ios - 尽管实现了sizeForItemAt，UICollectionViewFlowLayout单元格的大小从未改变

我有一个UICollectionViewCell，里面有一个UICollectionView和UICollectionViewFlowLayout。我正在使单元格成为集合 View 的委托(delegate)和数据源。我符合UICollectionViewDelegateFlowLayout并实现collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize(并返回正确的大小)。

但是，当我调用layout.itemSize.height时，我得到了默认的高度50。

class MyCell: UICollectionViewCell {
  fileprivate lazy var collectionView: UICollectionView = {
    let collectionView = UICollectionView(frame: .zero, collectionViewLayout: UICollectionViewFlowLayout())
    collectionView.delegate = self
    collectionView.dataSource = self
    collectionView.register(SomeOtherCell.self, forCellWithReuseIdentifier: SomeOtherCell.reuseIdentifier)
    collectionView.showsHorizontalScrollIndicator = false
    collectionView.showsVerticalScrollIndicator = false
    return collectionView
  }()

  fileprivate lazy var layout: UICollectionViewFlowLayout? = {
    let layout = collectionView.collectionViewLayout as? UICollectionViewFlowLayout
    layout?.scrollDirection = .horizontal
    layout?.sectionInset = UIEdgeInsets.zero
    return layout
  }()

  override func sizeThatFits(_ size: CGSize) -> CGSize {
  collectionView.frame = CGRect(x: layoutMargins.left, y: origin.y, width: width, height: layout.itemSize.height)
  // layout.itemSize.height is always 50
  }
}
extension MyCell: UICollectionViewDelegateFlowLayout {
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
    guard tags.count != 0 else {
        return .zero
    }

    let tag = Tag(text: tags[indexPath.item], index: indexPath.item)
    placeholderCell.configure(with: tag)
    let size = placeholderCell.getFrame(at: .zero, fitting: placeholderCell.width, alignedBy: semanticContentAttribute, apply: false).size
    collectionViewWidth += size.width
  // this size is always correct, that's what I want when I call  layout.itemSize.height
    return size
  }
}

最佳答案

问题

根据Apple documents for UICollectionViewFlowLayout's itemSize

它清楚地表明itemSize仅在委托(delegate)未实现collectionView(_:layout:sizeForItemAt :)方法且中没有提及itemSize将从collectionView(_:layout:sizeForItemAt:)返回值的情况下使用。 它们是2个不同的值。这就是为什么itemSize实现后sizeForItemAt不会改变的原因

因此，使用layout.itemSize.height时默认值为(50)才有意义。

您的问题有什么解决方案？

如果collectionView上的所有单元格都具有相同的大小，建议您为itemSize设置一个值，并且不要实现collectionView(_:layout:sizeForItemAt:)方法(删除UICollectionViewDelegateFlowLayout的扩展名)。

class MyCell: UICollectionViewCell {
  fileprivate lazy var collectionView: UICollectionView = {
    let collectionView = UICollectionView(frame: .zero, collectionViewLayout: UICollectionViewFlowLayout())
    // collectionView.delegate = self
    collectionView.dataSource = self
    collectionView.register(SomeOtherCell.self, forCellWithReuseIdentifier: SomeOtherCell.reuseIdentifier)
    collectionView.showsHorizontalScrollIndicator = false
    collectionView.showsVerticalScrollIndicator = false
    return collectionView
  }()

  fileprivate lazy var layout: UICollectionViewFlowLayout? = {
    let layout = collectionView.collectionViewLayout as? UICollectionViewFlowLayout
    layout?.scrollDirection = .horizontal
    layout?.sectionInset = UIEdgeInsets.zero
    // Calculate and change |itemSize| here
    layout?.itemSize = YOUR_CELL_SIZE_AFTER_CALCULATED
    return layout
  }()

  override func sizeThatFits(_ size: CGSize) -> CGSize {
    collectionView.frame = CGRect(x: layoutMargins.left, y: origin.y, width: width, height: layout.itemSize.height)
    // layout.itemSize.height is |YOUR_CELL_SIZE_AFTER_CALCULATED.height|
  }
}

如果每个单元格的collectionView大小不同，则需要使用UICollectionView's layoutAttributesForItem(at:) method。在IndexPath处获取UICollectionViewLayoutAttributes并使用layoutAttributes.size进行计算

layoutAttributes.size是从collectionView(_:layout:sizeForItemAt:)返回的值

let indexPath = // IndexPath you need to use to calculate
let layoutAttributes : UICollectionViewLayoutAttributes? = collectionView.layoutAttributesForItem(at: indexPath)
//  Use this value to calculate |collectionView| frame
print(layoutAttributes?.size)

关于ios - 尽管实现了sizeForItemAt，UICollectionViewFlowLayout单元格的大小从未改变，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/47894895/