You should better do it in the usual way of macOS programming.
(You have no need to define your own nib, you have no need to define global variables.)
Assuming you already added menu items Open view 1 and Open view 2 as in the screen shot, and the Custom Classes of the two view controllers are FirstViewController and SecondViewController.
(The first may be ViewController, then you may need to replace FirstViewController to ViewController in the following code.)
1. Create a new file MyWindowController.swift and define the IBActions for Open view 1 and Open view 2 in it.
MyWindowController.swift:
import Cocoa
class MyWindowController: NSWindowController {
@IBAction func openView1(_ sender :Any) {
guard let storyboard = self.storyboard else {
return
}
let firstViewController = storyboard.instantiateController(withIdentifier: "FirstView") as? FirstViewController
self.window?.contentViewController = firstViewController
}
@IBAction func openView2(_ sender :Any) {
guard let storyboard = self.storyboard else {
return
}
let secondViewController = storyboard.instantiateController(withIdentifier: "SecondView") as? SecondViewController
self.window?.contentViewController = secondViewController
}
}
2. In the Interface Builder, change the Custom Class of your Window Controller to MyWindowController.
3. Connect the action of the menu item Open view 1 to openView1: of First Responder.
(A pop up list is shown when you release the mouse button, you can choose openView1: from the list.)
4. Connect the action of the menu item Open view 2 to openView2: of First Responder as well.
5. Run your code.
If you find something wrong, please tell me. I did test.
