flutter-accessibility（Flutter 无障碍）

flutter-accessibility-and-adaptive-design

Goal

Implements, audits, and enforces accessibility (a11y) and adaptive design standards in Flutter applications. Ensures compliance with WCAG 2 and EN 301 549 by applying proper semantic roles, contrast ratios, tap target sizes, and assistive technology integrations. Constructs adaptive layouts that respond to available screen space and input modalities (touch, mouse, keyboard) without relying on hardware-specific checks or locked orientations.

Decision Logic

When implementing UI components, follow this decision tree to determine the required accessibility and adaptive design implementations:

1. Is the app targeting Flutter Web?

- Yes: Ensure SemanticsBinding.instance.ensureSemantics(); is called at startup. Explicitly map custom widgets to SemanticsRole to generate correct ARIA tags. - No: Proceed to standard mobile/desktop semantics.

1. Is the widget interactive?

- Yes: Wrap in Semantics with button: true or appropriate role. Ensure tap target is $\ge$ 48x48 logical pixels. - No: Ensure text contrast meets WCAG standards (4.5:1 for small text, 3.0:1 for large text).

1. Does the layout need to change based on screen size?

- Yes: Use LayoutBuilder or MediaQuery.sizeOf(context). Do NOT use MediaQuery.orientation or hardware type checks (e.g., isTablet). - No: Use standard flexible widgets (Expanded, Flexible) to fill available space.

1. Does the app support desktop/web input?

- Yes: Implement FocusableActionDetector, Shortcuts, and MouseRegion for hover states and keyboard traversal. - No: Focus primarily on touch targets and screen reader traversal order.

Instructions

1. Initialize Web Accessibility (If Applicable) For web targets, accessibility is disabled by default for performance. Force enable it at the entry point. import 'package:flutter/foundation.dart'; import 'package:flutter/semantics.dart'; void main() {runApp(const MyApp()); if (kIsWeb) {SemanticsBinding.instance.ensureSemantics();}}
2. Apply Semantic Annotations Use Semantics, MergeSemantics, and ExcludeSemantics to build a clean accessibility tree. For custom web components, explicitly define the SemanticsRole. Semantics(role: SemanticsRole.button, label: 'Submit Form', hint: 'Press to send your application', button: true, child: GestureDetector(onTap: _submit, child: const CustomButtonUI(),),)
3. Enforce Tap Target and Contrast Standards Ensure all interactive elements meet the 48x48 dp minimum (Android) or 44x44 pt minimum (iOS/Web). // Example of enforcing minimum tap target size ConstrainedBox(constraints: const BoxConstraints(minWidth: 48.0, minHeight: 48.0,), child: IconButton(icon: const Icon(Icons.info), onPressed: () {}, tooltip: 'Information', // Tooltip.message follows Tooltip.child in semantics tree),)
4. Implement Adaptive Layouts Use LayoutBuilder to respond to available space rather than device type. LayoutBuilder(builder: (context, constraints) {if (constraints.maxWidth > 600) {return const WideDesktopLayout();} else {return const NarrowMobileLayout();}},)
5. Implement Keyboard and Mouse Support Use FocusableActionDetector for custom controls to handle focus, hover, and keyboard shortcuts simultaneously. FocusableActionDetector(onFocusChange: (hasFocus) => setState(() => _hasFocus = hasFocus), onShowHoverHighlight: (hasHover) => setState(() => _hasHover = hasHover), actions: <Type, Action<Intent>>{ActivateIntent: CallbackAction<Intent>(onInvoke: (intent) {_performAction(); return null;},),}, child: MouseRegion(cursor: SystemMouseCursors.click, child: CustomWidget(isHovered: _hasHover, isFocused: _hasFocus),),)
6. Manage Focus Traversal Group related widgets using FocusTraversalGroup to ensure logical tab order for keyboard users. FocusTraversalGroup(policy: OrderedTraversalPolicy(), child: Column(children: [FocusTraversalOrder(order: const NumericFocusOrder(1.0), child: TextField(),), FocusTraversalOrder(order: const NumericFocusOrder(2.0), child: ElevatedButton(onPressed: () {}, child: Text('Submit')),),],),)
7. Validate Accessibility via Automated Tests STOP AND ASK THE USER: "Would you like me to generate widget tests to validate accessibility guidelines (contrast, tap targets) for your UI?" If yes, implement the AccessibilityGuideline API in the test suite: import 'package:flutter_test/flutter_test.dart'; void main() {testWidgets('Validates a11y guidelines', (WidgetTester tester) async {final SemanticsHandle handle = tester.ensureSemantics(); await tester.pumpWidget(const MyApp()); await expectLater(tester, meetsGuideline(androidTapTargetGuideline)); await expectLater(tester, meetsGuideline(iOSTapTargetGuideline)); await expectLater(tester, meetsGuideline(labeledTapTargetGuideline)); await expectLater(tester, meetsGuideline(textContrastGuideline)); handle.dispose();});}

Constraints

• Never lock device orientation. Apps must support both portrait and landscape modes to comply with accessibility standards.
• Never use hardware type checks (e.g., checking if the device is a phone or tablet) for layout decisions. Always use MediaQuery.sizeOf or LayoutBuilder.
• Never use MediaQuery.orientation near the top of the widget tree to switch layouts. Rely on available width/height breakpoints.
• Always provide Semantics labels for custom interactive widgets or images that convey meaning.
• Always use PageStorageKey on scrollable lists that do not change layout during orientation shifts to preserve scroll state.
• Do not consume infinite horizontal space for text fields or lists on large screens; constrain maximum widths for readability.
• Account for Tooltip semantics order: Tooltip.message is visited immediately *after* Tooltip.child in the semantics tree. Update tests accordingly.