Stretchy Header for UICollectionView - ios

Question

I am trying to create a stretchy header using a collectionview section header. There are a bunch of different ways to do this but I am just looking for the simplest and most straight forward method with clear insturctions on what to do. Has anybody seen a simple guide on doing this in Swift 3 or can you explain how this would be done here?
I don't think it should be that difficult. I would like to use the UICollectionViewDelegateFlowLayout and the ScrollviewDelegate becuase I think that would be the easiest way for it to be done.
Can I use the scrollViewDidScroll method to control height of the header as a user is scrolling. How would I change the height of the header manually? I know on the storyboard I can change it in the UICollectionView header size settings but how would I adjust it in code?

Answer 1

With Swift 5 / iOS 12.3, you can override shouldInvalidateLayout(forBoundsChange:) and layoutAttributesForElements(in:) methods inside a UICollectionViewFlowLayout subclass in order to create a stretchy header in a UICollectionView. The following sample code shows how to implement those methods:
CollectionViewController.swift
import UIKit
class CollectionViewController: UICollectionViewController {
let flowLayout = CustomFlowLayout()
override func viewDidLoad() {
super.viewDidLoad()
guard let collectionView = collectionView else { fatalError() }
collectionView.alwaysBounceVertical = true
collectionView.contentInsetAdjustmentBehavior = .always
collectionView.collectionViewLayout = flowLayout
collectionView.register(CollectionViewCell.self, forCellWithReuseIdentifier: "Cell")
collectionView.register(HeaderReusableView.self, forSupplementaryViewOfKind: UICollectionView.elementKindSectionHeader, withReuseIdentifier: "HeaderView")
}
override func collectionView(_ collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, at indexPath: IndexPath) -> UICollectionReusableView {
let headerView = collectionView.dequeueReusableSupplementaryView(ofKind: UICollectionView.elementKindSectionHeader, withReuseIdentifier: "HeaderView", for: indexPath) as! HeaderReusableView
return headerView
}
override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return 20
}
override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
return collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! CollectionViewCell
}
}
CustomFlowLayout.swift
import UIKit
class CustomFlowLayout: UICollectionViewFlowLayout {
let idealCellWidth: CGFloat = 100
let margin: CGFloat = 10
override init() {
super.init()
sectionInsetReference = .fromSafeArea
sectionInset = UIEdgeInsets(top: margin, left: margin, bottom: margin, right: margin)
headerReferenceSize = CGSize(width: 0, height: 80)
sectionHeadersPinToVisibleBounds = false
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
override func prepare() {
super.prepare()
guard let collectionView = collectionView else { return }
let availableWidth = collectionView.frame.width - collectionView.safeAreaInsets.left - collectionView.safeAreaInsets.right - sectionInset.left - sectionInset.right
let idealNumberOfCells = (availableWidth + minimumInteritemSpacing) / (idealCellWidth + minimumInteritemSpacing)
let numberOfCells = idealNumberOfCells.rounded(.down)
let cellWidth = (availableWidth + minimumInteritemSpacing) / numberOfCells - minimumInteritemSpacing
itemSize = CGSize(width: cellWidth, height: cellWidth)
}
override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
return true
}
override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
guard let collectionView = collectionView else { return nil }
guard let rectAttributes = super.layoutAttributesForElements(in: rect) else { return nil }
let offsetY = collectionView.contentOffset.y + collectionView.safeAreaInsets.top
if let firstHeader = rectAttributes.first(where: { $0.representedElementKind == UICollectionView.elementKindSectionHeader && offsetY < 0}) {
let origin = CGPoint(x: firstHeader.frame.origin.x, y: firstHeader.frame.minY - offsetY.magnitude)
let size = CGSize(width: firstHeader.frame.width, height: max(0, headerReferenceSize.height + offsetY.magnitude))
firstHeader.frame = CGRect(origin: origin, size: size)
}
return rectAttributes
}
}
CollectionViewCell.swift
import UIKit
class CollectionViewCell: UICollectionViewCell {
override init(frame: CGRect) {
super.init(frame: frame)
backgroundColor = .cyan
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}
HeaderReusableView.swift
import UIKit
class HeaderReusableView: UICollectionReusableView {
override init(frame: CGRect) {
super.init(frame: frame)
backgroundColor = .magenta
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}
Expected result: