Combine CoreData iOS Swift SwiftUI WatchOS

Fetching and displaying data on Watch app in SwiftUI

In previous post “Dynamic user notification on Apple Watch with SwiftUI” I was looking into how to add WatchOS app to an existing project and how to create dynamic notifications. This time, I am gonna cover basics how to fetch data from companion iOS app’s CoreData storage using WatchConnectivity framework and displaying the data in SwiftUI view.

Creating session between iOS and WatchOS app

iOS app uses CoreData for storing a list of plants with last and next watering dates. In the current setup, there is no web service and everything is stored locally on device. How to get the data in persistent store to WatchOS app?

We will use WatchConnectivity framework for making iOS and WatchOS app to be aware of each other. Connection is created by activating WCSession both in iOS and WatchOS app. Therefore the first step is to add a class managing WCSession to iOS project, let’s call it WatchConnectivityProvider (later, we’ll add similar class to WatchOS app as well). It’s main responsibility is to set up WCSession and handling WCSessionDelegate which includes fetching data from CoreData store. Therefore, one of the arguments is going to be NSPersistentContainer which gives us access to CoreData stack (access to the performBackgroundTask function).

final class WatchConnectivityProvider: NSObject, WCSessionDelegate {
    private let persistentContainer: NSPersistentContainer
    private let session: WCSession
    init(session: WCSession = .default, persistentContainer: NSPersistentContainer) {
        self.persistentContainer = persistentContainer
        self.session = session
        session.delegate = self

WCSession is activated by calling function activate() which will asynchronously activates it. The response of the activitation is returned by session(_:activationDidCompleteWith:error:) delegate method.

func connect() {
	guard WCSession.isSupported() else {
		os_log(.debug, log: .watch, "watch session is not supported")
	os_log(.debug, log: .watch, "activating watch session")
func session(_ session: WCSession, 
             activationDidCompleteWith activationState: WCSessionActivationState, 
             error: Error?) {
               log: .watch, 
               "did finish activating session %lu (error: %s)", 
               activationState == .activated, 
               error?.localizedDescription ?? "none") 

We’ll add similar class, but with different name, “PhoneConnectivityProvider” to WatchOS extension target. When both classes are created, we’ll need to initialise and call connect. This could be done in SceneDelegate (iOS) and ExtensionDelegate (WatchOS). Note that in iOS app we’ll need to implement two required delegate methods and for now, we can just log when those get called.

func sessionDidBecomeInactive(_ session: WCSession) {
	os_log(.debug, log: .watch, "session became inactive")
func sessionDidDeactivate(_ session: WCSession) {
	os_log(.debug, log: .watch, "session deactivated")

For testing the session we’ll first build and run iOS app and then WatchOS app. If everything goes well, Xcode logs message: “did finish activating session 1 (error: none)”. Meaning, session is up and running and we can send messages between apps. Side note, do not forget to build and run the app where changes were made.

Fetching plants from iOS app

As communication between iOS and WatchOS app relies on dictionaries, then step 1 is to define a set of shared keys what both apps use. This reduces the risk of mistyping keys. Therefore, let’s add a new file and include it in both iOS app target and WatchOS extension target.

struct WatchCommunication {
    static let requestKey = "request"
    static let responseKey = "response"
    enum Content: String {
        case allPlants

Step 2 is implementing a refreshAllPlants(completionHandler) function in PhoneConnectivityProvider (WatchOS app extension target) which sends a message to iOS app and waits for array of plants. WCSession has a function sendMessage(_:replyHandler:errorHandler:) which we can use for sending a dictionary to iOS app and wait for reply handler. We’ll define the message to have key WatchCommunication.requestKey and the value is raw value of WatchCommunication.Content.allPlants enum case. This schema can be easily expanded later on by adding more cases to the enum. In reply handler we expect to have an array of dictionaries describing all the plants. Let’s take a look on the full implementation for a moment and then discuss how dictionary was converted to Plant value type.

