No longer rebuild Routes when MediaQuery updates (#41763)

* No longer rebuild Routes when MediaQuery updates

fixes #37878

* Fix tests

* PR feedback

* Add missing endline

packages/flutter/lib/src/widgets/app.dart

@@ -706,8 +706,7 @@ class WidgetsApp extends StatefulWidget {
   _WidgetsAppState createState() => _WidgetsAppState();
 }
 
-class _WidgetsAppState extends State<WidgetsApp> implements WidgetsBindingObserver {
-
+class _WidgetsAppState extends State<WidgetsApp> with WidgetsBindingObserver {
   // STATE LIFECYCLE
 
   @override
@@ -731,13 +730,6 @@ class _WidgetsAppState extends State<WidgetsApp> implements WidgetsBindingObserv
     super.dispose();
   }
 
-  @override
-  void didChangeAppLifecycleState(AppLifecycleState state) { }
-
-  @override
-  void didHaveMemoryPressure() { }
-
-
   // NAVIGATOR
 
   GlobalKey<NavigatorState> _navigator;
@@ -995,46 +987,6 @@ class _WidgetsAppState extends State<WidgetsApp> implements WidgetsBindingObserv
     yield DefaultWidgetsLocalizations.delegate;
   }
 
-  // ACCESSIBILITY
-
-  @override
-  void didChangeAccessibilityFeatures() {
-    setState(() {
-      // The properties of window have changed. We use them in our build
-      // function, so we need setState(), but we don't cache anything locally.
-    });
-  }
-
-
-  // METRICS
-
-  @override
-  void didChangeMetrics() {
-    setState(() {
-      // The properties of window have changed. We use them in our build
-      // function, so we need setState(), but we don't cache anything locally.
-    });
-  }
-
-  @override
-  void didChangeTextScaleFactor() {
-    setState(() {
-      // The textScaleFactor property of window has changed. We reference
-      // window in our build function, so we need to call setState(), but
-      // we don't need to cache anything locally.
-    });
-  }
-
-  // RENDERING
-  @override
-  void didChangePlatformBrightness() {
-    setState(() {
-      // The platformBrightness property of window has changed. We reference
-      // window in our build function, so we need to call setState(), but
-      // we don't need to cache anything locally.
-    });
-  }
-
   // BUILDER
 
   bool _debugCheckLocalizations(Locale appLocale) {
@@ -1201,8 +1153,7 @@ class _WidgetsAppState extends State<WidgetsApp> implements WidgetsBindingObserv
       },
       child: DefaultFocusTraversal(
         policy: ReadingOrderTraversalPolicy(),
-        child: MediaQuery(
-          data: MediaQueryData.fromWindow(WidgetsBinding.instance.window),
+        child: _MediaQueryFromWindow(
           child: Localizations(
             locale: appLocale,
             delegates: _localizationsDelegates.toList(),
@@ -1213,3 +1164,77 @@ class _WidgetsAppState extends State<WidgetsApp> implements WidgetsBindingObserv
     );
   }
 }
+
+/// Builds [MediaQuery] from `window` by listening to [WidgetsBinding].
+///
+/// It is performed in a standalone widget to rebuild **only** [MediaQuery] and
+/// its dependents when `window` changes, instead of rebuilding the entire widget tree.
+class _MediaQueryFromWindow extends StatefulWidget {
+  const _MediaQueryFromWindow({Key key, this.child}) : super(key: key);
+
+  final Widget child;
+
+  @override
+  _MediaQueryFromWindowsState createState() => _MediaQueryFromWindowsState();
+}
+
+class _MediaQueryFromWindowsState extends State<_MediaQueryFromWindow> with WidgetsBindingObserver {
+  @override
+  void initState() {
+    super.initState();
+    WidgetsBinding.instance.addObserver(this);
+  }
+
+  // ACCESSIBILITY
+
+  @override
+  void didChangeAccessibilityFeatures() {
+    setState(() {
+      // The properties of window have changed. We use them in our build
+      // function, so we need setState(), but we don't cache anything locally.
+    });
+  }
+
+  // METRICS
+
+  @override
+  void didChangeMetrics() {
+    setState(() {
+      // The properties of window have changed. We use them in our build
+      // function, so we need setState(), but we don't cache anything locally.
+    });
+  }
+
+  @override
+  void didChangeTextScaleFactor() {
+    setState(() {
+      // The textScaleFactor property of window has changed. We reference
+      // window in our build function, so we need to call setState(), but
+      // we don't need to cache anything locally.
+    });
+  }
+
+  // RENDERING
+  @override
+  void didChangePlatformBrightness() {
+    setState(() {
+      // The platformBrightness property of window has changed. We reference
+      // window in our build function, so we need to call setState(), but
+      // we don't need to cache anything locally.
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return MediaQuery(
+      data: MediaQueryData.fromWindow(WidgetsBinding.instance.window),
+      child: widget.child,
+    );
+  }
+
+  @override
+  void dispose() {
+    WidgetsBinding.instance.removeObserver(this);
+    super.dispose();
+  }
+}

