在滚动时隐藏/显示收藏视图的标题

Question

我想在滚动时隐藏我的collectionView的Header单元格。当用户向下滚动一点时再显示一次。我试过使用UICollectionViewFlowLayout，但没有成功。

class FilterHeaderLayout: UICollectionViewFlowLayout {
override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
let layoutAttributes = super.layoutAttributesForElements(in: rect)
var safeAreaTop = 0.0
if #available(iOS 13.0, *) {
let window = UIApplication.shared.windows.first
safeAreaTop = window!.safeAreaInsets.top
}

layoutAttributes?.forEach({ attributes in
if attributes.representedElementKind == UICollectionView.elementKindSectionHeader{

guard let collectionView = collectionView else { return }

let width = collectionView.frame.width
let contentOfsetY = collectionView.contentOffset.y

if contentOfsetY < 0 {
//prevents header cell to drift away if user scroll all the way down

// 41 is height of a view in navigation bar. header cell needs to be below navigation bar.
attributes.frame = .init(x: 0, y: safeAreaTop+41+contentOfsetY, width: width, height: 50)
return
}

let scrollVelocity = collectionView.panGestureRecognizer.velocity(in: collectionView.superview)
if (scrollVelocity.y > 0.0) {
// show the header and make it stay at top
} else if (scrollVelocity.y < 0.0) {
// hide the header
}
attributes.frame = .init(x: 0, y: safeAreaTop+41+contentOfsetY, width: width, height: 50)
}
})

return layoutAttributes
}
override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
return true
}

}

Answer 1

这段代码已经很老了，但是让我们试试:

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

var offsetAdjustment = CGFloat.greatestFiniteMagnitude
let verticalOffset = proposedContentOffset.y
let targetRect = CGRect(origin: CGPoint(x: 0, y: proposedContentOffset.y), size: self.collectionView!.bounds.size)

for layoutAttributes in super.layoutAttributesForElements(in: targetRect)! {
let itemOffset = layoutAttributes.frame.origin.y
if (abs(itemOffset - verticalOffset) < abs(offsetAdjustment)) {
offsetAdjustment = itemOffset - verticalOffset
}
}

return CGPoint(x: proposedContentOffset.x, y: proposedContentOffset.y + offsetAdjustment)
}
override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
let attributesArray  = super.layoutAttributesForElements(in: rect)

attributesArray?.forEach({ (attributes) in
var length: CGFloat = 0
var contentOffset: CGFloat = 0
var position: CGFloat = 0
let collectionView = self.collectionView!

if (self.scrollDirection == .horizontal) {
length = attributes.size.width
contentOffset = collectionView.contentOffset.x
position = attributes.center.x - attributes.size.width / 2
} else {
length = attributes.size.height
contentOffset = collectionView.contentOffset.y
position = attributes.center.y - attributes.size.height / 2
}

if (position >= 0 && position <= contentOffset) {
let differ: CGFloat = contentOffset - position
let alphaFactor: CGFloat =  1 - differ / length
attributes.alpha = alphaFactor
attributes.transform3D = CATransform3DMakeTranslation(0, differ, 0)
} else if (position - contentOffset > collectionView.frame.height - Layout.model.height - Layout.model.spacing
&& position - contentOffset <= collectionView.frame.height) {
let differ: CGFloat = collectionView.frame.height - position + contentOffset
let alphaFactor: CGFloat =  differ / length
attributes.alpha = alphaFactor
attributes.transform3D = CATransform3DMakeTranslation(0, differ - length, 0)
attributes.zIndex = -1
} else {
attributes.alpha = 1
attributes.transform = CGAffineTransform.identity
}
})

return attributesArray
}
override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
return true
}