UICollectionViewFlowLayout cell size never changes despite sizeForItemAt implemented

Question

I have a UICollectionViewCell that has a UICollectionView with UICollectionViewFlowLayout inside. I am making the cell the delegate and the datasource of the collection view. I am conforming to UICollectionViewDelegateFlowLayout and implementing collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize (and returning the correct size).

However, when I call layout.itemSize.height I get the default height, 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
  }
}

Answer 1

Problem

• According to Apple documents for UICollectionViewFlowLayout's itemSize

  If the delegate does not implement the collectionView(_:layout:sizeForItemAt:) method, the flow layout uses the value in this property to set the size of each cell. This results in cells that all have the same size.

  The default size value is (50.0, 50.0).

• It says clearly that itemSize is only used when the delegate does not implement the collectionView(_:layout:sizeForItemAt:) method and no words mention that itemSize will return value from collectionView(_:layout:sizeForItemAt:). They are 2 different values. That's why itemSize won't change despite sizeForItemAt is implemented

  So it makes sense if layout.itemSize.height has default value (50) when you use it.

What is the solution for your problem?

• If all of cells on your collectionView have same size, I suggest to set a value for itemSize and don't implement collectionView(_:layout:sizeForItemAt:) method (Remove extension for UICollectionViewDelegateFlowLayout).

  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|
    }
  }
  

• If collectionView has different size for each cell, you need to use UICollectionView's layoutAttributesForItem(at:) method. Get UICollectionViewLayoutAttributes at an IndexPath and use layoutAttributes.size to calculate

  layoutAttributes.size is value returned from collectionView(_:layout:sizeForItemAt:)

  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)