UICollectionViewFlowLayoutのestimatedItemSizeを指定するとiOS12で表示がおかしい

問題

UICollectionViewFlowLayoutのestimatedItemSizeに以下を設定する。

        if let flowLayout = self.collectionView.collectionViewLayout as? UICollectionViewFlowLayout {
            flowLayout.estimatedItemSize = CGSize(width: 1, height: 1)
        }

もしくは

        if let flowLayout = self.collectionView.collectionViewLayout as? UICollectionViewFlowLayout {
            flowLayout.estimatedItemSize = UICollectionViewFlowLayout.automaticSize
        }

本来有効になるUICollectionViewCellのSelf-Sizing CellsがiOS12未満だと問題ないが、iOS12ではうまく動かない。

対応

class CollectionViewCell: UICollectionViewCell {
    
    @IBOutlet weak internal var label: UILabel!

    override func awakeFromNib() {
        super.awakeFromNib()
        
        self.contentView.translatesAutoresizingMaskIntoConstraints = false

        let leftConstraint = self.contentView.leftAnchor.constraint(equalTo: self.leftAnchor)
        let rightConstraint = self.contentView.rightAnchor.constraint(equalTo: self.rightAnchor)
        let topConstraint = self.contentView.topAnchor.constraint(equalTo: self.topAnchor)
        let bottomConstraint = self.contentView.bottomAnchor.constraint(equalTo: self.bottomAnchor)
        NSLayoutConstraint.activate([leftConstraint, rightConstraint, topConstraint, bottomConstraint])
    }

}

UICollectionViewCellのawakeFromNib()で上記の設定を行うことでうまく動作するようになる。

参考： UICollectionViewFlowLayout estimatedItemSize does not work properly with iOS12 though it works fine with iOS 11.*