Define accessibility settings data model and schema
epic-screen-reader-support-foundation-task-003 — Create the AccessibilitySettings data class with fields for screen reader override, announcement verbosity, sensitive field warning enabled flag, and preferred announcement delay. Define JSON serialization for local storage persistence using Dart's json_serializable or manual toJson/fromJson.
Acceptance Criteria
Technical Requirements
Implementation Notes
Prefer manual toJson/fromJson over code generation to avoid adding build_runner dependency unless it already exists in the project. If the project already uses freezed or json_serializable, use those for consistency. Store Duration as int milliseconds: 'announcementDelayMs': delay.inMilliseconds. For fromJson safety: use (json['screenReaderOverride'] as bool?) ??
false pattern for each field. Place this file in lib/src/accessibility/models/accessibility_settings.dart. The AnnouncementVerbosity enum should be in the same file or a sibling file. This model is purely a data container — keep it free of any Flutter widget or platform dependencies.
Testing Requirements
Write unit tests covering: (1) default construction produces expected defaults, (2) toJson produces correct map structure, (3) fromJson round-trips perfectly (serialize then deserialize equals original), (4) fromJson with missing keys uses defaults, (5) copyWith correctly overrides individual fields, (6) Duration millisecond serialization is correct. Use flutter_test. No mocking required for this pure data class.
Flutter's SemanticsService behaves differently between iOS (VoiceOver) and Android (TalkBack) in edge cases — e.g., announcement queuing, focus-gain timing, and attribute support. If the facade does not correctly abstract these differences, announcements may be silent or misfired on one platform, causing regression on the other platform to go unnoticed until device testing.
Mitigation & Contingency
Mitigation: Write platform-divergence unit tests early using SemanticsServiceFacade mocks. Validate announcement delivery on a physical iPhone (VoiceOver) and Android device (TalkBack) at the end of each sprint. Document known platform differences in the facade's inline API comments.
Contingency: If a platform difference cannot be abstracted cleanly, expose a platform-specific override path in the facade and implement targeted workarounds per platform, accepting the added complexity in exchange for correct behaviour.
Accessibility preferences stored in local storage may need new fields as higher-tier epics are implemented (e.g., announcement verbosity, sensitive-field guard toggle). Schema changes to an already-persisted store risk data migration failures or silent defaults on existing installs, breaking user preferences.
Mitigation & Contingency
Mitigation: Design the AccessibilitySettingsRepository with a versioned JSON schema from the start, using merge-with-defaults on read so new fields fall back gracefully. Define the full expected field list upfront based on all downstream epic requirements before writing the first record.
Contingency: If migration fails on a live install, fall back to full reset-to-defaults with a one-time in-app notification informing the user that accessibility preferences have been reset and inviting them to reconfigure.