How to manually self-size UITableView tableHeaderView/tableFooterView in iOS 11

SelfSizingTableHeaderAndTableFooterViews.swift

// For the best results, your tableHeaderView/tableFooterView should be a UITableViewHeaderFooterView with your content inside the contentView.
let tableHeaderView = UITableViewHeaderFooterView()
    
let fittingSize = CGSize(width: tableView.bounds.width - (tableView.safeAreaInsets.left + tableView.safeAreaInsets.right), height: 0)
let size = tableHeaderView.systemLayoutSizeFitting(fittingSize, withHorizontalFittingPriority: .required, verticalFittingPriority: .fittingSizeLevel)
tableHeaderView.frame = CGRect(origin: .zero, size: size)
tableView.tableHeaderView = tableHeaderView

// In particular, note that we call systemLayoutSizeFitting on the contentView of the UITableViewHeaderFooterView.
// When you set this view to the tableHeaderView/tableFooterView on the table view, the table view will preserve the existing size of its frame.
// If you need to change the size, remove the tableHeaderView/tableFooterView, set a new frame on it, then re-set it on the table view again.

// Inside a UITableViewController:

private func addTableViewHeader() {
    let header = UITableViewHeaderFooterView()
    let label = UILabel(text: "multi\nline\ntext", font: .systemFont(ofSize: 15), textColor: .white, textAlignment: .center, numberOfLines: 0)
    label.translatesAutoresizingMaskIntoConstraints = false
    header.contentView.addSubview(label)
    label.snp.makeConstraints { make in
        make.top.bottom.equalToSuperview().inset(20)
        make.left.right.equalTo(header.contentView.readableContentGuide)
    }
    tableView.tableHeaderView = header

    resizeTableHeaderView()
}

private func resizeTableHeaderView() {
    guard let header = tableView.tableHeaderView else { return }
    let fittingSize = CGSize(width: tableView.bounds.width - (tableView.safeAreaInsets.left + tableView.safeAreaInsets.right), height: 0)
    let size = header.systemLayoutSizeFitting(fittingSize, withHorizontalFittingPriority: .required, verticalFittingPriority: .fittingSizeLevel)
    header.frame = CGRect(origin: .zero, size: size)
    tableView.tableHeaderView = header
}

override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
    coordinator.animate(alongsideTransition: { [weak self] _ in
        self?.resizeTableHeaderView()
    })

    super.viewWillTransition(to: size, with: coordinator)
}