Setting tableHeaderView height dynamically

Determining the header's frame size using

header.systemLayoutSizeFitting(UILayoutFittingCompressedSize)

as suggested in the answers above didn't work for me when my header view consisted of a single multiline label. With the label's line break mode set to wrap, the text just gets cut off:

Instead, what did work for me was using the width of the table view and a height of 0 as the target size:

header.systemLayoutSizeFitting(CGSize(width: tableView.bounds.width, height: 0))

Putting it all together (I prefer to use an extension):

extension UITableView {
    func updateHeaderViewHeight() {
        if let header = self.tableHeaderView {
            let newSize = header.systemLayoutSizeFitting(CGSize(width: self.bounds.width, height: 0))
            header.frame.size.height = newSize.height
        }
    }
}

And call it like so:

override func viewWillLayoutSubviews() {
    super.viewWillLayoutSubviews()
    tableView.updateHeaderViewHeight()
}

Copied from this post. (Make sure you see it if you're looking for more details)

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()

    if let headerView = tableView.tableHeaderView {

        let height = headerView.systemLayoutSizeFitting(UIView.layoutFittingCompressedSize).height
        var headerFrame = headerView.frame

        //Comparison necessary to avoid infinite loop
        if height != headerFrame.size.height {
            headerFrame.size.height = height
            headerView.frame = headerFrame
            tableView.tableHeaderView = headerView
        }
    }
}

More condensed version of OP's answer, with the benefit of allowing layout to happen naturally (note this solution uses viewWillLayoutSubviews):

override func viewWillLayoutSubviews() {
    super.viewWillLayoutSubviews()

    if let header = tableView.tableHeaderView {
        let newSize = header.systemLayoutSizeFitting(UIView.layoutFittingCompressedSize)
        header.frame.size.height = newSize.height
    }
}

Thanks to TravMatth for the original answer.