Unwanted space between Collection View cells Compositional Layout

Question

I am using Collection View for a scene. I created a custom compositional layout which is like down below. However, while scrolling there is an unwanted space between the second part of the cells. It has occurred in different cell types. I checked the spacing or insets but I couldn't figure out the reason.

The compositional layout part:

struct UIHelper {

  static func createLayouts(section: [SectionType], sectionIndex: Int) -> NSCollectionLayoutSection {

      switch section[sectionIndex] {
      
      case .sevenDaysWeather(_):

        // MARK: - Item
        let itemSize = NSCollectionLayoutSize(widthDimension: .absolute(70), heightDimension: .absolute(124))
        let item = NSCollectionLayoutItem(layoutSize: itemSize)

        // MARK: - Group
        let groupSize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(1.0), heightDimension: .absolute(124))
        let group = NSCollectionLayoutGroup.horizontal(layoutSize: groupSize, subitems: [item])
        group.interItemSpacing = .fixed(12)

        // MARK: - Section
        let section = NSCollectionLayoutSection(group: group)
        section.orthogonalScrollingBehavior = .continuous

        // MARK: - Header
        let headerSize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(1.0), heightDimension: .absolute(24))
        let headerKind = UICollectionView.elementKindSectionHeader
        let headerElement = NSCollectionLayoutBoundarySupplementaryItem(layoutSize: headerSize, elementKind: headerKind, alignment: .top)
  
        section.boundarySupplementaryItems = [headerElement]
        section.contentInsets = NSDirectionalEdgeInsets(top: 12, leading: 16, bottom: 20, trailing: 0)
        return section
  }
}

The collection view part:

  
  func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    
    let type = sections[indexPath.section]
    
    switch type {
    case .sevenDaysWeather(let viewModels):
      guard let sevenDaysCell = collectionView.dequeueReusableCell(withReuseIdentifier: SevenDaysCollectionViewCell.identifer, for: indexPath) as? SevenDaysCollectionViewCell else { return UICollectionViewCell()}
      sevenDaysCell.configure(with: viewModels[indexPath.row])
      return sevenDaysCell
    }
  }
  
  func collectionView(_ collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, at indexPath: IndexPath) -> UICollectionReusableView {
    
    let header = collectionView.dequeueReusableSupplementaryView(ofKind: UICollectionView.elementKindSectionHeader, withReuseIdentifier: HeaderCollectionReusableView.identifier, for: indexPath) as! HeaderCollectionReusableView
    header.configure(section: sections, indexPath: indexPath)
    return header
  }
}

The wanted result:

The current result: Initial state Scrolled state

Edit: Normally I have two more sections in the collection view. In order to make the example more clear I trim those parts. But the logic was the same with the given example.

Answer 1

After a while, I realized that the problem was related to the width size of the layout group .

In the initial version that I posted, in the compositional layout group properties, I was using fractionalWidth in group size.

Like this:

// MARK: - Group

let groupSize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(1.0), heightDimension: .absolute(124))

After that, I change the group's width to absolute(widthValue). That fixed the unwanted space.

// MARK: - Group

let groupSize = NSCollectionLayoutSize(widthDimension: .absolute(686), heightDimension: .absolute(120))

Result: Initial

Scrolled

Explanation of absolute and fractional can be found in Apple's documentation

Note: I calculated the total widthValue like this:

(numberOfCells * cell widht) + ((numberOfCells - 1) * interSpacingValue)

Answer 2

I fixed it like this, but I wonder if there are a cleaner way:

private func makeHorizontalSection(environment: NSCollectionLayoutEnvironment) -> NSCollectionLayoutSection {
    // Calculations
    let size = CGSize(width: 100, height: 120)
    let insets: CGFloat = 23
    let itemSpacing: CGFloat = 4
    let availableWidth = environment.container.effectiveContentSize.width - 2 * insets
    let itemAndSpaceWidth = size.width + itemSpacing
    let numberOfColumns = floor(availableWidth / itemAndSpaceWidth)
    let groupWidth: CGFloat = numberOfColumns * itemAndSpaceWidth
    
    // Item
    let itemSize = NSCollectionLayoutSize(widthDimension: .absolute(size.width), heightDimension: .fractionalHeight(1))
    let item = NSCollectionLayoutItem(layoutSize: itemSize)
    
    // Group
    let groupSize = NSCollectionLayoutSize(widthDimension: .absolute(groupWidth), heightDimension: .absolute(size.height))
    let group = NSCollectionLayoutGroup.horizontal(layoutSize: groupSize, subitems: [item])
    group.interItemSpacing = .fixed(itemSpacing)
    
    // Section
    let section = NSCollectionLayoutSection(group: group)
    section.orthogonalScrollingBehavior = .continuous
    section.contentInsets = NSDirectionalEdgeInsets(top: 0, leading: insets, bottom: 0, trailing: insets)
    return section
}