在SplitView中使用单独的ViewController代替DetailViewController

Question

我有一个splitView。左侧（MasterViewController）是一张桌子，上面装有对象，用A，B，C，D表示，每个对象位于其自己的单元格中。因此，当选择A或B时，我希望splitview的右侧调出DetailViewController。但是C和D我希望右侧显示出自己的单独的View Controller来代替DetailViewController。这是我遇到的问题。

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == "showDetail" {
        if let indexPath = tableView.indexPathForSelectedRow {
            let object = dailyAgenda[indexPath.row]
            let controller = (segue.destination as! UINavigationController).topViewController as! DetailViewController
            controller.detailItem = object
            if object == "A" {
                controller.textDescription = "This is A"
            }
            else {
                controller.textDescription = indepthDescript[indexPath.row]
            }
        }}
    if segue.identifier == "CSegue" {
        let controller = (segue.destination as! UINavigationController).topViewController as! CViewController
    }
    if segue.identifier == "DSegue" {
        let controller = (segue.destination as! UINavigationController).topViewController as! DViewController
    }}
    override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
        if let indexPath = tableView.indexPathForSelectedRow {
        let object = dailyAgenda[indexPath.row]
        if object == "C" {
            self.performSegue(withIdentifier: "CSegue", sender: self)
        }
        if object == "D" {
            self.performSegue(withIdentifier: "DSegue", sender: self)
            }
        else {
            self.performSegue(withIdentifier: "showDetail", sender: self)
        }}}

每次我运行它时，A和B都起作用，但是当我尝试C或D时，我在这里遇到错误

在segue.identifier ==“ showDetail”下。我不知道为什么C和D调用showDetail标识符。任何帮助或解释将不胜感激。这是情节提要。