add physics to TabBarView (#11150)

packages/flutter/lib/src/material/tabs.dart

@@ -795,6 +795,7 @@ class TabBarView extends StatefulWidget {
     Key key,
     @required this.children,
     this.controller,
+    this.physics,
   }) : assert(children != null), super(key: key);
 
   /// This widget's selection and animation state.
@@ -806,6 +807,17 @@ class TabBarView extends StatefulWidget {
   /// One widget per tab.
   final List<Widget> children;
 
+  /// How the page view should respond to user input.
+  ///
+  /// For example, determines how the page view continues to animate after the
+  /// user stops dragging the page view.
+  ///
+  /// The physics are modified to snap to page boundaries using
+  /// [PageScrollPhysics] prior to being used.
+  ///
+  /// Defaults to matching platform conventions.
+  final ScrollPhysics physics;
+
   @override
   _TabBarViewState createState() => new _TabBarViewState();
 }
@@ -954,7 +966,7 @@ class _TabBarViewState extends State<TabBarView> {
       onNotification: _handleScrollNotification,
       child: new PageView(
         controller: _pageController,
-        physics: _kTabBarViewPhysics,
+        physics: widget.physics == null ? _kTabBarViewPhysics : _kTabBarViewPhysics.applyTo(widget.physics),
         children: _children,
       ),
     );

packages/flutter/test/material/tabs_test.dart

@@ -7,6 +7,7 @@ import 'dart:ui' show SemanticsFlags, SemanticsAction;
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/widgets.dart';
+import 'package:flutter/physics.dart';
 
 import '../rendering/mock_canvas.dart';
 import '../rendering/recording_canvas.dart';
@@ -128,6 +129,24 @@ class TabIndicatorRecordingCanvas extends TestRecordingCanvas {
   }
 }
 
+class TestScrollPhysics extends ScrollPhysics {
+  const TestScrollPhysics({ ScrollPhysics parent }) : super(parent: parent);
+  
+  @override
+  TestScrollPhysics applyTo(ScrollPhysics ancestor) {
+    return new TestScrollPhysics(parent: buildParent(ancestor));
+  }
+  
+  static final SpringDescription _kDefaultSpring = new SpringDescription.withDampingRatio(
+    mass: 0.5,
+    springConstant: 500.0,
+    ratio: 1.1,
+  );
+  
+  @override
+  SpringDescription get spring => _kDefaultSpring;
+}
+
 void main() {
   testWidgets('TabBar tap selects tab', (WidgetTester tester) async {
     final List<String> tabs = <String>['A', 'B', 'C'];
@@ -812,6 +831,53 @@ void main() {
     expect(tabController.index, 2);
   });
 
+  testWidgets('TabBarView scrolls end very close to a new page with custom physics', (WidgetTester tester) async {
+    final TabController tabController = new TabController(
+      vsync: const TestVSync(),
+      initialIndex: 1,
+      length: 3,
+    );
+
+    await tester.pumpWidget(
+      new SizedBox.expand(
+        child: new Center(
+          child: new SizedBox(
+            width: 400.0,
+            height: 400.0,
+            child: new TabBarView(
+              controller: tabController,
+              physics: const TestScrollPhysics(),
+              children: <Widget>[
+                const Center(child: const Text('0')),
+                const Center(child: const Text('1')),
+                const Center(child: const Text('2')),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(tabController.index, 1);
+
+    final PageView pageView = tester.widget(find.byType(PageView));
+    final PageController pageController = pageView.controller;
+    final ScrollPosition position = pageController.position;
+
+    // The TabBarView's page width is 400, so page 0 is at scroll offset 0.0,
+    // page 1 is at 400.0, page 2 is at 800.0.
+
+    expect(position.pixels, 400.0);
+
+    // Not close enough to switch to page 2
+    pageController.jumpTo(800.0 - 1.25 * position.physics.tolerance.distance);
+    expect(tabController.index, 1);
+
+    // Close enough to switch to page 2
+    pageController.jumpTo(800.0 - 0.75 * position.physics.tolerance.distance);
+    expect(tabController.index, 2);
+  });
+
   testWidgets('Scrollable TabBar with a non-zero TabController initialIndex', (WidgetTester tester) async {
     // This is a regression test for https://github.com/flutter/flutter/issues/9374