swift error "cannot assign value of type 'interface controller' to type HKWorkoutSessionDelegate'

问题

I have an error message arising for watchOS code to measure heart rate in HealthKit, that is working fine in a different Xcode project. I've compared the code and they seem the same. the file is InterfaceController.swift.

For the line 'self.workoutsession?.delegate = self' I get the red flag error "cannot assign value of type 'interface controller' to type HKWorkoutSessionDelegate" Any ideas?

Here is the code for the function. The last para is the function with the error (The prior code is for context). Would appreciate your help!

import WatchKit
import Foundation
import HealthKit


class InterfaceController: WKInterfaceController {

    @IBOutlet var label: WKInterfaceLabel!

    @IBOutlet private weak var heart: WKInterfaceImage!

    @IBOutlet var deviceLabel: WKInterfaceLabel!

    @IBOutlet var startStopButton: WKInterfaceButton!

    let healthStore = HKHealthStore()

    //State of the app - is the workout activated
    var workoutActive = false

    // define the activity type and location
    var workoutSession : HKWorkoutSession?
    let heartRateUnit = HKUnit(fromString: "count/min")
    var anchor = HKQueryAnchor(fromValue: Int(HKAnchoredObjectQueryNoAnchor))


    override func awakeWithContext(context: AnyObject?) {
        super.awakeWithContext(context)
    }

    override func willActivate() {
        super.willActivate()

        guard HKHealthStore.isHealthDataAvailable() == true else {
            label.setText("not available")
            return
        }

        guard let quantityType = HKQuantityType.quantityTypeForIdentifier(HKQuantityTypeIdentifierHeartRate) else {
            displayNotAllowed()
            return
        }

        let dataTypes = Set(arrayLiteral: quantityType)
        healthStore.requestAuthorizationToShareTypes(nil, readTypes: dataTypes) { (success, error) -> Void in
            if success == false {
                self.displayNotAllowed()
            }
        }
    }

    func displayNotAllowed() {
        label.setText("not allowed")
    }

    func workoutSession(workoutSession: HKWorkoutSession, didChangeToState toState: HKWorkoutSessionState, fromState: HKWorkoutSessionState, date: NSDate) {
        switch toState {
        case .Running:
            workoutDidStart(date)
        case .Ended:
            workoutDidEnd(date)
        default:
            print("Unexpected state \(toState)")
        }
    }

    func workoutSession(workoutSession: HKWorkoutSession, didFailWithError error: NSError) {
        // Do nothing for now
        NSLog("Workout error: \(error.userInfo)")
    }

    func workoutDidStart(date : NSDate) {
        if let query = createHeartRateStreamingQuery(date) {
            healthStore.executeQuery(query)
        } else {
            label.setText("cannot start")
        }
    }

    func workoutDidEnd(date : NSDate) {
        if let query = createHeartRateStreamingQuery(date) {
            healthStore.stopQuery(query)
            label.setText("---")
        } else {
            label.setText("cannot stop")
        }
    }

    // MARK: - Actions
    @IBAction func startBtnTapped() {
        if (self.workoutActive) {
            //finish the current workout
            self.workoutActive = false
            self.startStopButton.setTitle("Start")
            if let workout = self.workoutSession {
                healthStore.endWorkoutSession(workout)
            }
        } else {
            //start a new workout
            self.workoutActive = true
            self.startStopButton.setTitle("Stop")
            startWorkout()
        }

    }

    func startWorkout() {
        self.workoutSession = HKWorkoutSession(activityType: HKWorkoutActivityType.CrossTraining, locationType: HKWorkoutSessionLocationType.Indoor)

        self.workoutSession?.delegate = self

        healthStore.startWorkoutSession(self.workoutSession!)
    }