UICollectionView的Stretchy Header

Question

我正在尝试使用collectionview部分标题创建一个可伸缩的标头。有很多不同的方法可以做到这一点，但我只是在寻找最简单，最直接的方法，明确的做法是什么。有没有人在Swift 3中看过一个关于这样做的简单指南，或者你能解释一下这将如何在这里完成吗？

我认为不应该那么困难。我想使用UICollectionViewDelegateFlowLayout和ScrollviewDelegate因为我认为这将是最简单的方法。

我可以使用scrollViewDidScroll方法在用户滚动时控制标题的高度。如何手动更改标题的高度？我知道在故事板上我可以在UICollectionView标题大小设置中更改它，但我如何在代码中调整它？

Answer 1

您可以覆盖UICollectionViewFlowLayout子类中的shouldInvalidateLayout(forBoundsChange:)和layoutAttributesForElements(in:)方法，以便在UICollectionView中创建弹性标题。

以下Swift 4 / iOS 11完整代码显示了如何实现这些方法。

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: UICollectionElementKindSectionHeader, withReuseIdentifier: "HeaderView")
    }

    override func collectionView(_ collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, at indexPath: IndexPath) -> UICollectionReusableView {
        let headerView = collectionView.dequeueReusableSupplementaryView(ofKind: UICollectionElementKindSectionHeader, 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 == UICollectionElementKindSectionHeader && 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")
    }

}

预期结果：