Implement Supabase Realtime Approval Subscription
epic-expense-approval-workflow-coordinator-ui-task-004 — Build the RealtimeApprovalSubscription infrastructure component that subscribes to Supabase Realtime channels for the expense_claims table. Emit stream events when claims are submitted, updated, or actioned by other coordinators. Handle reconnection, channel cleanup, and coordinator-scoped filtering via RLS.
Acceptance Criteria
Technical Requirements
Execution Context
Tier 2 - 518 tasks
Can start after Tier 1 completes
Implementation Notes
Implement RealtimeApprovalSubscription as a service class injected via Riverpod (keeping it outside the widget tree). Use a StreamController.broadcast() internally and merge the initial snapshot observable with the Realtime channel stream using StreamGroup.merge() from the async package. For reconnection, listen to the Supabase RealtimeChannel's onClose callback and schedule a re-subscribe using a Timer with exponential backoff. Use a Set
This component is the real-time backbone for the coordinator review queue screen — keep it infrastructure-only with no business logic; routing decisions (auto-approve, escalate) belong in task-005.
Testing Requirements
Unit tests: mock the Supabase Realtime channel and verify the stream emits the correct ApprovalQueueEvent subtype for each raw Postgres change payload. Test that dispose() unsubscribes the channel. Test reconnection logic: simulate a channel error and verify re-subscription with backoff. Test that duplicate events (same id, same updated_at) are deduplicated.
Integration tests: connect to a test Supabase instance, insert and update rows, and assert the stream emits the correct events in order. Verify no events leak across chapters using two different coordinator JWTs. Use flutter_test with fake async timers for backoff testing.
Maintaining multi-select state across paginated list pages is architecturally complex in Flutter with Riverpod/BLoC. If the selection state is stored in the widget tree rather than the state layer, page transitions and list redraws can silently clear selections, causing coordinators to lose their multi-select and re-enter it.
Mitigation & Contingency
Mitigation: Store the selected claim ID set in a dedicated Riverpod StateNotifier outside the paginated list widget tree. The paginated list reads selection state from this provider and does not own it. Selection persists independently of list scroll position or page loads.
Contingency: If cross-page selection proves prohibitively complex, limit bulk selection to the currently visible page (add a clear warning in the UI) and prioritise single-page bulk approval for the initial release.
If a coordinator has the queue open while another coordinator approves claims from the same queue (possible in large organisations with shared chapter coverage), the Realtime update may arrive out of order or be missed during a reconnect, leaving the first coordinator's view stale and allowing them to attempt to approve an already-actioned claim.
Mitigation & Contingency
Mitigation: The ApprovalWorkflowService's optimistic locking (from the foundation epic) will catch the concurrent edit at the database level. The CoordinatorReviewQueueScreen should handle the resulting ConcurrencyException by removing the claim from the local list and showing a brief snackbar: 'This claim was already actioned by another coordinator.'
Contingency: Add a queue staleness indicator (a subtle 'last updated X seconds ago' label) and a manual refresh button as a fallback for coordinators who notice inconsistencies.
The end-to-end test requirement that a peer mentor receives a push notification within 30 seconds of coordinator approval depends on FCM delivery latency, which is outside the application's control and can vary significantly in CI/CD environments.
Mitigation & Contingency
Mitigation: Structure end-to-end tests to verify notification intent (correct FCM payload dispatched, correct Realtime event emitted) rather than actual device delivery timing. Use test doubles for FCM delivery in automated tests and reserve real-device delivery tests for manual pre-release validation.
Contingency: If notification timing requirements must be validated in automation, instrument the ApprovalNotificationService with a test hook that records dispatch timestamps and assert against those rather than actual FCM callbacks.