in (0..<count) {
let indexPath = IndexPath(item: item, section: 0)
let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
attributes.frame = CGRect(x: x, y: 0, width: itemWidth, height: itemHeight)
cache.append(attributes)
x += itemWidth + Layout.interItemSpacing
}
}
override var collectionViewContentSize: CGSize {
let width: CGFloat
if let maxX = cache.last?.frame.maxX {
width = maxX + Layout.horizontalPadding
} else {
width = collectionView!.width
}
return CGSize(width: width, height: collectionView!.height)
}
override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
return cache.first { $0.indexPath == indexPath }
}
override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
return cache.filter { $0.frame.intersects(rect) }
}
override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
let proposedEndFrame = CGRect(x: proposedContentOffset.x, y: 0, width: collectionView!.bounds.width, height: collectionView!.bounds.height)
let targetLayoutAttributes = cache.max { $0.frame.intersection(proposedEndFrame).width < $1.frame.intersection(proposedEndFrame).width }!
return CGPoint(x: targetLayoutAttributes.frame.minX - Layout.horizontalPadding, y: 0)
}
}
为了确保它在 Swift 版本（现在是 swift 5）中工作，我使用了来自@André Abreu的答案，我添加了一些更多信息：
当子类化 UICollectionViewFlowLayout 时，“覆盖 func awakeFromNib(){}”不起作用（不知道为什么）。相反，我使用了“覆盖 init(){super.init()}”
这是我放在类 SubclassFlowLayout: UICollectionViewFlowLayout {} 中的代码：
let padding: CGFloat = 16
override init() {
super.init()
self.minimumLineSpacing = padding
self.minimumInteritemSpacing = 2
self.scrollDirection = .horizontal
self.sectionInset = UIEdgeInsets(top: 0, left: padding, bottom: 0, right: 100) //right = "should set for footer" (Horizental)
}
required init?(coder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
var offsetAdjustment = CGFloat.greatestFiniteMagnitude
let leftInset = padding
let horizontalOffset = proposedContentOffset.x + leftInset // leftInset is for "where you want the item stop on the left"
let targetRect = CGRect(origin: CGPoint(x: proposedContentOffset.x, y: 0), size: self.collectionView!.bounds.size)
for layoutAttributes in super.layoutAttributesForElements(in: targetRect)! {
let itemOffset = layoutAttributes.frame.origin.x
if (abs(itemOffset - horizontalOffset) < abs(offsetAdjustment)) {
offsetAdjustment = itemOffset - horizontalOffset
}
}
let targetPoint = CGPoint(x: proposedContentOffset.x + offsetAdjustment, y: proposedContentOffset.y)
return targetPoint
}
子类化后，确保将其放入 ViewDidLoad() 中：
customCollectionView.collectionViewLayout = SubclassFlowLayout()
customCollectionView.isPagingEnabled = false
customCollectionView.decelerationRate = .fast //-> this for scrollView speed
对于那些在 Swift 中寻找解决方案的人：
class CustomCollectionViewFlowLayout: UICollectionViewFlowLayout {
private let collectionViewHeight: CGFloat = 200.0
private let screenWidth: CGFloat = UIScreen.mainScreen().bounds.width
override func awakeFromNib() {
super.awakeFromNib()
self.itemSize = CGSize(width: [InsertItemWidthHere], height: [InsertItemHeightHere])
self.minimumInteritemSpacing = [InsertItemSpacingHere]
self.scrollDirection = .Horizontal
let inset = (self.screenWidth - CGFloat(self.itemSize.width)) / 2
self.collectionView?.contentInset = UIEdgeInsets(top: 0,
left: inset,
bottom: 0,
right: inset)
}
override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
var offsetAdjustment = CGFloat.max
let horizontalOffset = proposedContentOffset.x + ((self.screenWidth - self.itemSize.width) / 2)
let targetRect = CGRect(x: proposedContentOffset.x, y: 0, width: self.screenWidth, height: self.collectionViewHeight)
var array = super.layoutAttributesForElementsInRect(targetRect)
for layoutAttributes in array! {
let itemOffset = layoutAttributes.frame.origin.x
if (abs(itemOffset - horizontalOffset) < abs(offsetAdjustment)) {
offsetAdjustment = itemOffset - horizontalOffset
}
}
return CGPoint(x: proposedContentOffset.x + offsetAdjustment, y: proposedContentOffset.y)
}
}
这是一个按单元格分页的演示（快速滚动时，不要跳过一个或多个单元格）：https ://github.com/ApesTalk/ATPagingByCell