Implement credential revocation flow when biometrics disabled
epic-biometric-session-authentication-ui-and-lifecycle-task-011 — Implement the revocation flow triggered when the user disables biometrics via the settings toggle. This must clear all stored biometric credentials and session tokens from SecureSessionStorage, sign out any active Supabase session, and redirect the user to the credential login screen with a confirmation message. Ensure the flow is atomic — either all credentials are cleared or none are.
Acceptance Criteria
Technical Requirements
Execution Context
Tier 2 - 518 tasks
Can start after Tier 1 completes
Implementation Notes
Implement the atomic local clear using a try/catch/rollback pattern: collect all storage keys to delete, attempt each deletion in sequence, and if any fails, re-write the previously deleted keys back (best-effort rollback). Log the failure and surface a RevocationException. Use a Completer-based mutex (or a simple bool _isRevoking flag with early return) to prevent concurrent invocations. Separate the Supabase sign-out call from the local storage clear so that network failures only affect remote invalidation.
The navigation to credential login screen should happen via a GoRouter redirect triggered by the BLoC emitting an unauthenticated state, rather than calling Navigator.pushNamedAndRemoveUntil directly — this keeps navigation logic in the router and prevents duplicate navigation events.
Testing Requirements
Write unit tests for SessionResumeManager.revokeAndSignOut(): (1) happy path — all steps succeed, assert all storage keys deleted and signOut called; (2) Supabase signOut throws NetworkException — assert local storage is still cleared and flow completes without rethrowing; (3) SecureSessionStorage.delete throws on biometric token key — assert rollback is attempted and a RevocationException is thrown; (4) concurrent call protection — calling revokeAndSignOut twice simultaneously should not result in a double sign-out. Write BLoC tests verifying the settings toggle invokes revokeAndSignOut and handles RevocationException by emitting an error state. Write integration test on TestFlight build: disable biometrics, restart app, verify login screen appears and biometric prompt is not shown.
Flutter's AppLifecycleState.resumed event fires in scenarios beyond simple user-initiated app-switching: it also fires when the native biometric dialog itself dismisses and returns control to Flutter, when system alerts (low battery, notifications) temporarily cover the app, and when the app is foregrounded by a deep link. Without careful debouncing and state tracking, SessionResumeManager can trigger multiple overlapping biometric prompts or prompt immediately after a just-completed authentication, creating a confusing loop.
Mitigation & Contingency
Mitigation: Implement a state machine inside SessionResumeManager with explicit states (idle, prompting, authenticated, awaiting-fallback) and guard all prompt triggers with a state check. Add a minimum inter-prompt interval of 3 seconds. Write widget tests that simulate rapid lifecycle event sequences and verify only one prompt is shown.
Contingency: If the state machine approach proves difficult to test or maintain, replace it with a simple boolean isPromptActive flag with a debounce timer, accepting slightly less precise semantics in exchange for simpler reasoning about concurrent lifecycle events.
The BiometricPromptOverlay appears on top of the existing app content when the app resumes. If focus is not correctly transferred to the overlay and returned to the underlying screen after authentication, screen reader users (VoiceOver/TalkBack) will either be unable to interact with the prompt or will lose their navigation position in the app after authentication completes, violating WCAG 2.2 focus management requirements and creating a broken experience for Blindeforbundet users.
Mitigation & Contingency
Mitigation: Use Flutter's FocusScope and FocusTrap utilities to capture focus within the overlay on presentation and restore the previous FocusNode after dismissal. Add a live region announcement (using the accessibility live region announcer component) when the overlay appears. Include dedicated VoiceOver and TalkBack test cases in the acceptance criteria.
Contingency: If FocusTrap behaviour proves unreliable across Flutter versions, implement the overlay as a full Navigator push to a modal route rather than an overlay widget, which gives Flutter's built-in modal semantics and focus management automatic WCAG-compliant behaviour.
The BiometricUnavailableBanner needs to deep-link to biometric enrollment settings on both iOS and Android. iOS uses a single URL scheme (app-settings:) that opens the app's settings page. Android has no universal URL for biometric settings — the correct Intent action (Settings.ACTION_BIOMETRIC_ENROLL) was introduced in API 30, with different fallback actions required for API 23-29. Using the wrong action on older Android devices either crashes or navigates to an unrelated settings screen.
Mitigation & Contingency
Mitigation: Build an Android API version check into LocalAuthIntegration that selects the correct Intent action based on the runtime SDK version. Test against Android API 23, 28, and 30+ in the CI matrix. For iOS, validate that the app-settings: URL scheme is correctly declared.
Contingency: If the Android settings Intent fragmentation cannot be resolved reliably for all target API levels, fall back to navigating to the top-level Settings screen (Settings.ACTION_SETTINGS) with an overlay instruction telling the user to navigate to 'Security > Biometrics' manually, ensuring the user always has a path to resolve the issue even if the deep link is imprecise.