Swift 已编译的不识别子类NSWindow实例

我像这样子类化了NSWindow

class MainWindow : NSWindow
{ var bo : Bo?

  override func keyDown(with event: NSEvent) {
    //super.keyDown(with: event)
    print("Keydown \(event.keyCode) - modifier \(event.modifierFlags)  char: \(event.characters)")

    if (bo != nil && event.characters == "+") { 
      // do something with bo
    }
  }

  public func setBo(_ bo : Bo)
  { self.bo = bo
  }

}

在AppDelegate中，我实例化了这样一个窗口

window = MainWindow(   
        contentRect: NSRect(x: 0, y: 0, width: 480, height: 300),
        styleMask: [.titled, .closable, .miniaturizable, .resizable, .fullSizeContentView],
        backing: .buffered, defer: false)

这很好，键盘事件会被打印出来

但是打电话的时候

window.setBo(bo)

---

编译器声明类型为“NSWindow”的

值在AppDelegate中没有成员“setBo”

更新声明为

@NSApplicationMain
class AppDelegate: NSObject, NSApplicationDelegate {

    var window: MainWindow!  // << here !!

@NSApplicationMain
类AppDelegate:NSObject，NSApplicationDelegate{
var窗口：主窗口//