Render prior vs current field value comparison rows
epic-bufdir-report-preview-screen-and-diff-task-008 — Inside BufdirPeriodDiffView, for each Bufdir field render a comparison row showing the prior-period value beside the current-period value with the calculated percentage delta. Use BufdirFieldRowWidget as the base for current values and add a secondary column for prior values. Display the delta as a formatted percentage string (e.g. '-12%', '+5%'). Handle null prior-period values gracefully with a 'N/A — no prior data' label.
Acceptance Criteria
Technical Requirements
Execution Context
Tier 3 - 413 tasks
Can start after Tier 2 completes
Implementation Notes
Extract the delta calculation into a pure function `double? calculatePercentageDelta(num? prior, num current)` returning null when prior is null or zero. Format the result with a `formatDelta(double?
delta)` helper that returns '+X%', '-X%', '0%', or 'N/A'. Create `BufdirDiffRowWidget` as a new widget that accepts `BufdirFieldDiffRow` (a data class holding fieldLabel, priorValue, currentValue, delta). Do not modify BufdirFieldRowWidget — compose it inside BufdirDiffRowWidget for the current-value column. Use a Row with Expanded children for the three-column layout to avoid overflow.
Ensure the 'N/A — no prior data' string is defined as a constant (or localised string key) so it can be targeted in tests.
Testing Requirements
Write flutter_test widget tests and pure Dart unit tests covering: (1) delta calculation utility — positive, negative, zero, and null-prior cases, (2) BufdirDeltaBadge renders '+5%' for positive, '-12%' for negative, '0%' for zero, 'N/A' for null prior, (3) BufdirDiffRowWidget renders all three columns with correct values from fixture data, (4) null prior-period value renders 'N/A — no prior data' in prior column and 'N/A' in delta column, (5) full BufdirPeriodDiffView widget test verifies correct number of rows rendered from a fixture with 5 fields. Use golden tests for the diff row layout if a design reference is available.
The preview screen must pass period, scope, and aggregation session state to the Bufdir Report Export screen via navigation arguments. If the export screen's navigation argument schema is not yet finalized when this epic begins, the handoff will require rework — potentially after TestFlight is already in use by coordinators.
Mitigation & Contingency
Mitigation: Define the shared BufdirExportNavigationArgs Dart class jointly with the Bufdir Report Export feature team before this epic starts. Store it in a shared models package both features depend on. Treat the class as an API contract with a minor-version bump policy.
Contingency: If the export screen's argument schema changes after the preview screen is implemented, the BufdirPreviewService session state model can be adapted with a compatibility shim. The preview screen itself requires only a one-line navigation call change.
The period diff view loads prior-period data on demand and renders it inline with the current report. On a large report (many sections, many fields) combined with slow Supabase connectivity, the diff overlay could block the UI or produce a janky re-render that degrades the coordinator experience during the pre-submission review.
Mitigation & Contingency
Mitigation: Load prior-period data in a background Riverpod FutureProvider that starts prefetching when the preview screen mounts (not when the user taps the diff toggle). Show a shimmer placeholder on each field row's prior-period column while loading. Cache prior-period data in BufdirPreviewRepository using the same local cache as current-period data.
Contingency: If diff view performance is unacceptable on TestFlight devices, disable the toggle for the initial TestFlight release and ship the diff view in the following sprint after profiling the Supabase query plan and adding an appropriate Postgres index on the prior-period data table.
The full preview screen will be reviewed by coordinators on TestFlight who will cross-reference it against their physical Bufdir reporting forms. Any section ordering difference or label mismatch discovered during UAT will require a fix before the feature can be signed off, potentially delaying the entire Bufdir reporting pipeline.
Mitigation & Contingency
Mitigation: Conduct a pre-TestFlight alignment review with at least one coordinator from NHF and one from HLF using a static screenshot of the preview screen layout. Obtain written sign-off on section order and field labels before distributing to the full test group of 5-8 people.
Contingency: If label mismatches are found during TestFlight UAT, update BufdirReportStructureMapper constants and rebuild. Since the mapper is a pure Dart class with no persisted state, corrections are deployed in the next TestFlight build with no database migration required.