Fix SnackBar assertions when configured with theme (#85150)

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

@@ -255,14 +255,6 @@ class SnackBar extends StatefulWidget {
     this.dismissDirection = DismissDirection.down,
   }) : assert(elevation == null || elevation >= 0.0),
        assert(content != null),
-       assert(
-         margin == null || behavior == SnackBarBehavior.floating,
-         'Margin can only be used with floating behavior',
-       ),
-       assert(
-         width == null || behavior == SnackBarBehavior.floating,
-         'Width can only be used with floating behavior',
-       ),
        assert(
          width == null || margin == null,
          'Width and margin can not be used together',
@@ -489,6 +481,26 @@ class _SnackBarState extends State<SnackBar> {
 
     final TextStyle? contentTextStyle = snackBarTheme.contentTextStyle ?? ThemeData(brightness: brightness).textTheme.subtitle1;
     final SnackBarBehavior snackBarBehavior = widget.behavior ?? snackBarTheme.behavior ?? SnackBarBehavior.fixed;
+    assert((){
+      // Whether the behavior is set through the constructor or the theme,
+      // assert that our other properties are configured properly.
+      if (snackBarBehavior != SnackBarBehavior.floating) {
+        String message(String parameter) {
+          final String prefix = '$parameter can only be used with floating behavior.';
+          if (widget.behavior != null) {
+            return '$prefix SnackBarBehavior.fixed was set in the SnackBar constructor.';
+          } else if (snackBarTheme.behavior != null) {
+            return '$prefix SnackBarBehavior.fixed was set by the inherited SnackBarThemeData.';
+          } else {
+            return '$prefix SnackBarBehavior.fixed was set by default.';
+          }
+        }
+        assert(widget.margin == null, message('Margin'));
+        assert(widget.width == null, message('Width'));
+      }
+      return true;
+    }());
+
     final bool isFloatingSnackBar = snackBarBehavior == SnackBarBehavior.floating;
     final double horizontalPadding = isFloatingSnackBar ? 16.0 : 24.0;
     final EdgeInsetsGeometry padding = widget.padding

packages/flutter/test/material/snack_bar_test.dart

@@ -816,9 +816,9 @@ void main() {
               return GestureDetector(
                 onTap: () {
                   Scaffold.of(context).showSnackBar(
-                    SnackBar(
-                      content: const Text('I am a snack bar.'),
-                      margin: const EdgeInsets.all(padding),
+                    const SnackBar(
+                      content: Text('I am a snack bar.'),
+                      margin: EdgeInsets.all(padding),
                       behavior: SnackBarBehavior.floating,
                     ),
                   );
@@ -994,8 +994,8 @@ void main() {
               return GestureDetector(
                 onTap: () {
                   ScaffoldMessenger.of(context).showSnackBar(
-                    SnackBar(
-                      content: const Text('I am a snack bar.'),
+                    const SnackBar(
+                      content: Text('I am a snack bar.'),
                       width: width,
                       behavior: SnackBarBehavior.floating,
                     ),
@@ -2420,6 +2420,106 @@ void main() {
     expect(find.text(snackBars[1]), findsNothing);
     expect(find.text(snackBars[2]), findsNothing);
   });
+
+  Widget _buildApp({
+    required SnackBarBehavior? behavior,
+    EdgeInsetsGeometry? margin,
+    double? width,
+  }) {
+    return MaterialApp(
+      home: Scaffold(
+        floatingActionButton: FloatingActionButton(
+          child: const Icon(Icons.send),
+          onPressed: () {},
+        ),
+        body: Builder(
+          builder: (BuildContext context) {
+            return GestureDetector(
+              onTap: () {
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
+                  behavior: behavior,
+                  margin: margin,
+                  width: width,
+                  content: const Text('I am a snack bar.'),
+                  duration: const Duration(seconds: 2),
+                  action: SnackBarAction(label: 'ACTION', onPressed: () {}),
+                ));
+              },
+              child: const Text('X'),
+            );
+          },
+        ),
+      ),
+    );
+  }
+
+  testWidgets('Setting SnackBarBehavior.fixed will still assert for margin', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/84935
+    await tester.pumpWidget(_buildApp(
+      behavior: SnackBarBehavior.fixed,
+      margin: const EdgeInsets.all(8.0),
+    ));
+    await tester.tap(find.text('X'));
+    await tester.pump(); // start animation
+    await tester.pump(const Duration(milliseconds: 750));
+    final AssertionError exception = tester.takeException() as AssertionError;
+    expect(
+      exception.message,
+      'Margin can only be used with floating behavior. SnackBarBehavior.fixed '
+      'was set in the SnackBar constructor.',
+    );
+  });
+
+  testWidgets('Default SnackBarBehavior will still assert for margin', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/84935
+    await tester.pumpWidget(_buildApp(
+      behavior: null,
+      margin: const EdgeInsets.all(8.0),
+    ));
+    await tester.tap(find.text('X'));
+    await tester.pump(); // start animation
+    await tester.pump(const Duration(milliseconds: 750));
+    final AssertionError exception = tester.takeException() as AssertionError;
+    expect(
+      exception.message,
+      'Margin can only be used with floating behavior. SnackBarBehavior.fixed '
+      'was set by default.',
+    );
+  });
+
+  testWidgets('Setting SnackBarBehavior.fixed will still assert for width', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/84935
+    await tester.pumpWidget(_buildApp(
+      behavior: SnackBarBehavior.fixed,
+      width: 5.0,
+    ));
+    await tester.tap(find.text('X'));
+    await tester.pump(); // start animation
+    await tester.pump(const Duration(milliseconds: 750));
+    final AssertionError exception = tester.takeException() as AssertionError;
+    expect(
+      exception.message,
+      'Width can only be used with floating behavior. SnackBarBehavior.fixed '
+      'was set in the SnackBar constructor.',
+    );
+  });
+
+  testWidgets('Default SnackBarBehavior will still assert for width', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/84935
+    await tester.pumpWidget(_buildApp(
+      behavior: null,
+      width: 5.0,
+    ));
+    await tester.tap(find.text('X'));
+    await tester.pump(); // start animation
+    await tester.pump(const Duration(milliseconds: 750));
+    final AssertionError exception = tester.takeException() as AssertionError;
+    expect(
+      exception.message,
+      'Width can only be used with floating behavior. SnackBarBehavior.fixed '
+      'was set by default.',
+    );
+  });
 }
 
