Implement ActivityTypeDonutChart widget
epic-coordinator-statistics-dashboard-ui-task-007 — Build the ActivityTypeDonutChart widget using the fl_chart adapter. Show activity type distribution as donut segments with a legend. Implement VoiceOver/TalkBack semantic labels on every segment announcing type name and percentage. Include loading skeleton and empty-state (no data for period). Colors must be chosen from the contrast-safe design token palette.
Acceptance Criteria
Technical Requirements
Execution Context
Tier 2 - 518 tasks
Can start after Tier 1 completes
Implementation Notes
Place the widget in `lib/features/statistics/widgets/activity_type_donut_chart.dart`. Use a Column layout: [PieChart (in SizedBox), SizedBox(height: 16), DonutLegendList]. Wrap the PieChart in a Stack to overlay the center total label — use Align(alignment: Alignment.center) with a IgnorePointer widget so the center label does not intercept touch events meant for the chart. For Semantics on segments, use the same overlay technique as MonthlyActivityBarChart: a post-frame callback reads each segment's bounding box and renders invisible Semantics widgets in a Stack above the chart.
For the percentage rounding, use `(count / total * 1000).round() / 10` to get one decimal place; ensure the sum of rounded percentages adjusts the largest segment to make total exactly 100%. On data change (key the PieChart on a hash of the data list) to trigger fl_chart's built-in animation. Confirm the DonutChartAdapter is provided via the same DI mechanism used across the feature to maintain consistency.
Testing Requirements
Widget tests (flutter_test): (1) Skeleton displayed when isLoading=true, PieChart absent. (2) EmptyState displayed when data=[]. (3) ErrorCard displayed when error non-null. (4) With 3 activity types, legend contains 3 rows with correct labels — verified via find.text().
(5) Tapping segment index 0 calls onSegmentTapped with correct activityTypeId. (6) Semantics tree includes labels for all segments and legend rows — use SemanticsController.simulateSemanticsAction. Golden tests for loading, empty, error, and 4-segment populated states at 375 px width. Accessibility audit: flutter_test's AccessibilityGuideline for minimum tap target size on each segment and legend row.
PeerMentorStatsList must handle rosters of 40+ mentors efficiently. A naive ListView implementation that re-renders all rows on filter change may cause frame drops on mid-range devices, degrading the experience for coordinators managing large chapters.
Mitigation & Contingency
Mitigation: Use ListView.builder with const constructors for row widgets. Profile the list with Flutter DevTools on a release build against a 60-mentor dataset before submitting for review. Implement sort in the BLoC layer, not in the widget.
Contingency: If frame drops persist, introduce pagination (load 20 mentors, scroll to load more) and add a search filter to reduce visible row count in practice.
The ActivityTypeDonutChart must render org-configured activity type labels from the org-labels system. If the terminology provider is not available or returns stale labels, chart segments will display raw key strings instead of human-readable organisation-specific names, confusing coordinators.
Mitigation & Contingency
Mitigation: Always resolve labels through the OrganizationLabelsProvider before passing data to the donut chart widget. Implement a fallback that formats the raw key as a readable string (e.g., 'peer_support' → 'Peer Support') if the provider returns null.
Contingency: If the org-labels system is unavailable, display the formatted fallback label and log a warning. Do not block chart rendering on label resolution — render with fallbacks immediately.
fl_chart widgets do not natively expose semantic labels for individual bars and donut segments. Without explicit Semantics wrappers, VoiceOver and TalkBack users will receive no meaningful chart information, failing accessibility requirements critical for Blindeforbundet and NHF deployments.
Mitigation & Contingency
Mitigation: Wrap each fl_chart widget in a Semantics widget with a descriptive label summarising the chart data. Implement the data table fallback toggle from the start, not as an afterthought. Validate with VoiceOver on iOS and TalkBack on Android during development.
Contingency: If fl_chart's rendering pipeline prevents semantic overlay from working reliably, replace chart widgets with a custom Canvas-based implementation that has full semantic control, or use a different charting library with better accessibility support.
The period filter state must be preserved when navigating away from the dashboard (e.g., drilling into a mentor's detail screen and pressing back). If the BLoC is re-created on navigation, the filter resets to default, forcing coordinators to re-select their period every time they drill down, degrading operational workflow efficiency.
Mitigation & Contingency
Mitigation: Provide the StatsBloc at a route level above the dashboard screen using a BlocProvider scoped to the statistics navigation shell, not inside the screen widget itself. Verify persistence with a widget integration test that navigates away and back.
Contingency: If route-level scoping is not achievable within the current navigation architecture, persist the last-used filter to a local session store and restore it on screen re-entry.