Detect volume button press

Using the second method, the value of the key path should be "outputVolume". That is the property we are observing.
So change the code to,

var outputVolumeObserve: NSKeyValueObservation?
let audioSession = AVAudioSession.sharedInstance()

func listenVolumeButton() {
    do {
        try audioSession.setActive(true)
    } catch {}

    outputVolumeObserve = audioSession.observe(\.outputVolume) { (audioSession, changes) in
        /// TODOs
    }
}