WCSessionDelegate

Watch kit controller (WKInterfaceController)

import WatchConnectivity

var watchSession : WCSession?

    override func awake(withContext context: Any?) {
        super.awake(withContext: context)
        // Configure interface objects here.
        startWatchSession()
    }

func startWatchSession(){

        if(WCSession.isSupported()){
            watchSession = WCSession.default()
            watchSession!.delegate = self
            watchSession!.activate()
        }
    }

//Callback in below delegate method when iOS app triggers event
func session(_ session: WCSession, didReceiveApplicationContext applicationContext: [String : Any]) {
        print("did ReceiveApplicationContext at watch")
    }