ios - detect up volume change swift

Question

I'm having problems detecting when someone presses up or down volume button. For the moment I just play a file but I want to know when the user presses the button to show an alert when the volume changes. I'm developing in Swift and I'm using AVFoundation to create this player. For the moment I can't find something that works in Swift. I'm very new to this language.

import UIKit
import AVFoundation

class ViewController: UIViewController {
    var backgroundMusicPlayer = AVAudioPlayer()

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        playBackgroundMusic("IronBacon.mp3")
    }

    func playBackgroundMusic(filename:String){
        let url = NSBundle.mainBundle().URLForResource(filename, withExtension: nil)
        print(url)
        guard let newUrl = url else{
            print("couldn't find file: (filename)")
            return
        }
        do{
            backgroundMusicPlayer = try AVAudioPlayer(contentsOfURL: newUrl)
            backgroundMusicPlayer.numberOfLoops = -1
            backgroundMusicPlayer.prepareToPlay()
        }catch let error as NSError{
            print(error.description)
        }
    }

    @IBAction func playPauseAction(sender: UIButton) {
        sender.selected = !sender.selected
        if sender.selected {
            backgroundMusicPlayer.play()
        } else {
            backgroundMusicPlayer.pause()
        }
    }

    func ShowAlert(title: String, message: String, dismiss: String) {
        let alertController = UIAlertController(title: title, message:
            message, preferredStyle: UIAlertControllerStyle.Alert)
        alertController.addAction(UIAlertAction(title: dismiss, style: UIAlertActionStyle.Default,handler: nil))
        self.presentViewController(alertController, animated: true, completion: nil)
    }

    func volumeUp(){
        ShowAlert( "example", message: "example", dismiss: "close")
    }

    func volumeDown(){
        ShowAlert( "example", message: "example", dismiss: "close")
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}

Answer 1

This should do the trick.

class ViewController: UIViewController {

    // MARK: Properties

    let notificationCenter = NSNotificationCenter.defaultCenter()

    // MARK: Lifecycle

    override func viewDidAppear(animated: Bool) {

        super.viewDidAppear(animated)

        notificationCenter.addObserver(self,
            selector: #selector(systemVolumeDidChange),
            name: "AVSystemController_SystemVolumeDidChangeNotification",
            object: nil
        )
    }

    override func viewDidDisappear(animated: Bool) {

        super.viewDidDisappear(animated)

        notificationCenter.removeObserver(self)
    }

    // MARK: AVSystemPlayer - Notifications

    func systemVolumeDidChange(notification: NSNotification) {

        print(notification.userInfo?["AVSystemController_AudioVolumeNotificationParameter"] as? Float)
    }
}