如何知道我的macOS应用程序是在后台还是在前台？

import Cocoa

class AppDelegate: NSObject, NSApplicationDelegate {
    func applicationDidFinishLaunching(_ aNotification: Notification) {
        // 注册通知
        let notificationCenter = NSWorkspace.shared.notificationCenter
        notificationCenter.addObserver(self, selector: #selector(appWillDeactivate), name: NSWorkspace.willDeactivateApplicationNotification, object: nil)
        notificationCenter.addObserver(self, selector: #selector(appDidActivate), name: NSWorkspace.didActivateApplicationNotification, object: nil)
    }
    
    @objc func appWillDeactivate(notification: NSNotification) {
        // 应用程序即将进入后台
        if let app = notification.userInfo?[NSWorkspace.applicationUserInfoKey] as? NSRunningApplication {
            print("\(app.localizedName ?? "") will deactivate")
        }
    }
    
    @objc func appDidActivate(notification: NSNotification) {
        // 应用程序已经进入前台
        if let app = notification.userInfo?[NSWorkspace.applicationUserInfoKey] as? NSRunningApplication {
            print("\(app.localizedName ?? "") did activate")
        }
    }
}

let appDelegate = AppDelegate()
NSApplication.shared.delegate = appDelegate
NSApplication.shared.run()