packages/flutter/test/material/app_test.dart

@@ -2,9 +2,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/semantics.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/material.dart';
+import 'package:mockito/mockito.dart';
 
 class StateMarker extends StatefulWidget {
   const StateMarker({ Key key, this.child }) : super(key: key);
@@ -392,6 +394,67 @@ void main() {
     );
   });
 
+  testWidgets("WidgetsApp don't rebuild routes when MediaQuery updates", (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/37878
+    int routeBuildCount = 0;
+    int dependentBuildCount = 0;
+
+    await tester.pumpWidget(WidgetsApp(
+      color: const Color.fromARGB(255, 255, 255, 255),
+      onGenerateRoute: (_) {
+        return PageRouteBuilder<void>(pageBuilder: (_, __, ___) {
+          routeBuildCount++;
+          return Builder(
+            builder: (BuildContext context) {
+              dependentBuildCount++;
+              MediaQuery.of(context);
+              return Container();
+            },
+          );
+        });
+      },
+    ));
+
+    expect(routeBuildCount, equals(1));
+    expect(dependentBuildCount, equals(1));
+
+    // didChangeMetrics
+    tester.binding.window.physicalSizeTestValue = const Size(42, 42);
+    addTearDown(tester.binding.window.clearPhysicalSizeTestValue);
+
+    await tester.pump();
+
+    expect(routeBuildCount, equals(1));
+    expect(dependentBuildCount, equals(2));
+
+    // didChangeTextScaleFactor
+    tester.binding.window.textScaleFactorTestValue = 42;
+    addTearDown(tester.binding.window.clearTextScaleFactorTestValue);
+
+    await tester.pump();
+
+    expect(routeBuildCount, equals(1));
+    expect(dependentBuildCount, equals(3));
+
+    // didChangePlatformBrightness
+    tester.binding.window.platformBrightnessTestValue = Brightness.dark;
+    addTearDown(tester.binding.window.clearPlatformBrightnessTestValue);
+
+    await tester.pump();
+
+    expect(routeBuildCount, equals(1));
+    expect(dependentBuildCount, equals(4));
+
+    // didChangeAccessibilityFeatures
+    tester.binding.window.accessibilityFeaturesTestValue = MockAccessibilityFeature();
+    addTearDown(tester.binding.window.clearAccessibilityFeaturesTestValue);
+
+    await tester.pump();
+
+    expect(routeBuildCount, equals(1));
+    expect(dependentBuildCount, equals(5));
+  });
+
   testWidgets('Can get text scale from media query', (WidgetTester tester) async {
     double textScaleFactor;
     await tester.pumpWidget(MaterialApp(
@@ -726,3 +789,5 @@ void main() {
     expect(themeAfterBrightnessChange.brightness, Brightness.dark);
   });
 }
+
+class MockAccessibilityFeature extends Mock implements AccessibilityFeatures {}