Integrate distance prefill cache into Distance Input Field
epic-mileage-reimbursement-entry-user-interface-task-002 — Connect DistanceInputFieldWidget to the DistancePrefillService so the field is pre-populated with the cached most-recent distance value on first render. Expose a prefillSource label (e.g. 'Last used: 23 km') as helper text below the field, and ensure the pre-filled value fires the onChanged callback immediately so downstream widgets are initialised.
Acceptance Criteria
Technical Requirements
Execution Context
Tier 1 - 540 tasks
Can start after Tier 0 completes
Implementation Notes
Introduce a minimal StatefulWidget wrapper (DistanceInputFieldConnector or convert task-001 widget to StatefulWidget) to handle the async prefill lifecycle. Use initState + mounted check before calling setState after the future resolves. If Riverpod is already in use for other features, expose DistancePrefillService as a Provider and read it with ref.watch/ref.read in the widget. Avoid FutureBuilder on the entire field to prevent layout jumps — instead initialise controller text after the future resolves inside setState.
The prefillSource label string should be constructed in the widget, not in the service, to keep service logic pure.
Testing Requirements
Write widget + unit tests: (1) mock DistancePrefillService returning 23.0 — field shows '23.0' and helper text shows 'Last used: 23 km'; (2) mock service returning null — field is empty, no helper text; (3) mock service throwing exception — field renders empty, no crash; (4) verify onChanged fires with 23.0 on first render when prefill value exists; (5) unit test DistancePrefillService save/load round-trip with fake SharedPreferences. Use Riverpod's ProviderContainer or mocktail for service injection in widget tests.
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.