Made the snackbar fallback theme include a full inverse colorScheme. (#37038)

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

@@ -6,6 +6,7 @@ import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 
 import 'button_theme.dart';
+import 'color_scheme.dart';
 import 'flat_button.dart';
 import 'material.dart';
 import 'scaffold.dart';
@@ -14,7 +15,6 @@ import 'theme.dart';
 import 'theme_data.dart';
 
 const double _singleLineVerticalPadding = 14.0;
-const Color _snackBarBackgroundColor = Color(0xFF323232);
 
 // TODO(ianh): We should check if the given text and actions are going to fit on
 // one line or not, and if they are, use the single-line layout, and if not, use
@@ -158,6 +158,8 @@ class _SnackBarActionState extends State<SnackBarAction> {
 ///    displayed snack bar, if any, and allows the next to be displayed.
 ///  * [SnackBarAction], which is used to specify an [action] button to show
 ///    on the snack bar.
+///  * [SnackBarThemeData], to configure the default property values for
+///    [SnackBar] widgets.
 ///  * <https://material.io/design/components/snackbars.html>
 class SnackBar extends StatelessWidget {
   /// Creates a snack bar.
@@ -184,7 +186,10 @@ class SnackBar extends StatelessWidget {
   /// Typically a [Text] widget.
   final Widget content;
 
-  /// The Snackbar's background color. By default the color is dark grey.
+  /// The Snackbar's background color. If not specified it will use
+  /// [ThemeData.snackBarTheme.backgroundColor]. If that is not specified
+  /// it will default to a dark variation of [ColorScheme.surface] for light
+  /// themes, or [ColorScheme.onSurface] for dark themes.
   final Color backgroundColor;
 
   /// The z-coordinate at which to place the snack bar. This controls the size
@@ -245,15 +250,40 @@ class SnackBar extends StatelessWidget {
     final MediaQueryData mediaQueryData = MediaQuery.of(context);
     assert(animation != null);
     final ThemeData theme = Theme.of(context);
+    final ColorScheme colorScheme = theme.colorScheme;
     final SnackBarThemeData snackBarTheme = theme.snackBarTheme;
-    // TODO(rami-a): Use a light theme if the app has a dark theme, https://github.com/flutter/flutter/issues/31418
-    final ThemeData darkTheme = ThemeData(
-      brightness: Brightness.dark,
-      accentColor: theme.accentColor,
-      accentColorBrightness: theme.accentColorBrightness,
+    final bool isThemeDark = theme.brightness == Brightness.dark;
+
+    // SnackBar uses a theme that is the opposite brightness from
+    // the surrounding theme.
+    final Brightness brightness = isThemeDark ? Brightness.light : Brightness.dark;
+    final Color themeBackgroundColor = isThemeDark
+      ? colorScheme.onSurface
+      : Color.alphaBlend(colorScheme.onSurface.withOpacity(0.80), colorScheme.surface);
+    final ThemeData inverseTheme = ThemeData(
+      brightness: brightness,
+      backgroundColor: themeBackgroundColor,
+      colorScheme: ColorScheme(
+        primary: colorScheme.onPrimary,
+        primaryVariant: colorScheme.onPrimary,
+        // For the button color, the spec says it should be primaryVariant, but for
+        // backward compatibility on light themes we are leaving it as secondary.
+        secondary: isThemeDark ? colorScheme.primaryVariant : colorScheme.secondary,
+        secondaryVariant: colorScheme.onSecondary,
+        surface: colorScheme.onSurface,
+        background: themeBackgroundColor,
+        error: colorScheme.onError,
+        onPrimary: colorScheme.primary,
+        onSecondary: colorScheme.secondary,
+        onSurface: colorScheme.surface,
+        onBackground: colorScheme.background,
+        onError: colorScheme.error,
+        brightness: brightness,
+      ),
       snackBarTheme: snackBarTheme,
     );
-    final TextStyle contentTextStyle = snackBarTheme.contentTextStyle ?? darkTheme.textTheme.subhead;
+
+    final TextStyle contentTextStyle = snackBarTheme.contentTextStyle ?? inverseTheme.textTheme.subhead;
     final SnackBarBehavior snackBarBehavior = behavior ?? snackBarTheme.behavior ?? SnackBarBehavior.fixed;
     final bool isFloatingSnackBar = snackBarBehavior == SnackBarBehavior.floating;
     final double snackBarPadding = isFloatingSnackBar ? 16.0 : 24.0;
@@ -297,7 +327,7 @@ class SnackBar extends StatelessWidget {
     );
 