 /// Start test for "SnackBar dismiss test".

packages/flutter/test/material/snack_bar_theme_test.dart

@@ -271,6 +271,97 @@ void main() {
     expect(floatingActionButtonOriginBottomCenter.dy > floatingActionButtonBottomCenter.dy, true);
     expect(snackBarTopCenter.dy > floatingActionButtonBottomCenter.dy, true);
   });
+
+  Widget _buildApp({
+    required SnackBarBehavior themedBehavior,
+    EdgeInsetsGeometry? margin,
+    double? width,
+  }) {
+    return MaterialApp(
+      theme: ThemeData(
+        snackBarTheme: SnackBarThemeData(behavior: themedBehavior),
+      ),
+      home: Scaffold(
+        floatingActionButton: FloatingActionButton(
+          child: const Icon(Icons.send),
+          onPressed: () {},
+        ),
+        body: Builder(
+          builder: (BuildContext context) {
+            return GestureDetector(
+              onTap: () {
+                ScaffoldMessenger.of(context).showSnackBar(SnackBar(
+                  margin: margin,
+                  width: width,
+                  content: const Text('I am a snack bar.'),
+                  duration: const Duration(seconds: 2),
+                  action: SnackBarAction(label: 'ACTION', onPressed: () {}),
+                ));
+              },
+              child: const Text('X'),
+            );
+          },
+        ),
+      ),
+    );
+  }
+
+  testWidgets('SnackBar theme behavior will assert properly for margin use', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/84935
+    // SnackBarBehavior.floating set in theme does not assert with margin
+    await tester.pumpWidget(_buildApp(
+      themedBehavior: SnackBarBehavior.floating,
+      margin: const EdgeInsets.all(8.0),
+    ));
+    await tester.tap(find.text('X'));
+    await tester.pump(); // start animation
+    await tester.pump(const Duration(milliseconds: 750));
+    AssertionError? exception = tester.takeException() as AssertionError?;
+    expect(exception, isNull);
+
+    // SnackBarBehavior.fixed set in theme will still assert with margin
+    await tester.pumpWidget(_buildApp(
+      themedBehavior: SnackBarBehavior.fixed,
+      margin: const EdgeInsets.all(8.0),
+    ));
+    await tester.tap(find.text('X'));
+    await tester.pump(); // start animation
+    await tester.pump(const Duration(milliseconds: 750));
+    exception = tester.takeException() as AssertionError;
+    expect(
+      exception.message,
+      'Margin can only be used with floating behavior. SnackBarBehavior.fixed '
+          'was set by the inherited SnackBarThemeData.',
+    );
+  });
+
+  testWidgets('SnackBar theme behavior will assert properly for width use', (WidgetTester tester) async {
+    // SnackBarBehavior.floating set in theme does not assert with width
+    await tester.pumpWidget(_buildApp(
+      themedBehavior: SnackBarBehavior.floating,
+      width: 5.0,
+    ));
+    await tester.tap(find.text('X'));
+    await tester.pump(); // start animation
+    await tester.pump(const Duration(milliseconds: 750));
+    AssertionError? exception = tester.takeException() as AssertionError?;
+    expect(exception, isNull);
+
+    // SnackBarBehavior.fixed set in theme will still assert with width
+    await tester.pumpWidget(_buildApp(
+      themedBehavior: SnackBarBehavior.fixed,
+      width: 5.0,
+    ));
+    await tester.tap(find.text('X'));
+    await tester.pump(); // start animation
+    await tester.pump(const Duration(milliseconds: 750));
+    exception = tester.takeException() as AssertionError;
+    expect(
+      exception.message,
+      'Width can only be used with floating behavior. SnackBarBehavior.fixed '
+      'was set by the inherited SnackBarThemeData.',
+    );
+  });
 }
 
 SnackBarThemeData _snackBarTheme() {