Add SegmentedButton expand feature (#142804)

fix #139486

The `expandedInsets` property enhancement for the `SegmentedButton` widget introduces flexibility in button layout design within Flutter applications. When `expandedInsets` is not null, this property allows the `SegmentedButton` to expand, filling the available horizontal space of its parent container and also have the specified insets, thus ensuring a uniform distribution of segment widths across the button. Conversely, with `expandedInsets` is null, the widget sizes itself based on the intrinsic sizes of its segments, preserving the natural width of each segment. This addition enhances the adaptability of the `SegmentedButton` to various UI designs, enabling developers to achieve both expansive and compact button layouts seamlessly.

### Setting expandedInsets = null
<img width="724" alt="image" src="https://github.com/flutter/flutter/assets/65075121/f173b327-a34b-49f0-a7b1-a212a376c5e3">

### Setting expandedInsets = EdgeInsets.zero
<img width="724" alt="image" src="https://github.com/flutter/flutter/assets/36861262/f141a3d3-80e3-4aeb-b7c8-d56ca77ca049">

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

@@ -130,6 +130,7 @@ class SegmentedButton<T> extends StatefulWidget {
     this.onSelectionChanged,
     this.multiSelectionEnabled = false,
     this.emptySelectionAllowed = false,
+    this.expandedInsets,
     this.style,
     this.showSelectedIcon = true,
     this.selectedIcon,
@@ -190,6 +191,13 @@ class SegmentedButton<T> extends StatefulWidget {
   /// [onSelectionChanged] will not be called.
   final bool emptySelectionAllowed;
 
+  /// Determines the segmented button's size and padding based on [expandedInsets].
+  ///
+  /// If null (default), the button adopts its intrinsic content size. When specified,
+  /// the button expands to fill its parent's space, with the [EdgeInsets]
+  /// defining the padding.
+  final EdgeInsets? expandedInsets;
+
   /// A static convenience method that constructs a segmented button
   /// [ButtonStyle] given simple values.
   ///
@@ -539,13 +547,17 @@ class SegmentedButtonState<T> extends State<SegmentedButton<T>> {
       surfaceTintColor: resolve<Color?>((ButtonStyle? style) => style?.surfaceTintColor),
       child: TextButtonTheme(
         data: TextButtonThemeData(style: segmentThemeStyle),
-        child: _SegmentedButtonRenderWidget<T>(
-          tapTargetVerticalPadding: tapTargetVerticalPadding,
-          segments: widget.segments,
-          enabledBorder: _enabled ? enabledBorder : disabledBorder,
-          disabledBorder: disabledBorder,
-          direction: direction,
-          children: buttons,
+        child: Padding(
+          padding: widget.expandedInsets ?? EdgeInsets.zero,
+          child: _SegmentedButtonRenderWidget<T>(
+            tapTargetVerticalPadding: tapTargetVerticalPadding,
+            segments: widget.segments,
+            enabledBorder: _enabled ? enabledBorder : disabledBorder,
+            disabledBorder: disabledBorder,
+            direction: direction,
+            isExpanded: widget.expandedInsets != null,
+            children: buttons,
+          ),
         ),
       ),
     );
@@ -588,6 +600,7 @@ class _SegmentedButtonRenderWidget<T> extends MultiChildRenderObjectWidget {
     required this.disabledBorder,
     required this.direction,
     required this.tapTargetVerticalPadding,
+    required this.isExpanded,
     required super.children,
   }) : assert(children.length == segments.length);
 
@@ -596,6 +609,7 @@ class _SegmentedButtonRenderWidget<T> extends MultiChildRenderObjectWidget {
   final OutlinedBorder disabledBorder;
   final TextDirection direction;
   final double tapTargetVerticalPadding;
+  final bool isExpanded;
 
   @override
   RenderObject createRenderObject(BuildContext context) {
@@ -605,6 +619,7 @@ class _SegmentedButtonRenderWidget<T> extends MultiChildRenderObjectWidget {
       disabledBorder: disabledBorder,
       textDirection: direction,
       tapTargetVerticalPadding: tapTargetVerticalPadding,
+      isExpanded: isExpanded,
     );
   }
 
