Implement screen reader simulation in AccessibilityTestHarness
epic-screen-reader-support-core-services-task-002 — Extend the AccessibilityTestHarness with a screen reader simulation layer that replays the semantic tree as a VoiceOver/TalkBack user would experience it. Implement methods to simulate focus traversal order, read out announcement sequences, and detect gaps in semantic coverage. Provide a simulateScreenReaderSession() utility that captures the full spoken output for a given widget tree.
Acceptance Criteria
Technical Requirements
Execution Context
Tier 1 - 540 tasks
Can start after Tier 0 completes
Implementation Notes
Use SemanticsBinding.instance.ensureSemantics() inside the harness setUp to force semantics on in the test environment — Flutter disables semantics by default in tests. Walk the semantic tree via SemanticsNode.visitChildren recursively, collecting nodes into a flat ordered list. To determine VoiceOver vs TalkBack traversal differences, check platform at test invocation and apply the appropriate sort comparator (VoiceOver: geometry-based reading order; TalkBack: child insertion order). For gap detection, flag any node where SemanticsNode.hasFlag(SemanticsFlag.isTextField) || hasAction(SemanticsAction.tap) but SemanticsNode.label.isEmpty.
Store the simulation result as an immutable value object to allow assertion helpers like expectUtteranceContains(session, 'Submit'). Avoid directly calling the private _semanticsOwner — use the public SemanticsBinding API. This task builds directly on the harness scaffolding from task-001; confirm that task's SemanticsHandle lifecycle management is reused here.
Testing Requirements
Unit tests using flutter_test: verify traversal order matches expected depth-first sequence for a hand-crafted widget tree; verify gap detection identifies an unlabelled IconButton; verify merged semantics are collapsed into a single utterance. Integration tests: run simulateScreenReaderSession() against the Activity Registration wizard and assert all form fields produce a label utterance. Edge-case tests: empty widget tree returns empty session; widget tree with only non-interactive decorative nodes returns zero focusable nodes. Target coverage: 90% line coverage on the simulation engine classes.
Flutter's build pipeline and SemanticsService.announce() operate asynchronously. Announcements triggered too early (before the semantic tree settles) may be swallowed silently on both platforms, causing acceptance criteria around the 500ms window to fail intermittently in CI and on device, which would block pilot launch.
Mitigation & Contingency
Mitigation: Implement the LiveRegionAnnouncer with a post-frame callback delay and an internal timing guard. Write integration tests using WidgetTester.pump() sequences that verify announcement delivery across multiple frame boundaries. Validate on physical devices at each sprint boundary, not only in CI.
Contingency: If consistent announcement timing cannot be achieved within Flutter's semantic pipeline, switch to a platform channel approach that calls native UIAccessibility.post (iOS) and AccessibilityManager.announce (Android) directly, bypassing Flutter's intermediary.
Flutter does not natively emit a focus-gain event to Dart code when VoiceOver or TalkBack moves focus to a specific widget. If the intercept mechanism for the SensitiveFieldWarningDialog relies on an unsupported or undocumented hook in the semantics tree, it may miss focus events for some field types or in some navigation contexts, leaving sensitive data unprotected.
Mitigation & Contingency
Mitigation: Prototype the focus-intercept mechanism during the first sprint of this epic, before building the dialog UI. Evaluate Flutter's SemanticsBinding.instance callbacks and custom SemanticsActions as intercept points. Document the chosen mechanism with platform compatibility notes.
Contingency: If no reliable focus-intercept is available, implement an alternative where sensitive fields show a static 'Screen reader active — tap to reveal' overlay instead of an OS dialog, which is less seamless but achieves equivalent privacy protection without relying on an unreliable event hook.
The AccessibilityTestHarness depends on internal flutter_test semantic tree APIs that can change between Flutter minor versions. If the project upgrades Flutter during this epic, the harness may break silently, causing CI accessibility tests to pass while actually skipping assertions.
Mitigation & Contingency
Mitigation: Pin the Flutter SDK version in pubspec.yaml for the duration of this epic. Document which flutter_test APIs are used and their stability tier. Add a canary test that explicitly fails if the semantic tree API surface changes.
Contingency: If a forced Flutter upgrade breaks the harness, prioritise patching the harness as a blocking task before any other epic work continues, using the canary test failure as the trigger.