Code Snippet from Page 32 of "Swift Development for the Apple Watch" (modified to accommodate breaking changes in swift/iOS since book was published)

wcsession_example.swift

//
//  InterfaceController.swift
//  Yo WatchKit Extension
//
//  Created by Christopher Pedersen on 11/7/18.
//  Copyright © 2018 Christopher Pedersen. All rights reserved.
//

import WatchKit
import Foundation
import WatchConnectivity


class InterfaceController: WKInterfaceController {
    
    override func awake(withContext context: Any?) {
        super.awake(withContext: context)
        
        // Configure interface objects here.
        
        let session = WCSession.default
        session.delegate = self
        session.activate()
    }
    
    
    override func willActivate() {
        // This method is called when watch view controller is about to be visible to user
        super.willActivate()
        
        // let session = WCSession.default
    }
    
    
    override func didDeactivate() {
        // This method is called when watch view controller is no longer visible
        super.didDeactivate()
    }

}

extension InterfaceController : WCSessionDelegate {
    @available(watchOSApplicationExtension 2.2, *)
    func session(_ session: WCSession, activationDidCompleteWith activationState: WCSessionActivationState, error: Error?) {
        
        print("Session activated!")
    }
    
    func sessionDidDeactivate(session: WCSession) {
        print("Session deactivated!")
    }
    
    func sessionDidBecomeInactive(session: WCSession) {
        print("Session became inactive!")
    }

    
    func sessionReachabilityDidChange(_ session: WCSession) {
        print("Reachability changed to \(session.isReachable)")
    }
}