Definitive UICollectionViewCell dynamic size

MyCell.swift

class FooCell: UICollectionViewCell {
    private var sizingOnlyWidthConstraint: NSLayoutConstraint? = nil

    func sizeWith(width: CGFloat, myString: String) -> CGSize {
        if sizingOnlyWidthConstraint == nil {
            sizingOnlyWidthConstraint = NSLayoutConstraint(item: self, attribute: .width, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: width)
            sizingOnlyWidthConstraint?.isActive = true
        }

        if sizingOnlyWidthConstraint!.constant != width {
            sizingOnlyWidthConstraint?.constant = width
        }

        self.commentBody = commentBody
        return self.systemLayoutSizeFitting(UILayoutFittingCompressedSize, 
                                            withHorizontalFittingPriority: UILayoutPriorityDefaultHigh, 
                                            verticalFittingPriority: UILayoutPriorityDefaultLow)
    }
}

MyViewController.swift

extension MyController: UICollectionViewDelegateFlowLayout {
    func collectionView(_ collectionView: UICollectionView,
                        layout collectionViewLayout: UICollectionViewLayout,
                        sizeForItemAt indexPath: IndexPath) -> CGSize {
        if templateCommentCellForSizing == nil {
            templateCommentCellForSizing = UserCommentCollectionViewCell(frame: CGRect(.zero)
        }
        
        return templateCommentCellForSizing!.sizeWith(width: availableWidth,
                                                      myString: myString)
    }
}