func refreshAllPlants(withCompletionHandler completionHandler: @escaping ([Plant]?) -> Void) {
	guard session.activationState == .activated else {
		os_log(.debug, log: .phone, "session is not active")
	let message = [WatchRequest.contentKey: WatchRequest.Content.allPlants.rawValue]
	session.sendMessage(message, replyHandler: { (payload) in
		let plantDictionaries = payload[WatchCommunication.requestKey] as? [[String: Any]]
		os_log(.debug, log: .phone, "received %lu plants", plantDictionaries?.count ?? 0)
		let decoder = JSONDecoder()
		decoder.dateDecodingStrategy = .secondsSince1970
		let plants = plantDictionaries?.compactMap({ Plant(dictionary: $0, decoder: decoder) })
		DispatchQueue.main.async {
	}, errorHandler: { error in
		os_log(.debug, log: .phone, "sending message failed: %s", error.localizedDescription)

iOS app deals with CoreData and Plant type is NSManagedObject subclass. WatchOS app extension defines its own Plant value type because it does not have CoreData stack. For converting dictionary to value type we can use approach described in “Storing struct in UserDefault”. Only addition is configuring the JSONDecoder to use dateDecodingStrategy secondsSince1970. Reason is that we’ll going to store dates as seconds since 1970. Converting dictionary to value type involves using JSONSerialization and it supports only NSString, NSNumber, NSArray, NSDictionary, or NSNull.

// Plant value type in WatchOS app extension
struct Plant: Identifiable, Decodable, DictionaryDecodable {
    let id: String
    let name: String
    let lastWateringDate: Date
    let nextWateringDate: Date
// Plant class in iOS app
final class Plant: NSManagedObject, Identifiable {
    @NSManaged var id: String
    @NSManaged var name: String
    @NSManaged var lastWateringDate: Date
    @NSManaged var nextWateringDate: Date

Step 3 is handling the message on the iOS app side and providing data for WatchOS app. What we need to do is implementing session delegate and fetching dictionary data from CoreData store. Let’s take a look on the full implementation and then break it down.

func session(_ session: WCSession, didReceiveMessage message: [String: Any], replyHandler: @escaping ([String: Any]) -> Void) {
	os_log(.debug, log: .watch, "did receive message: %s", message[WatchCommunication.requestKey] as? String ?? "unknown")
	guard let contentString = message[WatchCommunication.requestKey] as? String , let content = WatchCommunication.Content(rawValue: contentString) else {
	switch content {
	case .allPlants:
		persistentContainer.performBackgroundTask { (managedObjectContext) in            
			let all = Plant.allPlantsDictionaryRepresentation() as! [[String: Any]]
			// Replace Date with Double
			let converted = { (plantDictionary) -> [String: Any] in
				plantDictionary.mapValues { (value) -> Any in
					if let date = value as? Date {
						return date.timeIntervalSince1970
					else {
						return value
			let response = [WatchCommunication.responseKey: converted]

The first step is to look into the received dictionary and see which content is being asked by the WatchOS app. Then we’ll access persistent store, fetch dictionary representations of Plant, convert Date to seconds since 1970 (enabling WatchOS app to use JSONSerialization on the dictionary) and then sending the data back to WatchOS app. Note that getting Plants as dictionary is very simple with CoreData: we’ll make a fetch request with result type NSDictionary and set resultType property to .dictionaryResultType. For larger models we could also provide set of properties we need (propertiesToFetch) but at the moment, every property is added to the dictionary.

extension Plant {
	static let entityName = "Plant"
	static func makeDictionaryRequest() -> NSFetchRequest<NSDictionary> {
		return NSFetchRequest<NSDictionary>(entityName: entityName)
	static func allPlantsDictionaryRepresentation() -> [NSDictionary] {
		let request = makeDictionaryRequest()
		request.resultType = .dictionaryResultType
		do {
			return try request.execute()
		catch let nsError as NSError {
			os_log(.debug, log: .plants, "failed fetching all plants with error %s %s", nsError, nsError.userInfo)
			return []

Setting up UI in WatchOS app using SwiftUI

WatchOS app template in Xcode is hooked up in a way where Storyboard initialises HostingController which is responsible of providing initial SwiftUI view.