     final double elevation = this.elevation ?? snackBarTheme.elevation ?? 6.0;
-    final Color backgroundColor = this.backgroundColor ?? snackBarTheme.backgroundColor ?? _snackBarBackgroundColor;
+    final Color backgroundColor = this.backgroundColor ?? snackBarTheme.backgroundColor ?? inverseTheme.backgroundColor;
     final ShapeBorder shape = this.shape
       ?? snackBarTheme.shape
       ?? (isFloatingSnackBar ? RoundedRectangleBorder(borderRadius: BorderRadius.circular(4.0)) : null);
@@ -307,7 +337,7 @@ class SnackBar extends StatelessWidget {
       elevation: elevation,
       color: backgroundColor,
       child: Theme(
-        data: darkTheme,
+        data: inverseTheme,
         child: mediaQueryData.accessibleNavigation
             ? snackBar
             : FadeTransition(

packages/flutter/test/material/snack_bar_test.dart

@@ -2,8 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter_test/flutter_test.dart';
 
 void main() {
   testWidgets('SnackBar control test', (WidgetTester tester) async {
@@ -296,6 +297,87 @@ void main() {
     expect(tapCount, equals(1));
   });
 
+  testWidgets('Light theme SnackBar has dark background', (WidgetTester tester) async {
+    final ThemeData lightTheme = ThemeData.light();
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: lightTheme,
+        home: Scaffold(
+          body: Builder(
+              builder: (BuildContext context) {
+                return GestureDetector(
+                  onTap: () {
+                    Scaffold.of(context).showSnackBar(
+                      SnackBar(
+                        content: const Text('I am a snack bar.'),
+                        duration: const Duration(seconds: 2),
+                        action: SnackBarAction(
+                          label: 'ACTION',
+                          onPressed: () { },
+                        ),
+                      ),
+                    );
+                  },
+                  child: const Text('X'),
+                );
+              }
+          ),
+        ),
+      ),
+    );
+
+    await tester.tap(find.text('X'));
+    await tester.pump(); // start animation
+    await tester.pump(const Duration(milliseconds: 750));
+
+    final RenderPhysicalModel renderModel = tester.renderObject(
+        find.widgetWithText(Material, 'I am a snack bar.').first
+    );
+    // There is a somewhat complicated background color calculation based
+    // off of the surface color. For the default light theme it
+    // should be this value.
+    expect(renderModel.color, equals(const Color(0xFF333333)));
+  });
+
+  testWidgets('Dark theme SnackBar has light background', (WidgetTester tester) async {
+    final ThemeData darkTheme = ThemeData.dark();
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: darkTheme,
+        home: Scaffold(
+          body: Builder(
+            builder: (BuildContext context) {
+              return GestureDetector(
+                onTap: () {
+                  Scaffold.of(context).showSnackBar(
+                    SnackBar(
+                      content: const Text('I am a snack bar.'),
+                      duration: const Duration(seconds: 2),
+                      action: SnackBarAction(
+                        label: 'ACTION',
+                        onPressed: () { },
+                      ),
+                    ),
+                  );
+                },
+                child: const Text('X'),
+              );
+            }
+          ),
+        ),
+      ),
+    );
+
+    await tester.tap(find.text('X'));
+    await tester.pump(); // start animation
+    await tester.pump(const Duration(milliseconds: 750));
+
+    final RenderPhysicalModel renderModel = tester.renderObject(
+      find.widgetWithText(Material, 'I am a snack bar.').first
+    );
+    expect(renderModel.color, equals(darkTheme.colorScheme.onSurface));
+  });
+
   testWidgets('Snackbar labels can be colored', (WidgetTester tester) async {
     await tester.pumpWidget(
       MaterialApp(

packages/flutter/test/material/snack_bar_theme_test.dart

@@ -92,7 +92,7 @@ void main() {
     final RenderParagraph content = _getSnackBarTextRenderObject(tester, text);
 
     expect(content.text.style, Typography().white.subhead);
-    expect(material.color, const Color(0xFF323232));
+    expect(material.color, const Color(0xFF333333));
     expect(material.elevation, 6.0);
     expect(material.shape, null);
   });