Add `tabs_utils.dart` class (#143937)

This a test utility class for `tabs_test.dart` to prepare the class for Material 3 tests updates.

More info in https://github.com/flutter/flutter/issues/139076

packages/flutter/test/material/tabs_utils.dart

+// Copyright 2014 The Flutter 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/rendering.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+// This returns render paragraph of the Tab label text.
+RenderParagraph getTabText(WidgetTester tester, String text) {
+  return tester.renderObject<RenderParagraph>(find.descendant(
+    of: find.byWidgetPredicate((Widget widget) => widget.runtimeType.toString() == '_TabStyle'),
+    matching: find.text(text),
+  ));
+}
+
+// This creates and returns a TabController.
+TabController createTabController({
+  required int length,
+  required TickerProvider vsync,
+  int initialIndex = 0,
+  Duration? animationDuration,
+}) {
+  final TabController result = TabController(
+    length: length,
+    vsync: vsync,
+    initialIndex: initialIndex,
+    animationDuration: animationDuration,
+  );
+  addTearDown(result.dispose);
+  return result;
+}
+
+// This widget is used to test widget state in the tabs_test.dart file.
+class TabStateMarker extends StatefulWidget {
+  const TabStateMarker({ super.key, this.child });
+
+  final Widget? child;
+
+  @override
+  TabStateMarkerState createState() => TabStateMarkerState();
+}
+
+class TabStateMarkerState extends State<TabStateMarker> {
+  String? marker;
+
+  @override
+  Widget build(BuildContext context) {
+    return widget.child ?? Container();
+  }
+}
+
+// Tab controller builder for TabControllerFrame widget.
+typedef TabControllerFrameBuilder = Widget Function(BuildContext context, TabController controller);
+
+// This widget creates a TabController and passes it to the builder.
+class TabControllerFrame extends StatefulWidget {
+  const TabControllerFrame({
+    super.key,
+    required this.length,
+    this.initialIndex = 0,
+    required this.builder,
+  });
+
+  final int length;
+  final int initialIndex;
+  final TabControllerFrameBuilder builder;
+
+  @override
+  TabControllerFrameState createState() => TabControllerFrameState();
+}
+
+class TabControllerFrameState extends State<TabControllerFrame> with SingleTickerProviderStateMixin {
+  late TabController _controller;
+
+  @override
+  void initState() {
+    super.initState();
+    _controller = TabController(
+      vsync: this,
+      length: widget.length,
+      initialIndex: widget.initialIndex,
+    );
+  }
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return widget.builder(context, _controller);
+  }
+}
+
+// Test utility class to test tab indicator drawing.
+class TabIndicatorRecordingCanvas extends TestRecordingCanvas {
+  TabIndicatorRecordingCanvas(this.indicatorColor);
+
+  final Color indicatorColor;
+  late Rect indicatorRect;
+
+  @override
+  void drawLine(Offset p1, Offset p2, Paint paint) {
+    // Assuming that the indicatorWeight is 2.0, the default.
+    const double indicatorWeight = 2.0;
+    if (paint.color == indicatorColor) {
+      indicatorRect = Rect.fromPoints(p1, p2).inflate(indicatorWeight / 2.0);
+    }
+  }
+}
+
+// This creates a Fake implementation of ScrollMetrics.
+class TabMockScrollMetrics extends Fake implements ScrollMetrics { }
+
+class TabBarTestScrollPhysics extends ScrollPhysics {
+  const TabBarTestScrollPhysics({ super.parent });
+
+  @override
+  TabBarTestScrollPhysics applyTo(ScrollPhysics? ancestor) {
+    return TabBarTestScrollPhysics(parent: buildParent(ancestor));
+  }
+
+  @override
+  double applyPhysicsToUserOffset(ScrollMetrics position, double offset) {
+    return offset == 10 ? 20 : offset;
+  }
+
+  static final SpringDescription _kDefaultSpring = SpringDescription.withDampingRatio(
+    mass: 0.5,
+    stiffness: 500.0,
+    ratio: 1.1,
+  );
+
+  @override
+  SpringDescription get spring => _kDefaultSpring;
+}
+
+// This widget is used to log the lifecycle of the TabBarView children.
+class TabBody extends StatefulWidget {
+  const TabBody({
+    super.key,
+    required this.index,
+    required this.log,
+    this.marker = '',
+  });
+
+  final int index;
+  final List<String> log;
+  final String marker;
+
+  @override
+  State<TabBody> createState() => TabBodyState();
+}
+
+class TabBodyState extends State<TabBody> {
+  @override
+  void initState() {
+    widget.log.add('init: ${widget.index}');
+    super.initState();
+  }
+
+  @override
+  void didUpdateWidget(TabBody oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    // To keep the logging straight, widgets must not change their index.
+    assert(oldWidget.index == widget.index);
+  }
+
+  @override
+  void dispose() {
+    widget.log.add('dispose: ${widget.index}');
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Center(
+      child: widget.marker.isEmpty
+        ? Text('${widget.index}')
+        : Text('${widget.index}-${widget.marker}'),
+    );
+  }
+}
+
+// This widget is used to test the lifecycle of the TabBarView children with Ink widget.
+class TabKeepAliveInk extends StatefulWidget {
+  const TabKeepAliveInk({ super.key, required this.title });
+
+  final String title;
+
+  @override
+  State<StatefulWidget> createState() => _TabKeepAliveInkState();
+}
+
+class _TabKeepAliveInkState extends State<TabKeepAliveInk> with AutomaticKeepAliveClientMixin {
+  @override
+  bool get wantKeepAlive => true;
+
+  @override
+  Widget build(BuildContext context) {
+    super.build(context);
+    return Ink(
+      child: Text(widget.title),
+    );
+  }
+}
+
+// This widget is used to test the lifecycle of the TabBarView children.
+class TabAlwaysKeepAliveWidget extends StatefulWidget {
+  const TabAlwaysKeepAliveWidget({super.key});
+
+  static String text = 'AlwaysKeepAlive';
+
+  @override
+  State<TabAlwaysKeepAliveWidget> createState() => _TabAlwaysKeepAliveWidgetState();
+}
+
+class _TabAlwaysKeepAliveWidgetState extends State<TabAlwaysKeepAliveWidget> with AutomaticKeepAliveClientMixin {
+  @override
+  bool get wantKeepAlive => true;
+
+  @override
+  Widget build(BuildContext context) {
+    super.build(context);
+    return Text(TabAlwaysKeepAliveWidget.text);
+  }
+}