Detect main app state in its extension (NotificationService)

Context

I'm developing an iOS messaging app with a need to accurately manage badge counts for unread messages. When the app is running (either in foreground or background), I can query the server for the exact unread count within didReceiveRemoteNotification or userNotificationCenter(_:willPresent:) handlers.

Problem

However, I'm struggling with badge count management when the app is completely terminated. In this case, I need to rely on a NotificationService extension to increment the badge count. My concern is: how can my NotificationService extension detect whether the main app is running? I want to avoid badge count inconsistencies where:

• If the main app is running, it should handle badge counts via server queries
• If the main app is terminated, the NotificationService should increment badges

Question

What's the most reliable way for the NotificationService extension to detect the main app's running state, and what synchronization mechanisms should I implement to prevent data races when updating badge counts across these two contexts?

There are few ways you can achieve synchronization of your app and its extension but I think using UserDefaults via app group is the easiest:

In applicationDidBecomeActive:

let userDefaults = UserDefaults(suiteName: "com.yourapp.shared")
userDefaults?.set(true, forKey: "isAppActive")

In the NotificationService extension:

let userDefaults = UserDefaults(suiteName: "com.yourapp.shared")
if let isAppActive = userDefaults?.bool(forKey: "isAppActive"), !isAppActive {
    // increment badge count
}

In applicationWillTerminate:

let userDefaults = UserDefaults(suiteName: "com.yourapp.shared")
userDefaults?.set(false, forKey: "isAppActive")

However, note that this solution won't work as expected if your app is crashing as applicationWillTerminate won't be called in this case. So don't let it crash :)