@@ -633,11 +648,13 @@ class _RenderSegmentedButton<T> extends RenderBox with
     required OutlinedBorder disabledBorder,
     required TextDirection textDirection,
     required double tapTargetVerticalPadding,
+    required bool isExpanded,
   }) : _segments = segments,
        _enabledBorder = enabledBorder,
        _disabledBorder = disabledBorder,
        _textDirection = textDirection,
-       _tapTargetVerticalPadding = tapTargetVerticalPadding;
+       _tapTargetVerticalPadding = tapTargetVerticalPadding,
+       _isExpanded = isExpanded;
 
   List<ButtonSegment<T>> get segments => _segments;
   List<ButtonSegment<T>> _segments;
@@ -689,6 +706,16 @@ class _RenderSegmentedButton<T> extends RenderBox with
     markNeedsLayout();
   }
 
+  bool get isExpanded => _isExpanded;
+  bool _isExpanded;
+  set isExpanded(bool value) {
+    if (value == _isExpanded) {
+      return;
+    }
+    _isExpanded = value;
+    markNeedsLayout();
+  }
+
   @override
   double computeMinIntrinsicWidth(double height) {
     RenderBox? child = firstChild;
@@ -770,13 +797,18 @@ class _RenderSegmentedButton<T> extends RenderBox with
 
   Size _calculateChildSize(BoxConstraints constraints) {
     double maxHeight = 0;
-    double childWidth = constraints.minWidth / childCount;
     RenderBox? child = firstChild;
-    while (child != null) {
-      childWidth = math.max(childWidth, child.getMaxIntrinsicWidth(double.infinity));
-      child = childAfter(child);
+    double childWidth;
+    if (_isExpanded) {
+      childWidth = constraints.maxWidth / childCount;
+    } else {
+      childWidth = constraints.minWidth / childCount;
+      while (child != null) {
+        childWidth = math.max(childWidth, child.getMaxIntrinsicWidth(double.infinity));
+        child = childAfter(child);
+      }
+      childWidth = math.min(childWidth, constraints.maxWidth / childCount);
     }
-    childWidth = math.min(childWidth, constraints.maxWidth / childCount);
     child = firstChild;
     while (child != null) {
       final double boxHeight = child.getMaxIntrinsicHeight(childWidth);

packages/flutter/test/material/segmented_button_test.dart

@@ -6,6 +6,7 @@
 // machines.
 import 'dart:ui';
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -855,6 +856,59 @@ void main() {
       )
     );
   });
+
+  testWidgets('SegmentedButton expands to fill the available width when expandedInsets is not null', (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: Center(
+          child: SegmentedButton<int>(
+            segments: const <ButtonSegment<int>>[
+              ButtonSegment<int>(value: 1, label: Text('Segment 1')),
+              ButtonSegment<int>(value: 2, label: Text('Segment 2')),
+            ],
+           selected: const <int>{1},
+           expandedInsets: EdgeInsets.zero,
+          ),
+        ),
+      ),
+    ));
+
+    // Get the width of the SegmentedButton.
+    final RenderBox box = tester.renderObject(find.byType(SegmentedButton<int>));
+    final double segmentedButtonWidth = box.size.width;
+
+    // Get the width of the parent widget.
+    final double screenWidth = tester.getSize(find.byType(Scaffold)).width;
+
+    // The width of the SegmentedButton must be equal to the width of the parent widget.
+    expect(segmentedButtonWidth, equals(screenWidth));
+  });
+
+  testWidgets('SegmentedButton does not expand when expandedInsets is null', (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: Center(
+          child: SegmentedButton<int>(
+            segments: const <ButtonSegment<int>>[
+              ButtonSegment<int>(value: 1, label: Text('Segment 1')),
+              ButtonSegment<int>(value: 2, label: Text('Segment 2')),
+            ],
+            selected: const <int>{1},
+          ),
+        ),
+      ),
+    ));
+
+    // Get the width of the SegmentedButton.
+    final RenderBox box = tester.renderObject(find.byType(SegmentedButton<int>));
+    final double segmentedButtonWidth = box.size.width;
+
+    // Get the width of the parent widget.
+    final double screenWidth = tester.getSize(find.byType(Scaffold)).width;
+
+    // The width of the SegmentedButton must be less than the width of the parent widget.
+    expect(segmentedButtonWidth, lessThan(screenWidth));
+  }, skip: kIsWeb && !isCanvasKit); // https://github.com/flutter/flutter/issues/145527
 }
 
 Set<MaterialState> enabled = const <MaterialState>{};