watchOS Development

Comprehensive guidance for watchOS app development with SwiftUI, Watch Connectivity, and complications.

When This Skill Activates

Use this skill when the user:

• Is building a watchOS app or Watch extension
• Asks about Watch Connectivity (iPhone ↔ Watch sync)
• Needs help with complications or ClockKit
• Wants to implement watch-specific UI patterns
• Asks about WidgetKit complications or migrating from ClockKit to WidgetKit
• Wants to build watch face complications (accessoryCircular, accessoryRectangular, accessoryCorner, accessoryInline)
• Asks about HealthKit on watchOS, workout sessions, heart rate, or fitness tracking
• Needs Extended Runtime sessions for background workout tracking
• Wants to build watchOS widgets or Smart Stack widgets
• Asks about widget relevance, Smart Stack ordering, or widget suggestions
• Needs to share widgets cross-platform between iOS and watchOS

Key Principles

1. Watch-First Design

• Glanceable content - users look for seconds, not minutes
• Quick interactions - 2 seconds or less
• Essential information only - no scrolling walls of text
• Large touch targets - minimum 38pt height

2. Independent vs Companion

• Prefer independent Watch apps when possible
• Use Watch Connectivity for data sync, not as dependency
• Cache data locally for offline access
• Handle connectivity failures gracefully

3. Performance

• Minimize background work (battery)
• Use complication updates sparingly
• Prefer timeline-based content over live updates
• Keep views lightweight

Architecture Patterns

App Structure

@main
struct MyWatchApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

Navigation

// Use NavigationStack (watchOS 9+)
NavigationStack {
    List {
        NavigationLink("Item 1", value: Item.one)
        NavigationLink("Item 2", value: Item.two)
    }
    .navigationDestination(for: Item.self) { item in
        ItemDetailView(item: item)
    }
}

// TabView for main sections
TabView {
    HomeView()
    ActivityView()
    SettingsView()
}
.tabViewStyle(.verticalPage)

List Design

List {
    ForEach(items) { item in
        ItemRow(item: item)
    }
    .onDelete(perform: delete)
}
.listStyle(.carousel)  // For focused content
.listStyle(.elliptical)  // For browsing

Watch Connectivity

Session Setup

import WatchConnectivity

@Observable
final class WatchConnectivityManager: NSObject, WCSessionDelegate {
    static let shared = WatchConnectivityManager()

    private(set) var isReachable = false

    override init() {
        super.init()
        if WCSession.isSupported() {
            WCSession.default.delegate = self
            WCSession.default.activate()
        }
    }

    // Required delegate methods
    func session(_ session: WCSession, activationDidCompleteWith state: WCSessionActivationState, error: Error?) {
        isReachable = session.isReachable
    }

    #if os(iOS)
    func sessionDidBecomeInactive(_ session: WCSession) {}
    func sessionDidDeactivate(_ session: WCSession) {
        WCSession.default.activate()
    }
    #endif
}

Data Transfer Methods

Method	Use Case	Delivery
updateApplicationContext	Latest state (settings)	Overwrites previous
sendMessage	Real-time, both apps active	Immediate
transferUserInfo	Queued data	Guaranteed, in order
transferFile	Large data	Background transfer

// Application Context (most common)
func updateContext(_ data: [String: Any]) throws {
    try WCSession.default.updateApplicationContext(data)
}

// Real-time messaging
func sendMessage(_ message: [String: Any]) {
    guard WCSession.default.isReachable else { return }
    WCSession.default.sendMessage(message, replyHandler: nil)
}

// Receiving data
func session(_ session: WCSession, didReceiveApplicationContext context: [String: Any]) {
    Task { @MainActor in
        // Update UI with received data
    }
}

Complications

Timeline Provider

import ClockKit

struct ComplicationController: CLKComplicationDataSource {

    func getComplicationDescriptors(handler: @escaping ([CLKComplicationDescriptor]) -> Void) {
        let descriptor = CLKComplicationDescriptor(
            identifier: "myComplication",
            displayName: "My App",
            supportedFamilies: [.circularSmall, .modularSmall, .graphicCircular]
        )
        handler([descriptor])
    }

    func getCurrentTimelineEntry(
        for complication: CLKComplication,
        withHandler handler: @escaping (CLKComplicationTimelineEntry?) -> Void
    ) {
        let template = makeTemplate(for: complication.family)
        let entry = CLKComplicationTimelineEntry(date: .now, complicationTemplate: template)
        handler(entry)
    }
}