class HostingController: WKHostingController<PlantListView> {
    lazy private(set) var connectivityProvider: PhoneConnectivityProvider = {
        let provider = PhoneConnectivityProvider()
        return provider
    private lazy var listViewModel = PlantListViewModel(connectivityProvider: connectivityProvider)
    override var body: PlantListView {
        return PlantListView(viewModel: listViewModel)

PlantListView is a simple view showing a list of plants. It’s view model handles refreshing plants using the PhoneConnectivityProvider’s refreshAllPlants(withCompletionHandler:). SwiftUI view updates automatically when view model changes. This is because view model’s plants property uses @Published property wrapper, view model is ObservableObject and SwiftUI view uses ObservedObject property wrapper for view model (read more about refreshing SwiftUI view in MVVM in SwiftUI). Note that view model refreshes content as soon as SwiftUI view appears.

final class PlantListViewModel: ObservableObject {
    private let connectivityProvider: PhoneConnectivityProvider
    init(plants: [Plant] = [], connectivityProvider: PhoneConnectivityProvider) {
        self.plants = plants
        self.connectivityProvider = connectivityProvider
    @Published private(set) var plants: [Plant]
    func refresh() {
        connectivityProvider.refreshAllPlants { [weak self] (plants) in
            guard let plants = plants else { return }
            self?.plants = plants
struct PlantListView: View {
    @ObservedObject var viewModel: PlantListViewModel
    var body: some View {
        VStack {
            List(self.viewModel.plants) { plant in
                PlantCell(viewModel: PlantCellViewModel(plant: plant))
        }.onAppear {

PlantListView uses PlantCell for displaying individual views. PlantCell has two labels and makes itself as wide as possible.

struct PlantCell: View {
    let viewModel: PlantCellViewModel
    var body: some View {
        VStack(spacing: 4) {
            .frame(minWidth: 0, maxWidth: .greatestFiniteMagnitude)
struct PlantCellViewModel {
    let plant: Plant
    var title: String {
    private static let dateFormatter: DateFormatter = {
        let formatter = DateFormatter()
        formatter.dateFormat = DateFormatter.dateFormat(fromTemplate: "dMMMM", options: 0, locale: .current)
        return formatter
    var subtitle: String {
        let format = NSLocalizedString("PlantCellView_NextWatering", comment: "Next watering date.")
        return String(format: format, Self.dateFormatter.string(from: plant.nextWateringDate))


We added WCSessions to both iOS and WatchOS app and implemented delegate methods handling session and received messages. Then, we defined simple communication schema for communication and implemented refresh plants method in the WatchOS app and CoreData integration on the iOS app side. When data access was created, we added SwiftUI view displaying list of plants in the WatchOS app.

If this was helpful, please let me know on Mastodon@toomasvahter or Twitter @toomasvahter. Feel free to subscribe to RSS feed. Thank you for reading.

Example Project

WaterMyPlants (GitHub)

3 replies on “Fetching and displaying data on Watch app in SwiftUI”

Hi Toomas,
Thanks for putting this tutorial together! It is extremely helpful.

One thing is that I’m having some trouble running this demo. When I try to run the watch notification from the simulator I get this error:
2020-06-07 13:15:43.135359-0700 WaterMyPlantsWatch Extension[83212:7682344] [watch>phone] activating phone session
2020-06-07 13:15:43.136387-0700 WaterMyPlantsWatch Extension[83212:7682344] [watch>phone] session is not active
2020-06-07 13:15:43.156039-0700 WaterMyPlantsWatch Extension[83212:7682890] [watch>phone] did finish activating session 1 (error: none)
The data couldn’t be read because it is missing.
Fatal error: Unexpectedly found nil while unwrapping an Optional value: file /Users/user173145/Downloads/WaterMyPlants-master/WaterMyPlantsWatch Extension/DynamicNotification/NotificationController.swift, line 17
2020-06-07 13:15:43.302441-0700 WaterMyPlantsWatch Extension[83212:7682344] Fatal error: Unexpectedly found nil while unwrapping an Optional value: file /Users/user173145/Downloads/WaterMyPlants-master/WaterMyPlantsWatch Extension/DynamicNotification/NotificationController.swift, line 17

Is this a pairing issue, do I need to do something special to make it work? Sorry if this is a noob question – I’m just learning iOS development now.



Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s