Scaffold Real-Time Reimbursement Display stateful widget
epic-mileage-reimbursement-entry-user-interface-task-007 — Create the RealtimeReimbursementDisplayWidget as a StatefulWidget that accepts a distanceKm double and an orgRateNok double as inputs, and displays a formatted string such as 'NOK 4.90/km × 23 km = NOK 112.70'. On initial build with distance 0 or null, display a placeholder string like 'Enter distance to see reimbursement'. Apply design token text styles and colour theme for the display panel.
Acceptance Criteria
Technical Requirements
Implementation Notes
Keep the widget stateless in practice — all display logic derives purely from the two input props; the StatefulWidget shell is scaffolded now to accommodate the didUpdateWidget hook in task-008 without refactoring. Use NumberFormat from the intl package (already likely present) with locale 'nb_NO' or 'en_US' as agreed with the team for NOK formatting. The formatted string should be built in a pure helper method `_formatDisplay(double? distanceKm, double orgRateNok) -> String` so it can be unit-tested independently.
Apply design tokens via Theme.of(context).textTheme and the app's AppColors or token extension — never hardcode Color(0xFF...) values. The placeholder and result strings should be defined as string constants at the top of the file to support future localisation.
Testing Requirements
Write flutter_test widget tests covering: (1) placeholder rendered when distanceKm is null, (2) placeholder rendered when distanceKm is 0.0, (3) correct formatted string rendered for distanceKm=23.0 and orgRateNok=4.90 producing 'NOK 4.90/km × 23 km = NOK 112.70', (4) large value edge case distanceKm=999.0 renders without overflow. All tests use pumpWidget with a MaterialApp wrapper supplying the design token theme. No golden tests required at this stage.
Triggering MileageCalculationService on every keystroke in the distance field could cause frame drops on lower-end Android devices if the widget rebuild chain is too broad. Jank during real-time calculation would degrade the peer-mentor experience, particularly for users with motor impairments who may type slowly and need immediate feedback.
Mitigation & Contingency
Mitigation: Keep the reimbursement calculation inside the RealtimeReimbursementDisplay widget subtree only, using a StatefulWidget with a local state variable rather than lifting state to the BLoC. This limits rebuilds to a single widget. Profile on a low-end device (Qualcomm Snapdragon 450 class) before code review.
Contingency: If profiling shows frame drops, debounce the onChanged callback by 50ms using a Timer before updating the local state, which remains imperceptible to the user while eliminating excessive rebuilds.
The optional destination field requires balancing two competing requirements: WCAG 2.2 AA demands a clear label association for screen readers, while the privacy requirement means the label must communicate optionality and privacy sensitivity without exposing the user to undue pressure to fill it in. A poorly worded label could cause screen-reader users to misunderstand the field's optional nature or feel compelled to enter private location data.
Mitigation & Contingency
Mitigation: Draft label text and hint text with input from at least one screen-reader user during the accessibility review. Use Flutter's Semantics widget to provide a separate semantic label that is more descriptive than the visual placeholder. Test with TalkBack and VoiceOver before sign-off.
Contingency: If user testing reveals confusion, replace the inline hint with a tappable information icon that opens a brief explanation of why the field is optional and how the data is used, reducing cognitive load on the field label itself.
If OrgRateConfigRepository has not yet resolved the org rate when the form loads (e.g. slow network, first launch with empty cache), the reimbursement display cannot show a meaningful value. Showing '0' or an error state could confuse mentors and undermine trust in the feature.
Mitigation & Contingency
Mitigation: Display a loading skeleton in the reimbursement display widget until the rate is available. Once loaded, animate the value in. Show a subtle 'rate unavailable' message if the fetch times out, but allow form completion and submission (rate will be fetched server-side at submission time).
Contingency: If rate unavailability is frequent due to connectivity issues, store the last successfully fetched rate in SharedPreferences as an additional fallback so the widget can render a stale-but-indicative value with a caveat label.