Feature Flags

Feature Flags Generator

Generate a complete feature flag infrastructure with typed flag definitions, protocol-based providers (local, remote, composite), SwiftUI environment integration, an @Observable manager, and a debug menu for toggling flags at runtime.

When This Skill Activates

Use this skill when the user:

• Asks to "add feature flags" or "add feature toggles"
• Mentions A/B testing or gradual rollouts
• Asks about Firebase Remote Config or similar remote configuration
• Wants to disable features without shipping an app update
• Mentions "kill switches" or "feature gates"
• Wants to control features remotely for a subset of users
• Asks for a debug menu to toggle features during development

Pre-Generation Checks

1. Project Context Detection

• Check for existing feature flag implementations
• Check for Firebase Remote Config or third-party flag SDKs
• Identify source file locations (Sources/, App/, or root)
• Verify minimum deployment target (iOS 17+ / macOS 14+ for @Observable)

2. Conflict Detection

Search for existing feature flag code:

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Glob: **/*FeatureFlag*.swift, **/*FeatureToggle*.swift, **/*RemoteConfig*.swift
Grep: "FeatureFlag" or "FeatureToggle" or "RemoteConfig" or "isFeatureEnabled"

If existing feature flag code is found:

• Ask whether to replace or extend the existing implementation
• Check for flag names or enum cases that could conflict

If a third-party SDK (Firebase, LaunchDarkly, etc.) is detected:

• Ask if the user wants a standalone implementation or a wrapper around the SDK

3. Required Capabilities

Feature flags require:

• iOS 17+ / macOS 14+ deployment target (for @Observable manager)
• Network access entitlement if using remote flags
• No special Info.plist entries needed

Configuration Questions

Ask user via AskUserQuestion:

1. What features do you want to flag? (freeform)

   • Examples: new onboarding, premium paywall, experimental UI, dark mode v2
   • This determines the flag enum cases and their default values

2. What flag value types do you need?

   • Boolean only (feature on/off)
   • Boolean + String (on/off plus string configuration)
   • Boolean + String + Integer (full typed support)
   • Boolean + String + Integer + JSON (for complex configurations)

3. What provider architecture?

   • Local only -- UserDefaults-based with compile-time defaults
   • Remote only -- JSON endpoint with local caching
   • Composite (recommended) -- Local defaults with remote override; remote wins when available

4. Include debug menu?

   • Yes -- SwiftUI view for toggling flags at runtime (DEBUG builds only)
   • No -- Skip the debug view

5. Include SwiftUI environment integration?

   • Yes (recommended) -- Inject the flag manager via SwiftUI Environment
   • No -- Use the manager directly

Generation Process

Step 1: Determine File Locations

Check project structure:

• If Sources/ exists --> Sources/FeatureFlags/
• If App/ exists --> App/FeatureFlags/
• Otherwise --> FeatureFlags/

Step 2: Create Core Files

Generate these files based on configuration answers:

1. FeatureFlag.swift -- Flag enum with typed default values
2. FeatureFlagService.swift -- Protocol defining provider interface
3. LocalFeatureFlagProvider.swift -- UserDefaults-based provider with debug overrides
4. RemoteFeatureFlagProvider.swift -- URL-based provider with disk caching (if remote or composite)
5. CompositeFeatureFlagProvider.swift -- Combines local + remote; remote overrides local (if composite)
6. FeatureFlagManager.swift -- @Observable manager for SwiftUI
7. FeatureFlagEnvironmentKey.swift -- SwiftUI Environment integration (if requested)
8. FeatureFlagDebugView.swift -- Debug toggle view (if requested)

Step 3: Generate Code from Templates

Use the templates in templates.md and customize based on user answers:

• Replace placeholder flag cases with real feature names
• Set appropriate default values per flag
• Include or exclude remote/composite providers based on architecture choice
• Include or exclude typed value methods (string, int, JSON) based on type selection
• Include or exclude environment key and debug view

Output Format

After generation, provide:

Files Created

Copy code
Sources/FeatureFlags/
├── FeatureFlag.swift                    # Flag enum with typed defaults
├── FeatureFlagService.swift             # Provider protocol
├── LocalFeatureFlagProvider.swift       # UserDefaults-based provider
├── RemoteFeatureFlagProvider.swift      # URL-based provider (if remote/composite)
├── CompositeFeatureFlagProvider.swift   # Local + remote combiner (if composite)
├── FeatureFlagManager.swift             # @Observable manager for SwiftUI
├── FeatureFlagEnvironmentKey.swift      # SwiftUI Environment key (if requested)
└── FeatureFlagDebugView.swift           # Debug toggle menu (if requested)

