CupertinoPageScaffold dark mode (#40690)

packages/flutter/lib/src/cupertino/nav_bar.dart

@@ -372,7 +372,11 @@ class CupertinoNavigationBar extends StatefulWidget implements ObstructingPrefer
 
   /// True if the navigation bar's background color has no transparency.
   @override
-  bool get fullObstruction => backgroundColor == null ? null : backgroundColor.alpha == 0xFF;
+  bool shouldFullyObstruct(BuildContext context) {
+    final Color backgroundColor = CupertinoDynamicColor.resolve(this.backgroundColor, context)
+                               ?? CupertinoTheme.of(context).barBackgroundColor;
+    return backgroundColor.alpha == 0xFF;
+  }
 
   @override
   Size get preferredSize {

packages/flutter/lib/src/cupertino/page_scaffold.dart

@@ -4,6 +4,7 @@
 
 import 'package:flutter/widgets.dart';
 
+import 'colors.dart';
 import 'theme.dart';
 
 /// Implements a single iOS application page's layout.
@@ -110,8 +111,7 @@ class _CupertinoPageScaffoldState extends State<CupertinoPageScaffold> {
           ? existingMediaQuery.viewInsets.copyWith(bottom: 0.0)
           : existingMediaQuery.viewInsets;
 
-      final bool fullObstruction =
-        widget.navigationBar.fullObstruction ?? CupertinoTheme.of(context).barBackgroundColor.alpha == 0xFF;
+      final bool fullObstruction = widget.navigationBar.shouldFullyObstruct(context);
 
       // If navigation bar is opaquely obstructing, directly shift the main content
       // down. If translucent, let main content draw behind navigation bar but hint the
@@ -157,7 +157,8 @@ class _CupertinoPageScaffoldState extends State<CupertinoPageScaffold> {
 
     return DecoratedBox(
       decoration: BoxDecoration(
-        color: widget.backgroundColor ?? CupertinoTheme.of(context).scaffoldBackgroundColor,
+        color: CupertinoDynamicColor.resolve(widget.backgroundColor, context)
+            ?? CupertinoTheme.of(context).scaffoldBackgroundColor,
       ),
       child: Stack(
         children: <Widget>[
@@ -204,5 +205,5 @@ abstract class ObstructingPreferredSizeWidget extends PreferredSizeWidget {
   /// size.
   ///
   /// If false, this widget partially obstructs.
-  bool get fullObstruction;
+  bool shouldFullyObstruct(BuildContext context);
 }

packages/flutter/test/cupertino/scaffold_test.dart

@@ -6,6 +6,7 @@ import 'package:flutter/cupertino.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 import '../painting/mocks_for_image_cache.dart';
+import '../rendering/mock_canvas.dart';
 
 /// Integration tests testing both [CupertinoPageScaffold] and [CupertinoTabScaffold].
 void main() {
@@ -53,6 +54,53 @@ void main() {
     expect(tester.getRect(find.byType(Container)), const Rect.fromLTRB(0, 44, 800, 600));
   });
 
+  testWidgets('dark mode and obstruction work', (WidgetTester tester) async {
+    const Color dynamicColor = CupertinoDynamicColor.withBrightness(
+      color: Color(0xFFF8F8F8),
+      darkColor: Color(0xEE333333),
+    );
+
+    const CupertinoDynamicColor backgroundColor = CupertinoDynamicColor.withBrightness(
+      color: Color(0xFFFFFFFF),
+      darkColor: Color(0xFF000000),
+    );
+
+    BuildContext childContext;
+    Widget scaffoldWithBrightness(Brightness brightness) {
+      return Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: MediaQueryData(
+            platformBrightness: brightness,
+            viewInsets: const EdgeInsets.only(top: 20),
+          ),
+          child: CupertinoPageScaffold(
+            backgroundColor: backgroundColor,
+            navigationBar: const CupertinoNavigationBar(
+              middle: Text('Title'),
+              backgroundColor: dynamicColor,
+            ),
+            child: Builder(
+              builder: (BuildContext context) {
+                childContext = context;
+                return Container();
+              },
+            ),
+          ),
+        ),
+      );
+    }
+    await tester.pumpWidget(scaffoldWithBrightness(Brightness.light));
+
+    expect(MediaQuery.of(childContext).padding.top, 0);
+    expect(find.byType(CupertinoPageScaffold), paints..rect(color: backgroundColor.color));
+
+    await tester.pumpWidget(scaffoldWithBrightness(Brightness.dark));
+
+    expect(MediaQuery.of(childContext).padding.top, greaterThan(0));
+    expect(find.byType(CupertinoPageScaffold), paints..rect(color: backgroundColor.darkColor));
+  });
+
   testWidgets('Contents padding from viewInsets', (WidgetTester tester) async {
     await tester.pumpWidget(Directionality(
       textDirection: TextDirection.ltr,