Allow for customizing SnackBar's content TextStyle in its theme (#35075)

* Allow for customizing Snackbar's content TextStyle in its theme

* Address PR feedback

* Fix analyzer issues

* Address more PR feedback

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

@@ -253,6 +253,7 @@ class SnackBar extends StatelessWidget {
       accentColorBrightness: theme.accentColorBrightness,
       snackBarTheme: snackBarTheme,
     );
+    final TextStyle contentTextStyle = snackBarTheme.contentTextStyle ?? darkTheme.textTheme.subhead;
     final SnackBarBehavior snackBarBehavior = behavior ?? snackBarTheme.behavior ?? SnackBarBehavior.fixed;
     final bool isFloatingSnackBar = snackBarBehavior == SnackBarBehavior.floating;
     final double snackBarPadding = isFloatingSnackBar ? 16.0 : 24.0;
@@ -263,7 +264,7 @@ class SnackBar extends StatelessWidget {
         child: Container(
           padding: const EdgeInsets.symmetric(vertical: _singleLineVerticalPadding),
           child: DefaultTextStyle(
-            style: darkTheme.textTheme.subhead,
+            style: contentTextStyle,
             child: content,
           ),
         ),

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

@@ -55,6 +55,7 @@ class SnackBarThemeData extends Diagnosticable {
     this.backgroundColor,
     this.actionTextColor,
     this.disabledActionTextColor,
+    this.contentTextStyle,
     this.elevation,
     this.shape,
     this.behavior,
@@ -77,6 +78,11 @@ class SnackBarThemeData extends Diagnosticable {
   /// otherwise.
   final Color disabledActionTextColor;
 
+  /// Used to configure the [DefaultTextStyle] for the [SnackBar.content] widget.
+  ///
+  /// If null, [SnackBar] defines its default.
+  final TextStyle contentTextStyle;
+
   /// Default value for [SnackBar.elevation].
   ///
   /// If null, [SnackBar] uses a default of 6.0.
@@ -102,6 +108,7 @@ class SnackBarThemeData extends Diagnosticable {
     Color backgroundColor,
     Color actionTextColor,
     Color disabledActionTextColor,
+    TextStyle contentTextStyle,
     double elevation,
     ShapeBorder shape,
     SnackBarBehavior behavior,
@@ -110,6 +117,7 @@ class SnackBarThemeData extends Diagnosticable {
       backgroundColor: backgroundColor ?? this.backgroundColor,
       actionTextColor: actionTextColor ?? this.actionTextColor,
       disabledActionTextColor: disabledActionTextColor ?? this.disabledActionTextColor,
+      contentTextStyle: contentTextStyle ?? this.contentTextStyle,
       elevation: elevation ?? this.elevation,
       shape: shape ?? this.shape,
       behavior: behavior ?? this.behavior,
@@ -127,6 +135,7 @@ class SnackBarThemeData extends Diagnosticable {
       backgroundColor: Color.lerp(a?.backgroundColor, b?.backgroundColor, t),
       actionTextColor: Color.lerp(a?.actionTextColor, b?.actionTextColor, t),
       disabledActionTextColor: Color.lerp(a?.disabledActionTextColor, b?.disabledActionTextColor, t),
+      contentTextStyle: TextStyle.lerp(a?.contentTextStyle, b?.contentTextStyle, t),
       elevation: lerpDouble(a?.elevation, b?.elevation, t),
       shape: ShapeBorder.lerp(a?.shape, b?.shape, t),
       behavior: t < 0.5 ? a.behavior : b.behavior,
@@ -139,6 +148,7 @@ class SnackBarThemeData extends Diagnosticable {
       backgroundColor,
       actionTextColor,
       disabledActionTextColor,
+      contentTextStyle,
       elevation,
       shape,
       behavior,
@@ -155,6 +165,7 @@ class SnackBarThemeData extends Diagnosticable {
     return typedOther.backgroundColor == backgroundColor
         && typedOther.actionTextColor == actionTextColor
         && typedOther.disabledActionTextColor == disabledActionTextColor
+        && typedOther.contentTextStyle == contentTextStyle
         && typedOther.elevation == elevation
         && typedOther.shape == shape
         && typedOther.behavior == behavior;
@@ -166,6 +177,7 @@ class SnackBarThemeData extends Diagnosticable {
     properties.add(ColorProperty('backgroundColor', backgroundColor, defaultValue: null));
     properties.add(ColorProperty('actionTextColor', actionTextColor, defaultValue: null));
     properties.add(ColorProperty('disabledActionTextColor', disabledActionTextColor, defaultValue: null));
+    properties.add(DiagnosticsProperty<TextStyle>('contentTextStyle', contentTextStyle, defaultValue: null));
     properties.add(DoubleProperty('elevation', elevation, defaultValue: null));
     properties.add(DiagnosticsProperty<ShapeBorder>('shape', shape, defaultValue: null));
     properties.add(DiagnosticsProperty<SnackBarBehavior>('behavior', behavior, defaultValue: null));

