How to detect another instance of a Swift app is already running

Question

I am trying to detect in my Swift app if an instance is already running, and if so, refrain the user from launching another instance. I am using NSRunningApplication class to detect such behavior but am having trouble getting it to work properly. I have read through some of the older posts and know that flock() is a lower-level way to detect so but I would prefer to "do it the Swift way" if possible. Here is a snippet of the code:

func applicationDidFinishLaunching(_ aNotification: Notification) {
    // Check if app has already been launched. If so, alert user and terminate the new session
    let bundleID = Bundle.main.bundleIdentifier!

    if NSRunningApplication.runningApplications(withBundleIdentifier: bundleID).count > 1 {
        // warn and terminate
        NSApp.terminate(nil)
    }

    // keep going and finish launching the app
    ...
}

It appears that the count returned is always of value 1 regardless of how many sessions I am trying to open. Any idea what goes wrong, or am I not calling this check in the right place?

Additionally, my app also allows the user to double-click on a file with my defined file extension to launch the app. I assume that if I can get the above working then it should also work for this scenario as well?

UPDATE:

After reviewing the launch behavior of my app using Activity Monitor, I think I have determined the source of the observed behavior.

Full disclosure, my app creates a new process to launch a console app, and will wait until this process completes and terminates. Updated code snippets below:

func applicationDidFinishLaunching(_ aNotification: Notification) {
    // Check if app has already been launched. If so, alert user and terminate the new session
    let bundleID = Bundle.main.bundleIdentifier!

    let num_instances = NSRunningApplication.runningApplications(withBundleIdentifier: bundleID).count

    // Debug dialog box to see instances count
    let alert = NSAlert()
    alert.messageText = "msg"
    alert.informativeText = "num instances running: \(num_instances)"
    alert.alertStyle = NSAlert.Style.critical
    alert.addButton(withTitle: "OK")
    alert.runModal()

    if num_instances > 1 {
        // warn and terminate
        NSApp.terminate(nil)
    }

    // keep going and finish launching the app
    let task = Process()
    task.launchPath = "myConsoleApp"
    task.launch()
    task.waitUntilExit()
    let status = task.terminationStatus

    if status == 0 {
        // Done with the console app, exit
        ...
    } else {
        // something's wrong; report to user
        ...
    }
 }

Now, when the app is first launched, there are two processes created initially: mySwiftApp and myConsoleApp. However, with the presence of the debug dialog box, the mySwiftApp process is terminated after myConsoleApp has launched. As a result, the instance count for mySwiftApp is now 0. Therefore, subsequent launch of mySwiftApp will be successful and now there are two myConsoleApp processes running on the system. It is not clear to me why the presence of the alert box will terminate the mySwiftApp process but I suspect that it sends a termination signal to my app and changes its termination status.

If the debug dialog box is not there, due to the waitUnitlExit() setup, any attempt to launch the second instance will not be seen and essentially got dropped to the floor. Although this is the "desired" behavior since the second instance cannot be launched, it will be nice to have a way to notify the user.

Any idea what's the best way to do so?