size the top header dynamically for autolayout after values have been set

DynamicHeaderSizeForAutolayout.swift

// MAKE SURE YOU HAVE TOP TO BOTTOM CONSTRAINTS SET ON ALL OF THE OBJECTS IN THE HEADER VIEW!!!

// better as an extension
func sizeHeaderToFit() {
    
    if let tableViewHeader = self.tableHeaderView {
      let headerView = tableViewHeader
      
      headerView.setNeedsLayout()
      headerView.layoutIfNeeded()
      
      let height = headerView.systemLayoutSizeFitting(UILayoutFittingCompressedSize).height
      var frame = headerView.frame
      frame.size.height = height
      headerView.frame = frame
      
      self.tableHeaderView = headerView
    }
  }

// see: http://roadfiresoftware.com/2015/05/how-to-size-a-table-header-view-using-auto-layout-in-interface-builder


// all this is unnecessary
func layoutTableHeaderView() {
    
    guard let headerView = tableView.tableHeaderView else { return }
    headerView.translatesAutoresizingMaskIntoConstraints = false
    
    let headerWidth = headerView.bounds.size.width;
    let temporaryWidthConstraints = NSLayoutConstraint.constraints(withVisualFormat: "[headerView(width)]", options: NSLayoutFormatOptions(rawValue: UInt(0)), metrics: ["width": headerWidth], views: ["headerView": headerView])
    
    headerView.addConstraints(temporaryWidthConstraints)
    
    headerView.setNeedsLayout()
    headerView.layoutIfNeeded()
    
    let headerSize = headerView.systemLayoutSizeFitting(UILayoutFittingCompressedSize)
    let height = headerSize.height
    var frame = headerView.frame
    
    frame.size.height = height
    headerView.frame = frame
    
    self.tableView.tableHeaderView = headerView
    
    headerView.removeConstraints(temporaryWidthConstraints)
    headerView.translatesAutoresizingMaskIntoConstraints = true
    
  }