[Stepper] Add Alternative Label on horizontal-type Stepper (#91496)

* Add label on Step

* Add _buildLabelText, _labelStyle

* Update _buildHorizontal with _buildLabelText

* Fix Redundant Center widgets

* Fix Unnecessary Column Widget

* Fix Unnecessary import

* Fix Unnecessary Container

* Set label style to bodyText1 to match title style

* Update test for Alternative Label

* Fix code format

* Update label docstring

* Update to test textstyle of label

* Update to Test `TextStyles` of Label

* Fix typo in inline comments

* Update for Stepper type to `horizontal`

* Restore `_buildVerticalHeader`

* Update docstring, `Only [StepperType.horizontal]`

* Update for Readability

* Add `_isLabel` method

* Update `Horizontal height` with `_isLabel`

* Update to make `_buildIcon` centered

* Fix for `curly_braces_in_flow_control_structures`

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

@@ -138,6 +138,7 @@ class Step {
     required this.content,
     this.state = StepState.indexed,
     this.isActive = false,
+    this.label,
   }) : assert(title != null),
        assert(content != null),
        assert(state != null);
@@ -162,6 +163,10 @@ class Step {
 
   /// Whether or not the step is active. The flag only influences styling.
   final bool isActive;
+
+  /// Only [StepperType.horizontal], Optional widget that appears under the [title].
+  /// By default, uses the `bodyText1` theme.
+  final Widget? label;
 }
 
 /// A material stepper widget that displays progress through a sequence of
@@ -353,6 +358,15 @@ class _StepperState extends State<Stepper> with TickerProviderStateMixin {
     return Theme.of(context).brightness == Brightness.dark;
   }
 
+  bool _isLabel() {
+    for (final Step step in widget.steps) {
+      if (step.label != null) {
+        return true;
+      }
+    }
+    return false;
+  }
+
   Widget _buildLine(bool visible) {
     return Container(
       width: visible ? 1.0 : 0.0,
@@ -573,6 +587,27 @@ class _StepperState extends State<Stepper> with TickerProviderStateMixin {
     }
   }
 
+  TextStyle _labelStyle(int index) {
+    final ThemeData themeData = Theme.of(context);
+    final TextTheme textTheme = themeData.textTheme;
+
+    assert(widget.steps[index].state != null);
+    switch (widget.steps[index].state) {
+      case StepState.indexed:
+      case StepState.editing:
+      case StepState.complete:
+        return textTheme.bodyText1!;
+      case StepState.disabled:
+        return textTheme.bodyText1!.copyWith(
+          color: _isDark() ? _kDisabledDark : _kDisabledLight,
+        );
+      case StepState.error:
+        return textTheme.bodyText1!.copyWith(
+          color: _isDark() ? _kErrorDark : _kErrorLight,
+        );
+    }
+  }
+
   Widget _buildHeaderText(int index) {
     return Column(
       crossAxisAlignment: CrossAxisAlignment.start,
@@ -598,6 +633,17 @@ class _StepperState extends State<Stepper> with TickerProviderStateMixin {
     );
   }
 
+  Widget _buildLabelText(int index) {
+    if (widget.steps[index].label != null) {
+      return AnimatedDefaultTextStyle(
+        style: _labelStyle(index),
+        duration: kThemeAnimationDuration,
+        child: widget.steps[index].label!,
+      );
+    }
+    return const SizedBox();
+  }
+
   Widget _buildVerticalHeader(int index) {
     return Container(
       margin: const EdgeInsets.symmetric(horizontal: 24.0),
@@ -709,9 +755,14 @@ class _StepperState extends State<Stepper> with TickerProviderStateMixin {
           child: Row(
             children: <Widget>[
               SizedBox(
-                height: 72.0,
-                child: Center(
-                  child: _buildIcon(i),
+                height: _isLabel() ? 104.0 : 72.0,
+                child: Column(
+                  mainAxisAlignment: MainAxisAlignment.center,
+                  children: <Widget>[
+                    if (widget.steps[i].label != null) const SizedBox(height: 24.0,),
+                    Center(child: _buildIcon(i)),
+                    if (widget.steps[i].label != null) SizedBox(height : 24.0, child: _buildLabelText(i),),
+                  ],
                 ),
               ),
               Container(

packages/flutter/test/material/stepper_test.dart

@@ -1144,6 +1144,90 @@ testWidgets('Stepper custom indexed controls test', (WidgetTester tester) async
 
      expect(material.margin, equals(margin));
    });
+
+  testWidgets('Stepper with Alternative Label', (WidgetTester tester) async {
+    int index = 0;
+    late TextStyle bodyText1Style;
+    late TextStyle bodyText2Style;
+    late TextStyle captionStyle;
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Material(
+          child: StatefulBuilder(
+              builder: (BuildContext context, StateSetter setState) {
+            bodyText1Style = Theme.of(context).textTheme.bodyText1!;
+            bodyText2Style = Theme.of(context).textTheme.bodyText2!;
+            captionStyle = Theme.of(context).textTheme.caption!;
+            return Stepper(
+              type: StepperType.horizontal,
+              currentStep: index,
+              onStepTapped: (int i) {
+                setState(() {
+                  index = i;
+                });
+              },
+              steps: <Step>[
+                Step(
+                  title: const Text('Title 1'),
+                  content: const Text('Content 1'),
+                  label: Text('Label 1', style: Theme.of(context).textTheme.caption),
+                ),
+                Step(
+                  title: const Text('Title 2'),
+                  content: const Text('Content 2'),
+                  label: Text('Label 2', style: Theme.of(context).textTheme.bodyText1),
+                ),
+                Step(
+                  title: const Text('Title 3'),
+                  content: const Text('Content 3'),
+                  label: Text('Label 3', style: Theme.of(context).textTheme.bodyText2),
+                ),
+              ],
+            );
+          }),
+        ),
+      ),
+    );
+
+    // Check Styles of Label Text Widgets before tapping steps
+    final Text label1TextWidget =
+        tester.widget<Text>(find.text('Label 1'));
+    final Text label3TextWidget =
+        tester.widget<Text>(find.text('Label 3'));
+
+    expect(captionStyle, label1TextWidget.style);
+    expect(bodyText2Style, label3TextWidget.style);
+
+    late Text selectedLabelTextWidget;
+    late Text nextLabelTextWidget;
+
+    // Tap to Step1 Label then, `index` become 0
+    await tester.tap(find.text('Label 1'));
+    expect(index, 0);
+
+    // Check Styles of Selected Label Text Widgets and Another Label Text Widget
+    selectedLabelTextWidget =
+        tester.widget<Text>(find.text('Label ${index + 1}'));
+    expect(captionStyle, selectedLabelTextWidget.style);
+    nextLabelTextWidget =
+        tester.widget<Text>(find.text('Label ${index + 2}'));
+    expect(bodyText1Style, nextLabelTextWidget.style);
+
+
+    // Tap to Step2 Label then, `index` become 1
+    await tester.tap(find.text('Label 2'));
+    expect(index, 1);
+
+    // Check Styles of Selected Label Text Widgets and Another Label Text Widget
+    selectedLabelTextWidget =
+        tester.widget<Text>(find.text('Label ${index + 1}'));
+    expect(bodyText1Style, selectedLabelTextWidget.style);
+
+    nextLabelTextWidget =
+        tester.widget<Text>(find.text('Label ${index + 2}'));
+    expect(bodyText2Style, nextLabelTextWidget.style);
+  });
 }
 
 class _TappableColorWidget extends StatefulWidget {