Add tests/matchers for automatic accessibility testing (#20462)

\

packages/flutter_test/lib/flutter_test.dart

@@ -46,6 +46,7 @@ library flutter_test;
 
 export 'dart:async' show Future;
 
+export 'src/accessibility.dart';
 export 'src/all_elements.dart';
 export 'src/binding.dart';
 export 'src/controller.dart';

packages/flutter_test/lib/src/matchers.dart

@@ -17,9 +17,11 @@ import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/services.dart';
 
+import 'accessibility.dart';
 import 'binding.dart';
 import 'finders.dart';
 import 'goldens.dart';
+import 'widget_tester.dart' show WidgetTester;
 
 /// Asserts that the [Finder] matches no widgets in the widget tree.
 ///
@@ -461,6 +463,37 @@ Matcher matchesSemanticsData({
   );
 }
 
+/// Asserts that the currently rendered widget meets the provided accessibility
+/// `guideline`.
+///
+/// This matcher requires the result to be awaited and for semantics to be
+/// enabled first.
+///
+/// ## Sample code
+///
+/// ```dart
+/// final SemanticsHandle handle = tester.ensureSemantics();
+/// await meetsGuideline(tester, meetsGuideline(textContrastGuideline));
+/// handle.dispose();
+/// ```
+///
+/// Supported accessibility guidelines:
+///
+///   * [androidTapTargetGuideline], for Android minimum tapable area guidelines.
+///   * [iOSTapTargetGuideline], for iOS minimum tapable area guidelines.
+///   * [textContrastGuideline], for WCAG minimum text contrast guidelines.
+AsyncMatcher meetsGuideline(AccessibilityGuideline guideline) {
+  return new _MatchesAccessibilityGuideline(guideline);
+}
+
+/// The inverse matcher of [meetsGuideline].
+///
+/// This is needed because the [isNot] matcher does not compose with an
+/// [AsyncMatcher].
+AsyncMatcher doesNotMeetGuideline(AccessibilityGuideline guideline) {
+  return new _DoesNotMatchAccessibilityGuideline(guideline);
+}
+
 class _FindsWidgetMatcher extends Matcher {
   const _FindsWidgetMatcher(this.min, this.max);
 
@@ -1660,3 +1693,41 @@ class _MatchesSemanticsData extends Matcher {
     return mismatchDescription.add(matchState['failure']);
   }
 }
+
+class _MatchesAccessibilityGuideline extends AsyncMatcher {
+  _MatchesAccessibilityGuideline(this.guideline);
+
+  final AccessibilityGuideline guideline;
+
+  @override
+  Description describe(Description description) {
+    return description.add(guideline.description);
+  }
+
+  @override
+  Future<String> matchAsync(covariant WidgetTester tester) async {
+    final Evaluation result = await guideline.evaluate(tester);
+    if (result.passed)
+      return null;
+    return result.reason;
+  }
+}
+
+class _DoesNotMatchAccessibilityGuideline extends AsyncMatcher {
+  _DoesNotMatchAccessibilityGuideline(this.guideline);
+
+  final AccessibilityGuideline guideline;
+
+  @override
+  Description describe(Description description) {
+    return description.add('Does not ' + guideline.description);
+  }
+
+  @override
+  Future<String> matchAsync(covariant WidgetTester tester) async {
+    final Evaluation result = await guideline.evaluate(tester);
+    if (result.passed)
+      return 'Failed';
+    return null;
+  }
+}
\ No newline at end of file

