Make NSCollectionViewItem Responsive Width and Height

I had a hard time figuring out how to make NSCollectionViewItem width to be responsive. The problem was that I wanted my NSCollectionViewItem to follow its parent width. Maybe I didn't google the right keywords, I didn't find any solution on the internet. I found the solution after reading NSCollectionViewDelegateFlowLayout, it wasn't straight forward. When the window is resized, NSCollectionView will follow the its parent width by default, but not the cells inside it. Turns out that we need to invalidate the cells layout manually and tell NSCollectionViewDelegateFlowLayout our cells preferred size.

extension MyCollectionViewController: NSCollectionViewDelegateFlowLayout {
  func collectionView(
    _ collectionView: NSCollectionView,
    layout collectionViewLayout: NSCollectionViewLayout,
    sizeForItemAt indexPath: IndexPath
  ) -> NSSize {
    // Here we're telling that we want our cell width to
    // be equal to our collection view width
    // and height equals to 70
    return CGSize(width: collectionView.bounds.width, height: 70)
  }
}

There's one catch here, remember we need to invalidate the cells layout manually

class MyCollectionViewController: NSViewController {
  @IBOutlet weak var collectionView: NSCollectionView!

  // This method will be called everytime window is resized
  override func viewWillLayout() {
    super.viewWillLayout()

    // When we're invalidating the collection view layout
    // it will call `collectionView(_:layout:sizeForItemAt:)` method
    collectionView.collectionViewLayout?.invalidateLayout()
  }
}

There it is, hope it helps 🍪