Implement session validation and routing in SessionResumeManager
epic-biometric-session-authentication-ui-and-lifecycle-task-003 — Read the stored session from SecureSessionStorage on resume, validate expiry timestamp, and route to the appropriate screen: BiometricPromptOverlay if session is valid and biometrics available, or full credential login if session is expired or biometrics are unavailable. Handle null/missing session gracefully by routing to login.
Acceptance Criteria
Technical Requirements
Execution Context
Tier 1 - 540 tasks
Can start after Tier 0 completes
Implementation Notes
Define a private `_resolveResume()` method that encapsulates the full validation and routing logic, called from within the debounce timer callback (task-002). Keep routing logic thin: resolve a `SessionResumeDestination` enum value (biometricPrompt, credentialLogin) from the validation, then dispatch a single navigation call. This keeps the routing logic testable without a real navigation stack. For expiry parsing, store the expiry as an ISO-8601 UTC string in SecureSessionStorage (e.g.
`2026-03-26T12:00:00Z`) and parse with `DateTime.parse(expiry).toUtc()`. Do not use Unix epoch integers — they are harder to debug. Prefer a Result/Either pattern or typed exceptions for SecureSessionStorage read errors rather than try/catch at the call site, to keep _resolveResume() readable. Ensure the Supabase Auth session expiry field is used as the source of truth for the stored expiry — do not create a separate expiry timestamp.
Testing Requirements
Unit tests using flutter_test with mocked SecureSessionStorage and BiometricAuthService. Test cases: (1) null session → routes to login; (2) missing session key → routes to login; (3) expired session → deletes session then routes to login; (4) valid session + biometrics available → routes to BiometricPromptOverlay; (5) valid session + biometrics unavailable → routes to login; (6) expired session with timezone offset → correctly identifies expiry using UTC; (7) SecureSessionStorage read throws exception → routes to login (defensive fallback). Integration test: boot app with expired token stored in secure storage, put app to background and resume, verify login screen appears and token is cleared.
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.