CompositionalLayoutPlayground.swift

//: A UIKit based Playground for presenting user interface

import UIKit
import PlaygroundSupport

private let reuseIdentifier = "reuse"

/// A playground for trying different UICOllectionViewCompositionalLayout techniques.
class MyViewController : UIViewController, UICollectionViewDataSource {
  private let data: [[UIColor]] = [
    [.red, .orange, .yellow, .green],
    [.blue, .black]
  ]

  /// A layout of independent lines. Each line scrolls horizontally.
  private let horizontalScrollingLinesLayout: UICollectionViewCompositionalLayout = {
    let item = NSCollectionLayoutItem(
      layoutSize: NSCollectionLayoutSize(
        widthDimension: .fractionalWidth(1),
        heightDimension: .fractionalHeight(1)
      )
    )
    let group = NSCollectionLayoutGroup.horizontal(
      layoutSize: NSCollectionLayoutSize(widthDimension: .absolute(100), heightDimension: .absolute(100)),
      subitems: [item]
    )
    let section = NSCollectionLayoutSection(group: group)
    section.orthogonalScrollingBehavior = .continuous
    section.interGroupSpacing = 10
    section.contentInsets = NSDirectionalEdgeInsets(top: 10, leading: 10, bottom: 0, trailing: 10)
    return UICollectionViewCompositionalLayout(section: section)
  }()

  private lazy var collectionView: UICollectionView = {
    let view = UICollectionView(frame: .zero, collectionViewLayout: horizontalScrollingLinesLayout)
    view.register(UICollectionViewCell.self, forCellWithReuseIdentifier: reuseIdentifier)
    view.dataSource = self

    // DON'T DO THIS
    //    view.contentInset = UIEdgeInsets(top: 10, left: 10, bottom: 10, right: 10)
    return view
  }()

  func numberOfSections(in collectionView: UICollectionView) -> Int {
    data.count
  }

  func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    data[section].count
  }

  func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath)
    cell.backgroundColor = data[indexPath.section][indexPath.row]
    return cell
  }

  override func loadView() {
    view = collectionView
  }
}
// Present the view controller in the Live View window
PlaygroundPage.current.liveView = MyViewController()