WidgetKit Complications (watchOS 9+)

import WidgetKit
import SwiftUI

struct MyComplication: Widget {
    var body: some WidgetConfiguration {
        StaticConfiguration(
            kind: "MyComplication",
            provider: ComplicationProvider()
        ) { entry in
            ComplicationView(entry: entry)
        }
        .configurationDisplayName("My Complication")
        .supportedFamilies([
            .accessoryCircular,
            .accessoryRectangular,
            .accessoryCorner,
            .accessoryInline
        ])
    }
}

UI Components

Digital Crown

@State private var crownValue = 0.0

ScrollView {
    // Content
}
.focusable()
.digitalCrownRotation($crownValue)

Haptic Feedback

WKInterfaceDevice.current().play(.click)
WKInterfaceDevice.current().play(.success)
WKInterfaceDevice.current().play(.failure)

Now Playing

import WatchKit

NowPlayingView()  // Built-in now playing controls

Workout Apps

import HealthKit

@Observable
class WorkoutManager {
    let healthStore = HKHealthStore()
    var session: HKWorkoutSession?
    var builder: HKLiveWorkoutBuilder?

    func startWorkout(type: HKWorkoutActivityType) async throws {
        let config = HKWorkoutConfiguration()
        config.activityType = type
        config.locationType = .outdoor

        session = try HKWorkoutSession(healthStore: healthStore, configuration: config)
        builder = session?.associatedWorkoutBuilder()

        session?.startActivity(with: .now)
        try await builder?.beginCollection(at: .now)
    }
}

Best Practices

Performance

• Use @Observable over ObservableObject (watchOS 10+)
• Limit background refreshes
• Cache images locally
• Use lazy loading for lists

Battery

• Minimize location updates
• Use scheduled background tasks
• Prefer complications over frequent refreshes
• Batch network requests

User Experience

• Always show loading states
• Provide haptic feedback
• Support keyboard input
• Use clear iconography

Testing

Simulator

• Test with different watch sizes
• Verify complications in all families
• Test Watch Connectivity with paired iPhone simulator

On Device

• Test battery impact
• Verify haptics feel appropriate
• Test in different lighting conditions

Decision Tree

Choose the right reference file based on what the user needs:

What are you building?
|
+- iPhone <-> Watch data sync
|  -> watch-connectivity.md
|     +- Session management, application context, real-time messaging
|     +- File transfers, offline caching, complication push updates
|
+- Watch face complications
|  -> complications.md
|     +- ClockKit (legacy) vs WidgetKit (modern) complications
|     +- Migration from ClockKit to WidgetKit
|     +- Complication families (circular, rectangular, corner, inline)
|     +- Timeline providers, reload strategies, gauges
|
+- Health / fitness / workout tracking
|  -> health-fitness.md
|     +- HealthKit authorization and data types
|     +- HKWorkoutSession and HKLiveWorkoutBuilder
|     +- Real-time heart rate, calories, distance
|     +- Extended Runtime sessions, route tracking
|
+- watchOS widgets / Smart Stack
|  -> widgets-for-watch.md
|     +- Smart Stack configuration and relevance
|     +- Cross-platform widget sharing (iOS + watchOS)
|     +- watchOS-specific design (dark background, small screen)
|
+- General watchOS app development
   -> This file (SKILL.md)
      +- App structure, navigation, lists
      +- Digital Crown, haptics, Now Playing

Reference Files

File	Content
watch-connectivity.md	iPhone <-> Watch sync, session management, data transfer, offline caching
complications.md	ClockKit to WidgetKit migration, complication families, timeline providers, gauges
health-fitness.md	HealthKit, workout sessions, heart rate, Extended Runtime, route tracking, privacy
widgets-for-watch.md	Smart Stack widgets, relevance, cross-platform sharing, watchOS design

External References

• watchOS Human Interface Guidelines
• Watch Connectivity
• ClockKit
• WidgetKit
• HealthKit Workouts