Implement biometric toggle in app settings
epic-biometric-session-authentication-ui-and-lifecycle-task-010 — Add a biometric authentication toggle switch to the app settings screen. When toggled on, trigger the opt-in flow; when toggled off, invoke the revocation flow that clears stored biometric credentials from SecureSessionStorage. Display the current hardware capability status below the toggle using BiometricUnavailableBanner if unavailable.
Acceptance Criteria
Technical Requirements
Execution Context
Tier 2 - 518 tasks
Can start after Tier 1 completes
Implementation Notes
Model the biometric settings section as a dedicated Cubit (BiometricSettingsCubit) rather than mixing state into the general settings Cubit. On init, load BiometricPreference and BiometricUnavailableReason in parallel. Pass the reason to BiometricUnavailableBanner and conditionally render it based on hardware availability (not on opt-in status). The toggle widget itself should be stateless — drive it entirely from BLoC state.
Treat the toggle as a command dispatcher: toggling on emits EnableBiometricsRequested event, toggling off emits DisableBiometricsRequested event. The BLoC handles confirmation logic or delegates it. Ensure the revocation flow (task-011) is injectable so this widget can call it without direct coupling.
Testing Requirements
Write BLoC/Cubit unit tests: (1) initial state loads biometric preference and hardware status; (2) when hardware absent, state emits biometricDisabled with correct reason; (3) toggle-on action triggers opt-in call and emits biometricEnabled on success; (4) toggle-on action emits error state on opt-in failure and reverts preference. Write widget tests: (1) toggle is disabled and BiometricUnavailableBanner is rendered when hardware absent; (2) toggle shows correct initial value from preference; (3) tapping toggle calls the correct BLoC event; (4) confirmation dialog appears before revocation; (5) cancelling confirmation leaves toggle on.
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.