Xcode Segue“无法转换类型的值”

Question

我有两个不同的视图控制器，详细信息和搜索。转到 Detail 的 segues 工作正常，但转到 Search 的 segues 不断使应用程序崩溃。我花了两个小时阅读类似的问题，但似乎都没有相同的问题：

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

    let detailVC = segue.destination as! DetailViewController

    if segue.identifier == "newDocSegue"   {
        // Create a new document and pass it to the detail view.
        detailVC.doc = Document()
        detailVC.isAddAction = true
    }

    if segue.identifier == "editDocSegue"    {
        // Load the existing document into the detail view, for editing.
        let indexPath = tableView.indexPath(for: sender as! UITableViewCell)!
        detailVC.doc = Document[indexPath.row]
        detailVC.isAddAction = false
    }
     else if segue.identifier == "searchSegue" {
        shouldPerformSegue(withIdentifier: "searchSegue", sender: Any?.self)
    }

}

Answer 1

您的问题出在：

let detailVC = segue.destination as! DetailViewController

如果你的目的地的类型是SearchViewController 会导致错误

试试这个：

if let detailVC = segue.destination as? DetailViewController

您应该了解as? 和as!

Answer 2

你的问题是

让 detailVC = segue.destination 为！ DetailViewController

线。这将尝试将任何 VC（包括您的搜索 VC）转换为 DetailViewController。试试这个：

let detailVC : UIViewController

if segue.identifier == "newDocSegue"   {
    // Create a new document and pass it to the detail view.
    detailVC = segue.destination as! DetailViewController
    detailVC.doc = Document()
    detailVC.isAddAction = true
}

if segue.identifier == "editDocSegue"    {
    // Load the existing document into the detail view, for editing.
    detailVC = segue.destination as! ???  // not sure what type you want here
    let indexPath = tableView.indexPath(for: sender as! UITableViewCell)!
    detailVC.doc = Document[indexPath.row]
    detailVC.isAddAction = false
}
 else if segue.identifier == "searchSegue" {
    detailVC = segue.destination as! SearchViewController
    shouldPerformSegue(withIdentifier: "searchSegue", sender: Any?.self)
}