ios – UISearchController搜索栏宽度不变

class MainViewController: UIViewController,UITableViewDelegate,UITableViewDatasource,UISearchResultsupdating,UISearchBarDelegate {

     var searchController = UISearchController(searchResultsController: nil)
     override func viewDidLoad() {
         super.viewDidLoad()
         searchController.searchBar.delegate = self
         searchController.searchResultsupdater = self
         searchController.dimsBACkgroundDuringPresentation = false
         searchController.searchBar.sizeToFit()
         tableView.tableHeaderView = searchController.searchBar
         definesPresentationContext = true
    }}

我试图用searchController.searchBar.frame = CGRectMake(0,200,200)之类的东西手动设置框架,但宽度仍然在屏幕之外.故事板中不存在搜索栏,但tableView不存在.