UITableView-header-dynamic-height.swift

    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()

        guard let headerView = tableView.tableHeaderView else {
            return
        }

        // The table view header is created with the frame size set in
        // the Storyboard. Calculate the new size and reset the header
        // view to trigger the layout.
        // Calculate the minimum height of the header view that allows
        // the text label to fit its preferred width.
        let size = headerView.systemLayoutSizeFitting(UIView.layoutFittingCompressedSize)

        if headerView.frame.size.height != size.height {
            headerView.frame.size.height = size.height

            // Need to set the header view property of the table view
            // to trigger the new layout. Be careful to only do this
            // once when the height changes or we get stuck in a layout loop.
            tableView.tableHeaderView = headerView

            // Now that the table view header is sized correctly have
            // the table view redo its layout so that the cells are
            // correcly positioned for the new header size.
            // This only seems to be necessary on iOS 9.
            tableView.layoutIfNeeded()
        }
    }