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Window(s) is a more important concept for macOS than in iOS, because a mac app can have multiple windows while iOS has only 1 key window.

Window has certain functionalities, many styles, but it is cumbersome to create. So we are used to creating NSViewController as a scene.

If you were to create a modal window conveniently, eg. using presentingViewController.presentAsModalWindow(anotherViewController), you will need some work to handle window events.

This code handles the window close event, which can happen in 2 ways:

1. User press ESC
2. User clicks on the red close button

class AnotherViewController: NSViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        view.window?.delegate = self
    }

    // Press ESC
    override func cancelOperation(_ sender: Any?) {
        do {
            try save()
            dismiss(sender)
        } catch {
            presentError(error)
        }
    }

    private func save() throws {
        // Validate and throw errors
    }

}

extension AnotherViewController: NSWindowDelegate {

    // Click window close button
    func windowShouldClose(_ sender: NSWindow) -> Bool {
        do {
            try save()
            return true
        } catch {
            presentError(error)
            return false
        }
    }

}

The code is pretty self explanatory.

The gist is that Cocoa NSViewController becomes the window’s delegate.

NSWindowDelegate has more events, such as windowWillClose, windowWillMiniaturize, customWindowsToEnterFullScreen etc, that you can make use of.

cancelOperation is the keyboard shortcut of pressing ESC. You have to call dismiss explicitly, which will NOT ask for windowShouldClose.