packages/flutter/test/material/snack_bar_theme_test.dart

@@ -17,6 +17,7 @@ void main() {
     expect(snackBarTheme.backgroundColor, null);
     expect(snackBarTheme.actionTextColor, null);
     expect(snackBarTheme.disabledActionTextColor, null);
+    expect(snackBarTheme.contentTextStyle, null);
     expect(snackBarTheme.elevation, null);
     expect(snackBarTheme.shape, null);
     expect(snackBarTheme.behavior, null);
@@ -40,6 +41,7 @@ void main() {
       backgroundColor: const Color(0xFFFFFFFF),
       actionTextColor: const Color(0xFF0000AA),
       disabledActionTextColor: const Color(0xFF00AA00),
+      contentTextStyle: const TextStyle(color: Color(0xFF123456)),
       elevation: 2.0,
       shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(2.0)),
       behavior: SnackBarBehavior.floating,
@@ -54,6 +56,7 @@ void main() {
       'backgroundColor: Color(0xffffffff)',
       'actionTextColor: Color(0xff0000aa)',
       'disabledActionTextColor: Color(0xff00aa00)',
+      'contentTextStyle: TextStyle(inherit: true, color: Color(0xff123456))',
       'elevation: 2.0',
       'shape: RoundedRectangleBorder(BorderSide(Color(0xff000000), 0.0, BorderStyle.none), BorderRadius.circular(2.0))',
       'behavior: SnackBarBehavior.floating'
@@ -61,6 +64,7 @@ void main() {
   });
 
   testWidgets('Passing no SnackBarThemeData returns defaults', (WidgetTester tester) async {
+    const String text = 'I am a snack bar.';
     await tester.pumpWidget(MaterialApp(
       home: Scaffold(
         body: Builder(
@@ -68,7 +72,7 @@ void main() {
             return GestureDetector(
               onTap: () {
                 Scaffold.of(context).showSnackBar(SnackBar(
-                  content: const Text('I am a snack bar.'),
+                  content: const Text(text),
                   duration: const Duration(seconds: 2),
                   action: SnackBarAction(label: 'ACTION', onPressed: () {}),
                 ));
@@ -85,13 +89,16 @@ void main() {
     await tester.pump(const Duration(milliseconds: 750));
 
     final Material material = _getSnackBarMaterial(tester);
+    final RenderParagraph content = _getSnackBarTextRenderObject(tester, text);
 
+    expect(content.text.style, Typography().white.subhead);
     expect(material.color, const Color(0xFF323232));
     expect(material.elevation, 6.0);
     expect(material.shape, null);
   });
 
   testWidgets('SnackBar uses values from SnackBarThemeData', (WidgetTester tester) async {
+    const String text = 'I am a snack bar.';
     final SnackBarThemeData snackBarTheme = _snackBarTheme();
 
     await tester.pumpWidget(MaterialApp(
@@ -102,7 +109,7 @@ void main() {
               return GestureDetector(
                 onTap: () {
                   Scaffold.of(context).showSnackBar(SnackBar(
-                    content: const Text('I am a snack bar.'),
+                    content: const Text(text),
                     duration: const Duration(seconds: 2),
                     action: SnackBarAction(label: 'ACTION', onPressed: () {}),
                   ));
@@ -120,7 +127,9 @@ void main() {
 
     final Material material = _getSnackBarMaterial(tester);
     final RawMaterialButton button = _getSnackBarButton(tester);
+    final RenderParagraph content = _getSnackBarTextRenderObject(tester, text);
 
+    expect(content.text.style, snackBarTheme.contentTextStyle);
     expect(material.color, snackBarTheme.backgroundColor);
     expect(material.elevation, snackBarTheme.elevation);
     expect(material.shape, snackBarTheme.shape);
@@ -265,9 +274,10 @@ void main() {
 SnackBarThemeData _snackBarTheme() {
   return SnackBarThemeData(
     backgroundColor: Colors.orange,
+    actionTextColor: Colors.green,
+    contentTextStyle: const TextStyle(color: Colors.blue),
     elevation: 12.0,
     shape: BeveledRectangleBorder(borderRadius: BorderRadius.circular(12)),
-    actionTextColor: Colors.green,
   );
 }
 
@@ -288,3 +298,10 @@ RawMaterialButton _getSnackBarButton(WidgetTester tester) {
     ).first,
   );
 }
+
+RenderParagraph _getSnackBarTextRenderObject(WidgetTester tester, String text) {
+  return tester.renderObject(find.descendant(
+    of: find.byType(SnackBar),
+    matching: find.text(text),
+  ));
+}