Cognitive Load Rule Engine core implementation
epic-cognitive-accessibility-foundation-task-006 — Build the CognitiveLoadRuleEngine service class that enforces cognitive accessibility constraints at runtime. Implement rules: ≤5 wizard steps per flow, ≤3 primary choices per screen, maximum one primary CTA per screen, and no nested modals. Expose a validateScreen() method returning a list of violations with plain-language descriptions. Integrate as a development-mode assertion layer that logs warnings and blocks navigation on violations.
Acceptance Criteria
Technical Requirements
Execution Context
Tier 1 - 540 tasks
Can start after Tier 0 completes
Implementation Notes
Implement as a pure Dart singleton with no Flutter widget dependencies to maximise testability. Use kDebugMode from 'package:flutter/foundation.dart' to gate assertion behaviour. Define ScreenConfig as a simple immutable Dart class (not a widget) with fields: stepCount, primaryChoiceCount, primaryCtaCount, hasNestedModal, screenId. Violations should be collected — not thrown — so a single validateScreen() call returns all issues at once (fail-all, not fail-fast).
Keep rule logic in private methods (_validateStepCount, etc.) for easy extension. Avoid any async code — the engine is a synchronous validation service. Consider making constants overridable via a CognitiveLoadRuleConfig object passed at engine initialisation, to allow org-specific threshold customisation in future without breaking the API.
Testing Requirements
Unit tests using flutter_test (dart:test). Test each of the 4 rules independently with boundary values (e.g., exactly 5 steps = pass, 6 steps = fail). Test validateScreen() with a fully compliant ScreenConfig returns empty list. Test that kDebugMode assertion fires — use FlutterError.onError override in test harness to capture assertion failures.
Test release-mode path by mocking kDebugMode constant. Test CognitiveViolation model serialization. Minimum 90% line coverage on rule engine class. No integration or e2e tests required for this pure service.
The error message registry and help content registry both depend on bundled JSON assets loaded at startup. If asset loading fails silently (e.g. malformed JSON, missing pubspec asset declaration), the entire plain-language layer falls back to empty strings or raw error codes, breaking the accessibility guarantee app-wide.
Mitigation & Contingency
Mitigation: Implement eager validation of both assets during app initialisation with an assertion failure in debug mode and a structured error log in release mode. Add integration tests that verify asset loading in the Flutter test harness on every CI run.
Contingency: Ship a hardcoded minimum-viable fallback message set directly in Dart code so the app always has at least a safe generic message, preventing a blank or code-only error surface.
The AccessibilityDesignTokenEnforcer relies on dart_code_metrics custom lint rules. If the lint toolchain is not already configured in the project's CI pipeline, integrating a new linting plugin may cause unexpected build failures or require significant CI configuration work beyond the estimated scope.
Mitigation & Contingency
Mitigation: Audit the existing dart_code_metrics configuration in the project before starting implementation. Scope the lint rules to a separate Dart package that can be integrated incrementally, starting with the most critical rule (hard-coded colors) and adding others in subsequent iterations.
Contingency: Fall back to Flutter test-level assertions (using the cognitive-accessibility-audit utility) to catch violations in CI if the lint plugin integration is delayed, preserving enforcement coverage without blocking the epic.
WizardDraftRepository must choose between shared_preferences and Hive for local persistence. Choosing the wrong store for the data volume (e.g. shared_preferences for complex nested wizard state) can lead to serialisation bugs or performance degradation, particularly on lower-end Android devices used by some NHF members.
Mitigation & Contingency
Mitigation: Define a clean repository interface first and implement shared_preferences as the initial backend. Profile serialisation round-trip time with a realistic wizard state payload (≈10 fields) before committing to either store.
Contingency: Swap the persistence backend behind the repository interface without touching wizard UI code, which is possible precisely because the repository abstraction isolates the storage detail.
The AccessibilityDesignTokenEnforcer scope could expand significantly if a large portion of existing widgets use hard-coded values. Discovering widespread violations during this epic would force either a major refactor or a decision to exclude legacy components, potentially reducing the enforcer's coverage and value.
Mitigation & Contingency
Mitigation: Run a preliminary audit of existing widgets using a simple grep for hard-coded hex colors and raw pixel values before implementation begins. Use the results to set a realistic remediation boundary for this epic and log all out-of-scope violations as tracked tech-debt items.
Contingency: Scope the enforcer to new and modified components only (via file-path filters in dart_code_metrics config), shipping a partial but immediately valuable coverage rather than blocking the epic on full-codebase remediation.