statusBarApp.swift

class MenuDelegate: NSObject, NSMenuDelegate {
  func menuWillOpen(_ menu: NSMenu) {
    print("Menu opened!")
    // do things
  }
}

@NSApplicationMain
class AppDelegate: NSObject, NSApplicationDelegate {
  var statusItem: NSStatusItem?
  var statusBar: NSStatusBar? 
  // our menu delegate
  var menuDelegate: MenuDelegate?
  
  func applicationDidFinishLaunching(_ aNotification: Notification) {
    // get a reference to the system wide status bar
    self.statusBar = NSStatusBar.system
    self.statusItem = self.statusBar?.statusItem(withLength: NSStatusItem.variableLength)
    
    // setup the button which sits in the status bar (like your wifi icon)
    if let button = self.statusItem?.button {
      // use image from our asset named 'status-bar-icon'
      button.image = NSImage(named: NSImage.Name("status-bar-icon"))
    }
    
    self.statusItem?.menu = NSMenu(title: "Menu")
    // instantiate the menu delegate
    self.menuDelegate = MenuDelegate()
    
    if let menu = self.statusItem?.menu {
      // add menu items
      ...
      menu.delegate = self.menuDelegate
    }
  }
}