Add an adaptive visual density static function, and add it to… (#51921)

Adds a VisualDensity.adaptivePlatformDensity static function that returns different values for visual density based on the defaultTargetPlatform. Returns compact for desktop platforms, and a default visual density for other platforms.

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

@@ -1762,6 +1762,24 @@ class VisualDensity with Diagnosticable {
   /// It corresponds to a density value of -2 in both axes.
   static const VisualDensity compact = VisualDensity(horizontal: -2.0, vertical: -2.0);
 
+  /// Returns a visual density that is adaptive based on the [defaultTargetPlatform].
+  ///
+  /// For desktop platforms, this returns [compact], and for other platforms,
+  /// it returns a default-constructed [VisualDensity].
+  static VisualDensity get adaptivePlatformDensity {
+    switch (defaultTargetPlatform) {
+      case TargetPlatform.android:
+      case TargetPlatform.iOS:
+      case TargetPlatform.fuchsia:
+        break;
+      case TargetPlatform.linux:
+      case TargetPlatform.macOS:
+      case TargetPlatform.windows:
+        return compact;
+    }
+    return const VisualDensity();
+  }
+
   /// Copy the current [VisualDensity] with the given values replacing the
   /// current values.
   VisualDensity copyWith({

packages/flutter/test/material/theme_data_test.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -178,6 +179,20 @@ void main() {
     expect(theme.applyElevationOverlayColor, isTrue);
   });
 
+  testWidgets('VisualDensity.adaptivePlatformDensity returns adaptive values', (WidgetTester tester) async {
+    switch (debugDefaultTargetPlatformOverride) {
+      case TargetPlatform.android:
+      case TargetPlatform.iOS:
+      case TargetPlatform.fuchsia:
+        expect(VisualDensity.adaptivePlatformDensity, equals(const VisualDensity()));
+        break;
+      case TargetPlatform.linux:
+      case TargetPlatform.macOS:
+      case TargetPlatform.windows:
+        expect(VisualDensity.adaptivePlatformDensity, equals(VisualDensity.compact));
+    }
+  }, variant: TargetPlatformVariant.all());
+
   testWidgets('ThemeData.copyWith correctly creates new ThemeData with all copied arguments', (WidgetTester tester) async {
 
     final SliderThemeData sliderTheme = SliderThemeData.fromPrimaryColors(

packages/flutter_tools/templates/app/lib/main.dart.tmpl

@@ -36,6 +36,10 @@ class MyApp extends StatelessWidget {
         // Notice that the counter didn't reset back to zero; the application
         // is not restarted.
         primarySwatch: Colors.blue,
+        // This makes the visual density adapt to the platform that you run
+        // the app on. For desktop platforms, the controls will be smaller and
+        // closer together (more dense) than on mobile platforms.
+        visualDensity: VisualDensity.adaptivePlatformDensity,
       ),
       home: MyHomePage(title: 'Flutter Demo Home Page'),
     );