packages/flutter_test/test/accessibility_test.dart

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('text contrast guideline', () {
+    testWidgets('black text on white background - Text Widget - direct style', (WidgetTester tester) async {
+      final SemanticsHandle handle = tester.ensureSemantics();
+      await tester.pumpWidget(_boilerplate(
+        const Text(
+          'this is a test',
+          style: TextStyle(fontSize: 14.0, color: Colors.black),
+        ),
+      ));
+      await expectLater(tester, meetsGuideline(textContrastGuideline));
+      handle.dispose();
+    });
+
+    testWidgets('white text on black background - Text Widget - direct style', (WidgetTester tester) async {
+      final SemanticsHandle handle = tester.ensureSemantics();
+      await tester.pumpWidget(_boilerplate(
+        new Container(
+          width: 200.0,
+          height: 200.0,
+          color: Colors.black,
+          child: const Text(
+            'this is a test',
+            style: TextStyle(fontSize: 14.0, color: Colors.white),
+          ),
+        ),
+      ));
+      await expectLater(tester, meetsGuideline(textContrastGuideline));
+      handle.dispose();
+    });
+
+    testWidgets('black text on white background - Text Widget - inherited style', (WidgetTester tester) async {
+      final SemanticsHandle handle = tester.ensureSemantics();
+      await tester.pumpWidget(_boilerplate(
+        new DefaultTextStyle(
+          style: const TextStyle(fontSize: 14.0, color: Colors.black),
+          child: new Container(
+            color: Colors.white,
+            child: const Text('this is a test'),
+          ),
+        ),
+      ));
+      await expectLater(tester, meetsGuideline(textContrastGuideline));
+      handle.dispose();
+    });
+
+    testWidgets('white text on black background - Text Widget - inherited style', (WidgetTester tester) async {
+      final SemanticsHandle handle = tester.ensureSemantics();
+      await tester.pumpWidget(_boilerplate(
+        new DefaultTextStyle(
+          style: const TextStyle(fontSize: 14.0, color: Colors.white),
+          child: new Container(
+            width: 200.0,
+            height: 200.0,
+            color: Colors.black,
+            child: const Text('this is a test'),
+          ),
+        ),
+      ));
+      await expectLater(tester, meetsGuideline(textContrastGuideline));
+      handle.dispose();
+    });
+
+    testWidgets('Material text field - amber on amber', (WidgetTester tester) async {
+      final SemanticsHandle handle = tester.ensureSemantics();
+      await tester.pumpWidget(new MaterialApp(
+        home: new Scaffold(
+          body: new Container(
+            width: 200.0,
+            height: 200.0,
+            color: Colors.amberAccent,
+            child: new TextField(
+              style: const TextStyle(color: Colors.amber),
+              controller: new TextEditingController(text: 'this is a test'),
+            ),
+          ),
+        ),
+      ));
+      await expectLater(tester, doesNotMeetGuideline(textContrastGuideline));
+      handle.dispose();
+    });
+
+    testWidgets('Material text field - default style', (WidgetTester tester) async {
+      final SemanticsHandle handle = tester.ensureSemantics();
+      await tester.pumpWidget(new MaterialApp(
+          home: new Scaffold(
+            body: new Center(
+              child: new TextField(
+                controller: new TextEditingController(text: 'this is a test'),
+              ),
+            ),
+          ),
+        ),
+      );
+      await expectLater(tester, meetsGuideline(textContrastGuideline));
+      handle.dispose();
+    });
+
+    testWidgets('grey text on white background fails with correct message', (WidgetTester tester) async {
+      final SemanticsHandle handle = tester.ensureSemantics();
+      await tester.pumpWidget(_boilerplate(
+        new Container(
+          width: 200.0,
+          height: 200.0,
+          color: Colors.yellow,
+          child: const Text(
+            'this is a test',
+            style: TextStyle(fontSize: 14.0, color: Colors.yellowAccent),
+          ),
+        ),
+      ));
+      final Evaluation result = await textContrastGuideline.evaluate(tester);
+      expect(result.passed, false);
+      expect(result.reason,
+        'SemanticsNode#21(Rect.fromLTRB(300.0, 200.0, 500.0, 400.0), label: "this is a test",'
+        ' textDirection: ltr):\nExpected contrast ratio of at least '
+        '4.5 but found 1.17 for a font size of 14.0. '
+        'The computed foreground color was: Color(0xfffafafa), '
+        'The computed background color was: Color(0xffffeb3b)\n'
+        'See also: https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html');
+      handle.dispose();
+    });
+  });
+
+  group('tap target size guideline', () {
+    testWidgets('Tappable box at 48 by 48', (WidgetTester tester) async {
+      final SemanticsHandle handle = tester.ensureSemantics();
+      await tester.pumpWidget(_boilerplate(
+        new SizedBox(
+          width: 48.0,
+          height: 48.0,
+          child: new GestureDetector(
+            onTap: () {},
+          ),
+        ),
+      ));
+      await expectLater(tester, meetsGuideline(androidTapTargetGuideline));
+      handle.dispose();
+    });
+
+    testWidgets('Tappable box at 47 by 48', (WidgetTester tester) async {
+      final SemanticsHandle handle = tester.ensureSemantics();
+      await tester.pumpWidget(_boilerplate(
+        new SizedBox(
+          width: 47.0,
+          height: 48.0,
+          child: new GestureDetector(
+            onTap: () {},
+          ),
+        ),
+      ));
+      await expectLater(tester, doesNotMeetGuideline(androidTapTargetGuideline));
+      handle.dispose();
+    });
+
+    testWidgets('Tappable box at 48 by 47', (WidgetTester tester) async {
+      final SemanticsHandle handle = tester.ensureSemantics();
+      await tester.pumpWidget(_boilerplate(
+        new SizedBox(
+          width: 48.0,
+          height: 47.0,
+          child: new GestureDetector(
+            onTap: () {},
+          ),
+        ),
+      ));
+      await expectLater(tester, doesNotMeetGuideline(androidTapTargetGuideline));
+      handle.dispose();
+    });
+
+    testWidgets('Tappable box at 48 by 48 shrunk by transform', (WidgetTester tester) async {
+      final SemanticsHandle handle = tester.ensureSemantics();
+      await tester.pumpWidget(_boilerplate(
+        new Transform.scale(
+          scale: 0.5, // should have new height of 24 by 24.
+          child: new SizedBox(
+            width: 48.0,
+            height: 48.0,
+            child: new GestureDetector(
+              onTap: () {},
+            ),
+          ),
+        ),
+      ));
+      await expectLater(tester, doesNotMeetGuideline(androidTapTargetGuideline));
+      handle.dispose();
+    });
+
+    testWidgets('Too small tap target fails with the correct message', (WidgetTester tester) async {
+      final SemanticsHandle handle = tester.ensureSemantics();
+      await tester.pumpWidget(_boilerplate(
+        new SizedBox(
+          width: 48.0,
+          height: 47.0,
+          child: new GestureDetector(
+            onTap: () {},
+          ),
+        ),
+      ));
+      final Evaluation result = await androidTapTargetGuideline.evaluate(tester);
+      expect(result.passed, false);
+      expect(result.reason,
+        'SemanticsNode#36(Rect.fromLTRB(376.0, 276.5, 424.0, 323.5), actions: [tap]): expected tap '
+        'target size of at least Size(48.0, 48.0), but found Size(48.0, 47.0)\n'
+        'See also: https://support.google.com/accessibility/android/answer/7101858?hl=en');
+      handle.dispose();
+    });
+  });
+}
+
+Widget _boilerplate(Widget child) {
+  return new MaterialApp(
+    home: new Scaffold(body: new Center(child: child)),
+  );
+}