Integration Steps

1. Initialize the manager in your App struct or entry point:

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import SwiftUI

@main
struct MyApp: App {
    @State private var featureFlagManager: FeatureFlagManager

    init() {
        // Local only
        let provider = LocalFeatureFlagProvider()

        // Or composite (remote overrides local)
        // let provider = CompositeFeatureFlagProvider(
        //     local: LocalFeatureFlagProvider(),
        //     remote: RemoteFeatureFlagProvider(
        //         endpoint: URL(string: "https://api.example.com/flags")!
        //     )
        // )

        _featureFlagManager = State(initialValue: FeatureFlagManager(provider: provider))
    }

    var body: some Scene {
        WindowGroup {
            ContentView()
                .environment(featureFlagManager)
        }
    }
}

2. Use flags in your views:

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struct ContentView: View {
    @Environment(FeatureFlagManager.self) private var flags

    var body: some View {
        VStack {
            if flags.isEnabled(.newOnboarding) {
                NewOnboardingView()
            } else {
                LegacyOnboardingView()
            }
        }
    }
}

3. Refresh remote flags (if using remote or composite):

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// Refresh on app launch or periodically
Task {
    try await featureFlagManager.refresh()
}

4. Add debug menu (if generated, DEBUG builds only):

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#if DEBUG
NavigationLink("Feature Flags") {
    FeatureFlagDebugView()
        .environment(featureFlagManager)
}
#endif

Testing Instructions

1. Unit test providers independently: Each provider conforms to FeatureFlagService and can be tested in isolation.
2. Mock provider for previews and tests:

   swiftCopy code
   final class MockFeatureFlagProvider: FeatureFlagService {
       var overrides: [FeatureFlag: Bool] = [:]
   
       func isEnabled(_ flag: FeatureFlag) -> Bool {
           overrides[flag] ?? flag.defaultValue
       }
       // ... implement remaining protocol methods
   }
   

3. Debug menu: Run in DEBUG builds, navigate to the debug menu, and toggle flags to verify behavior.
4. Remote provider: Use a local JSON file served via a test server or mock URLProtocol to test remote fetching.

Common Patterns

Boolean Flags (Kill Switches)

The most common pattern. Enable or disable a feature entirely.

swiftCopy code
if flags.isEnabled(.premiumPaywall) {
    PremiumPaywallView()
}

String Flags (Copy Variants / A/B Testing)

Use string values to serve different text or configuration strings remotely.

swiftCopy code
let welcomeMessage = flags.stringValue(.welcomeMessage) ?? "Welcome!"
Text(welcomeMessage)

Integer Flags (Thresholds / Limits)

Control numeric parameters like retry counts, page sizes, or rate limits.

swiftCopy code
let maxRetries = flags.intValue(.maxRetries) ?? 3

JSON Flags (Complex Configuration)

For structured configuration that changes server-side.

swiftCopy code
struct PaywallConfig: Codable {
    let title: String
    let trialDays: Int
    let showTestimonials: Bool
}

if let config: PaywallConfig = flags.jsonValue(.paywallConfig) {
    PaywallView(config: config)
}

Gradual Rollout

Combine feature flags with user segmentation.

swiftCopy code
// Server returns different flag values per user segment
// The flag is simply on/off from the client perspective
if flags.isEnabled(.newCheckoutFlow) {
    NewCheckoutView()
} else {
    LegacyCheckoutView()
}

Gotchas

• Stale flags: Always provide sensible local defaults. If the remote fetch fails, the app must still function correctly with local values.
• Flag cleanup: After a feature is fully rolled out, remove the flag enum case, delete related conditional code, and clean up remote configuration. Stale flags accumulate technical debt.
• Thread safety: The generated FeatureFlagManager is @MainActor-isolated. Access it on the main thread or via @Environment in SwiftUI views. The providers use Sendable-conforming storage.
• Testing both paths: When a flag controls a UI branch, write tests (or at least manual test plans) for both the enabled and disabled paths. It is easy to forget the disabled path once a flag has been on for weeks.
• Debug overrides in production: The debug override mechanism uses #if DEBUG guards. Double-check that debug toggles never leak into release builds.
• Cache invalidation: The remote provider caches to disk. Set an appropriate cacheDuration (default 5 minutes). For time-sensitive flags, call refresh() explicitly.
• UserDefaults key collisions: All flag keys are prefixed with ff_ to avoid collisions with other UserDefaults entries in the app.

References

• templates.md -- Production-ready Swift templates for all generated files
• Feature Toggles (Martin Fowler)
• Firebase Remote Config