How to tell whether user opened app using push notification?

How can I tell whether user opened an iOS app using push notification? I want to track using analytics when the user opens the app through push notification. I’m using pusher.com to send the push notification. What app delegate function can I add my amplitude tracking code?

Here’s what I have

func application(_ application: UIApplication,
                 didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    if let notification = launchOptions?[.remoteNotification] as? [AnyHashable: Any] {
        Amplitude.instance().logEvent("Push Opened", withEventProperties: notification)
    }
    return true
}

Unfortunately it doesn’t cover when the app is in the background. How can I cover that?

application(_:didFinishLaunchingWithOptions:)

That method is only call on a app launch, so that means you already handled "Cold-Start" cases.

To handle background scenario:

1. Set UNUserNotificationCenter delegate

func application(_ application: UIApplication,
                     didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

        // 1) Set UNUserNotificationCenter delegate
        UNUserNotificationCenter.current().delegate = self
        /* the rest of the code */
        ...
     
        return true
    }

2. Track events in userNotificationCenter(_:didReceive:withCompletionHandler:)

extension AppDelegate: UNUserNotificationCenterDelegate {
    func userNotificationCenter(_ center: UNUserNotificationCenter, didReceive response: UNNotificationResponse, withCompletionHandler completionHandler: @escaping () -> Void) {
        let userInfo = response.notification.request.content.userInfo
        /* Track Events here!!! */
        
        ...
        completionHandler()
    }
}

As the document of the delegation method says:

The method will be called on the delegate when the user responded to the notification by opening the application, dismissing the notification or choosing a UNNotificationAction. The delegate must be set before the application returns from application:didFinishLaunchingWithOptions:.

In my opinion, I think we can remove the tracking in the application(_:didFinishLaunchingWithOptions:), it's not tested, please verify.