AVSystemController_SystemVolumeDidChangeNotification not giving callback for the first time

Question

I want to check if both the volume buttons are working fine. So I set the observer AVSystemController_SystemVolumeDidChangeNotification to check that.

NotificationCenter.default.addObserver(self, selector: #selector(volumeCallback(notification:)), name: NSNotification.Name("AVSystemController_SystemVolumeDidChangeNotification"), object: nil)

Given is volumeCallback method:

@objc private func volumeCallback(notification: NSNotification) {
    // check if app is in forground
    guard UIApplication.shared.applicationState == .active else {
        return
    }

    //get volume level
    if let userInfo = notification.userInfo {
        if let volumeChangeType = userInfo["AVSystemController_AudioVolumeChangeReasonNotificationParameter"] as? String {
            if volumeChangeType == "ExplicitVolumeChange" {
                print("value changed")

                let level = userInfo["AVSystemController_AudioVolumeNotificationParameter"] as? Float
                guard let volLevel = level else {
                    return
                }
                // my work here
            }
        }
    }
}

Now the problem is, I am not getting callback in volumeCallback for the first installation of the app. The weird thing is, this method is being called when the app is in background, but not being called in foreground.

I am using iPhone 5s (iOS 10.3.3).

I don't understand what is the problem in this code. Any help will be appreciated.

Answer 1

This can be easily done with key-value observer as AVAudioSession provides outputVolume property. Check here.

You can just add observer on this property and get callbacks.

Here's a simple way of doing this in Swift 5:

// Audio session object
private let session = AVAudioSession.sharedInstance()
// Observer
private var progressObserver: NSKeyValueObservation!

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    do {
        try session.setActive(true, options: .notifyOthersOnDeactivation)
    } catch {
        print("cannot activate session")
    }

    progressObserver = session.observe(\.outputVolume) { [weak self] (session, value) in
        print(session.outputVolume)
    }

    return true
}