UISeachController委托方法不叫，搜索栏不能成为第一个响应

Question

我有一些困难，让搜索栏在我searchcontroller成为firstResponder。我注意到没有被称为委托方法，但是当我打字筛选用户的列表搜索栏按预期工作。

searchcontroller的定义：

lazy var searchController: UISearchController = {
    let searchController = UISearchController(searchResultsController: nil)
    searchController.searchResultsUpdater = self
    searchController.obscuresBackgroundDuringPresentation = false
    searchController.searchBar.placeholder = "Search"
    return searchController
}()

设置它：

private func setupSearchController() {
    self.navigationItem.searchController = searchController
    searchController.definesPresentationContext = true
    searchController.delegate = self
    searchController.isActive = true
    searchController.searchBar.delegate = self
    searchController.searchBar.becomeFirstResponder()
}

我试图从另一个这项建议SO问题，但委托方法不被称为：

func didPresentSearchController(searchController: UISearchController) {
    UIView.animate(withDuration: 0.1, animations: { () -> Void in }) { (completed) -> Void in
        searchController.searchBar.becomeFirstResponder()
    }
}

Answer 1

问题是你试图访问UI元素（searchbarcontroller）之前，用户界面是完全加载。这可以通过两种方式来完成

1. 使用主队列显示键盘

   private func setupSearchController() {
        self.navigationItem.searchController = searchController
        searchController.definesPresentationContext = true
        searchController.delegate = self
        searchController.isActive = true
        searchController.searchBar.delegate = self
        DispatchQueue.main.async {
              self.searchController.searchBar.becomeFirstResponder()
        } 
   }

采用这种方法键盘将只显示在viewDidLoad中一次

2. 在viewDidAppear显示键盘

   override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        self.searchController.searchBar.becomeFirstResponder()
   }

通过这种方法，键盘会始终显示时画面出现。