NSWorkspace didMountNotification / didUnmountNotification not triggered - swift

Question

I am in the process of creating a new macOS application that is going to perform some action whenever a USB device (or other storage device) has been attached to the computer. It also needs to run silently in the background.
For the USB part, I have added the following class to the project:
import Foundation
import AppKit
class UsbDetector {
init() {
print("Hello from UsbDetector")
let notificationCenter = NSWorkspace.shared.notificationCenter
notificationCenter.addObserver(self, selector: #selector(didMount(_:)), name: NSWorkspace.didMountNotification, object: nil)
notificationCenter.addObserver(self, selector: #selector(didUnmount(_:)), name: NSWorkspace.didUnmountNotification, object: nil)
}
#objc func didMount(_ notification: NSNotification) {
print("Hello from mounted")
if let devicePath = notification.userInfo!["NSDevicePath"] as? String {
print("Mounted: \(devicePath)")
}
}
#objc func didUnmount(_ notification: NSNotification) {
print("Hello from unmounted")
if let devicePath = notification.userInfo!["NSDevicePath"] as? String {
print("Unmounted: \(devicePath)")
}
}
}
I instantiate the UsbDetector from AppDelegate.swift as follows:
import Cocoa
import SwiftUI
#NSApplicationMain
class AppDelegate: NSObject, NSApplicationDelegate {
var window: NSWindow!
func applicationDidFinishLaunching(_ aNotification: Notification) {
print("Hello from AppDelegate")
// Start UsbDetector
_ = UsbDetector()
// Create the SwiftUI view that provides the window contents.
let contentView = ContentView()
// Create the window and set the content view.
window = NSWindow(
contentRect: NSRect(x: 0, y: 0, width: 480, height: 300),
styleMask: [.titled, .closable, .miniaturizable, .resizable, .fullSizeContentView],
backing: .buffered, defer: false)
window.center()
window.setFrameAutosaveName("Main Window")
window.contentView = NSHostingView(rootView: contentView)
window.makeKeyAndOrderFront(nil)
}
func applicationWillTerminate(_ aNotification: Notification) {
// Insert code here to tear down your application
}
}
To make it run in the background, I have added the following key to the Info.plist file of the project:
<key>LSBackgroundOnly</key>
<true/>
When the app is started, it is visible in the Activity Monitor but nowhere else, which is fine. But when I attach / detach a USB stick, the NSWorkspace event is not triggered. All that gets printed to the console is:
Hello from AppDelegate
Hello from UsbDetector
I am using Xcode 11.5 and macOS Catalina 10.15.5.
Can someone tell me what I am doing wrong here? Thanks in advance!