Make TabBarView/PageView accessible (#12386)

* Make TabBarView/PageView accessible

* review fedback

packages/flutter/lib/src/rendering/sliver_multi_box_adaptor.dart

@@ -290,25 +290,25 @@ abstract class RenderSliverMultiBoxAdaptor extends RenderSliver
     switch (constraints.normalizedGrowthDirection) {
       case GrowthDirection.forward:
         super.visitChildrenForSemantics((RenderObject child) {
-          // The sliver is overlapped at the leading edge.
-          final Offset bottomLeftInViewport = MatrixUtils.transformPoint(
-              child.getTransformTo(parent), child.semanticBounds.bottomLeft
+          // The sliver is overlapped at the leading edge; check if trailing edge is visible.
+          final Offset bottomRightInViewport = MatrixUtils.transformPoint(
+              child.getTransformTo(parent), child.semanticBounds.bottomRight
           );
           final double endOverlap = constraints.overlap;
-          if ((constraints.axis == Axis.vertical && bottomLeftInViewport.dy > endOverlap) ||
-              (constraints.axis == Axis.horizontal && bottomLeftInViewport.dx > endOverlap))
+          if ((constraints.axis == Axis.vertical && bottomRightInViewport.dy > endOverlap) ||
+              (constraints.axis == Axis.horizontal && bottomRightInViewport.dx > endOverlap))
             visitor(child);
         });
         break;
       case GrowthDirection.reverse:
         super.visitChildrenForSemantics((RenderObject child) {
-          // The sliver is overlapped at the trailing edge.
-          final Offset topRightInViewport = MatrixUtils.transformPoint(
-              child.getTransformTo(parent), child.semanticBounds.topRight
+          // The sliver is overlapped at the trailing edge; check if leading edge is visible.
+          final Offset topLeftInViewport = MatrixUtils.transformPoint(
+              child.getTransformTo(parent), child.semanticBounds.topLeft
           );
           final double startOverlap = constraints.remainingPaintExtent - constraints.overlap;
-          if ((constraints.axis == Axis.vertical && topRightInViewport.dy < startOverlap) ||
-              (constraints.axis == Axis.horizontal && topRightInViewport.dx < startOverlap))
+          if ((constraints.axis == Axis.vertical && topLeftInViewport.dy < startOverlap) ||
+              (constraints.axis == Axis.horizontal && topLeftInViewport.dx < startOverlap))
             visitor(child);
         });
         break;

packages/flutter/test/widgets/page_view_test.dart

@@ -4,8 +4,10 @@
 
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 
+import 'semantics_tester.dart';
 import 'states.dart';
 
 const Duration _frameDuration = const Duration(milliseconds: 100);
@@ -505,4 +507,43 @@ void main() {
     ));
     expect(controller2.page, 0);
   });
+
+  testWidgets('PageView exposes semantics of children', (WidgetTester tester) async {
+    final SemanticsTester semantics = new SemanticsTester(tester);
+
+    final PageController controller = new PageController();
+    await tester.pumpWidget(new Directionality(
+      textDirection: TextDirection.ltr,
+      child: new PageView(
+          controller: controller,
+          children: new List<Widget>.generate(3, (int i) {
+            return new Semantics(
+              child: new Text('Page #$i'),
+              container: true,
+            );
+          })
+        ),
+    ));
+    expect(controller.page, 0);
+
+    expect(semantics, includesNodeWith(label: 'Page #0'));
+    expect(semantics, isNot(includesNodeWith(label: 'Page #1')));
+    expect(semantics, isNot(includesNodeWith(label: 'Page #2')));
+
+    controller.jumpToPage(1);
+    await tester.pumpAndSettle();
+
+    expect(semantics, isNot(includesNodeWith(label: 'Page #0')));
+    expect(semantics, includesNodeWith(label: 'Page #1'));
+    expect(semantics, isNot(includesNodeWith(label: 'Page #2')));
+
+    controller.jumpToPage(2);
+    await tester.pumpAndSettle();
+
+    expect(semantics, isNot(includesNodeWith(label: 'Page #0')));
+    expect(semantics, isNot(includesNodeWith(label: 'Page #1')));
+    expect(semantics, includesNodeWith(label: 'Page #2'));
+
+    semantics.dispose();
+  });
 }