This code makes the TableView Footer be at the bottom of the screen if the cells doesn't fulfil the screen, and below the cells if it fills the screen

TableView Footer at Bottom.swift

/**
Returns the remaining space for the TableView's footer .
- Parameters:
  - tableView: The TableView.
- Returns:
  - The remaining space between the last cell and the footerView in CGFloat.
- Important:
The TableView top and bottom constraints should be 0 to the SuperView.
You should set the estimate row height (storyboard or programmatically).
*/
func calculateSpaceForFooterView(_ tableView: UITableView) -> CGFloat {
  let verticalMargin = tableView.directionalLayoutMargins.bottom + tableView.directionalLayoutMargins.top + tableView.contentInset.top + tableView.contentInset.bottom
  let screenHeight = tableView.bounds.size.height - verticalMargin
  if tableView.contentSize.height < screenHeight {
      var footerHeight = screenHeight - tableView.contentSize.height

      if let tableFooterView = tableView.tableFooterView {
          footerHeight += tableFooterView.directionalLayoutMargins.bottom + tableFooterView.directionalLayoutMargins.top
      }

      if footerHeight < tableView.sectionFooterHeight {
          return tableView.sectionFooterHeight
      } else {
          return footerHeight
      }
  } else {
      return tableView.sectionFooterHeight
  }
}