WatchKit App Lifecycle

WatchKit App Lifecycle

Lifecycle Architecture

watchOS uses two lifecycle models:

SwiftUI App Protocol (Modern)

@main
struct MyWatchApp: App {
    @WKApplicationDelegateAdaptor var appDelegate: MyAppDelegate

    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

WKApplicationDelegate

Use for lifecycle events not covered by SwiftUI's scenePhase. Note: WKExtensionDelegate was renamed to WKApplicationDelegate in Xcode 14.

Scene Phase States

Note: On watchOS, .inactive does NOT mean the app isn't running.

Background Execution Modes

WKExtendedRuntimeSession Types

Critical Anti-Patterns

1. Heavy Work in Lifecycle Methods

// BAD: Slows resume time
func applicationDidBecomeActive() {
    loadAllDataFromDisk()
    syncWithServer()
}

// GOOD: Defer to background
func applicationDidBecomeActive() {
    Task.detached(priority: .background) {
        await self.prefetchData()
    }
}

2. Reading scenePhase in Sheets

// BAD: Always returns .active in sheets
struct SettingsSheet: View {
    @Environment(\.scenePhase) var scenePhase  // Broken!
}

// GOOD: Pass from root view
struct ContentView: View {
    @Environment(\.scenePhase) var scenePhase

    var body: some View {
        Button("Settings") { showSettings = true }
            .sheet(isPresented: $showSettings) {
                SettingsSheet(scenePhase: scenePhase)
            }
    }
}

3. Starting Extended Sessions from Background

// BAD: Cannot start from background
func applicationDidEnterBackground() {
    let session = WKExtendedRuntimeSession()
    session.start()  // Error!
}

// GOOD: Start while active
func startMindfulnessSession() {
    guard WKApplication.shared().applicationState == .active else { return }
    extendedSession = WKExtendedRuntimeSession()
    extendedSession?.start()
}

4. Not Recovering Workout Sessions

// BAD: handleActiveWorkoutRecovery NOT called on reboot
class AppDelegate: NSObject, WKApplicationDelegate {
    func handleActiveWorkoutRecovery() {
        recoverWorkout()
    }
}

// GOOD: Check in applicationDidFinishLaunching
func applicationDidFinishLaunching() {
    Task {
        do {
            let (session, builder) = try await HKHealthStore().recoverActiveWorkoutSession()
            workoutManager.resume(session: session, builder: builder)
        } catch {
            // No session to recover
        }
    }
}

5. Network Calls During Background Transition

// BAD: Not enough time
func applicationWillResignActive() {
    URLSession.shared.dataTask(with: url) { ... }  // Won't complete
}

// GOOD: Use expiring activity
func applicationWillResignActive() {
    ProcessInfo.processInfo.performExpiringActivity(withReason: "Sync") { expired in
        guard !expired else { return }
        self.quickSync()
    }
}

Background App Refresh

Correct Pattern

func handle(_ backgroundTasks: Set<WKRefreshBackgroundTask>) {
    for task in backgroundTasks {
        if let refreshTask = task as? WKApplicationRefreshBackgroundTask {
            // 1. Schedule next FIRST
            scheduleNextRefresh()

            // 2. Use download task (not data task)
            let config = URLSessionConfiguration.background(withIdentifier: "com.app.refresh")
            let session = URLSession(configuration: config, delegate: self, delegateQueue: nil)
            session.downloadTask(with: url).resume()

            // 3. Complete this task
            refreshTask.setTaskCompletedWithSnapshot(false)
        }
    }
}

func scheduleNextRefresh() {
    // At least 5 minutes in future
    let preferredDate = Date().addingTimeInterval(5 * 60)
    WKApplication.shared().scheduleBackgroundRefresh(
        withPreferredDate: preferredDate,
        userInfo: nil
    ) { _ in }
}

Review Questions

1. Is SwiftUI App protocol used with @WKApplicationDelegateAdaptor for lifecycle events?
2. Is scenePhase read from root view (not sheets/modals)?
3. Are extended runtime sessions started only while app is active?
4. Is HKHealthStore().recoverActiveWorkoutSession() called in applicationDidFinishLaunching?
5. Are background tasks scheduled at least 5 minutes apart?
6. Is URLSessionDownloadTask (not DataTask) used for background network?
7. Is next refresh scheduled BEFORE completing current task?