Add OS settings deep link and credential login fallback to BiometricUnavailableBanner
epic-biometric-session-authentication-ui-and-lifecycle-task-008 — Implement the one-tap deep link in BiometricUnavailableBanner that opens the OS biometric settings screen (using url_launcher with the platform-specific settings URI for iOS and Android). Add a second action button for direct navigation to credential login. Both actions must be keyboard-accessible and screen-reader labelled.
Acceptance Criteria
Technical Requirements
Execution Context
Tier 1 - 540 tasks
Can start after Tier 0 completes
Implementation Notes
Perform the canLaunchUrl check asynchronously during widget init (or in the parent BLoC) and pass a bool prop showSettingsLink to the banner to keep build() synchronous. Abstract url_launcher behind an interface (e.g. UrlLauncherService) so widget tests can inject a mock. iOS URI options in order of reliability: (1) App-prefs:root=TOUCHID_PASSCODE (works iOS 15+), (2) x-apple-prefs:root=TOUCHID_PASSCODE, (3) app-settings: (opens Settings root).
Test all three on real devices. For Android, use intent:#Intent;action=android.settings.BIOMETRIC_ENROLL;end encoded as a URI. Wrap launchUrl in a try/catch and show a ScaffoldMessenger snackbar on failure. Reference the terminology system for all button labels.
Testing Requirements
Mock url_launcher via a platform channel mock or a wrapper interface. Write widget tests: (1) when canLaunchUrl returns true and reason is notEnrolled, assert 'Open settings' button is visible and tapping it calls launchUrl with the correct platform URI; (2) when canLaunchUrl returns false, assert button is hidden; (3) assert credential login button is always present; (4) assert both buttons have correct Semantics labels. Write integration tests on a physical iOS device (TestFlight build) verifying that tapping the settings deep link actually opens the correct OS screen.
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.