When the app enters the detached state, clear the record of the last SystemChrome style sent to the host (#153586)

This ensures that the style will be sent again after the host is reattached

Fixes flutter/flutter#150418

Author: jason-simmons

packages/flutter/lib/src/services/binding.dart

@@ -27,6 +27,7 @@ import 'raw_keyboard.dart' show RawKeyboard;
 import 'restoration.dart';
 import 'service_extensions.dart';
 import 'system_channels.dart';
+import 'system_chrome.dart';
 import 'text_input.dart';
 
 export 'dart:ui' show ChannelBuffers, RootIsolateToken;
@@ -284,7 +285,10 @@ mixin ServicesBinding on BindingBase, SchedulerBinding {
   Future<String?> _handleLifecycleMessage(String? message) async {
     final AppLifecycleState? state = _parseAppLifecycleMessage(message!);
     final List<AppLifecycleState> generated = _generateStateTransitions(lifecycleState, state!);
-    generated.forEach(handleAppLifecycleStateChanged);
+    for (final AppLifecycleState stateChange in generated) {
+      handleAppLifecycleStateChanged(stateChange);
+      SystemChrome.handleAppLifecycleStateChanged(stateChange);
+    }
     return null;
   }
 

packages/flutter/lib/src/services/system_chrome.dart

@@ -641,6 +641,17 @@ abstract final class SystemChrome {
     });
   }
 
+  /// Called by the binding during a transition to a new app lifecycle state.
+  static void handleAppLifecycleStateChanged(AppLifecycleState state) {
+    // When the app is detached, clear the record of the style sent to the host
+    // so that it will be sent again when the app is reattached.
+    if (state == AppLifecycleState.detached) {
+      scheduleMicrotask(() {
+        _latestStyle = null;
+      });
+    }
+  }
+
   static SystemUiOverlayStyle? _pendingStyle;
 
   /// The last style that was set using [SystemChrome.setSystemUIOverlayStyle].

packages/flutter/test/services/system_chrome_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 'dart:ui' as ui show AppLifecycleState;
 
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -200,4 +201,31 @@ void main() {
       'systemNavigationBarContrastEnforced: null})',
     );
   });
+
+  testWidgets('SystemChrome handles detached lifecycle state', (WidgetTester tester) async {
+    final List<MethodCall> log = <MethodCall>[];
+
+    TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger.setMockMethodCallHandler(SystemChannels.platform, (MethodCall methodCall) async {
+      log.add(methodCall);
+      return null;
+    });
+
+    const SystemUiOverlayStyle systemUiOverlayStyle = SystemUiOverlayStyle();
+    SystemChrome.setSystemUIOverlayStyle(systemUiOverlayStyle);
+    await tester.idle();
+    expect(log.length, equals(1));
+
+    // Setting the same state should not send another message to the host.
+    SystemChrome.setSystemUIOverlayStyle(systemUiOverlayStyle);
+    await tester.idle();
+    expect(log.length, equals(1));
+
+    // The state should be sent again if the app was detached.
+    SystemChrome.handleAppLifecycleStateChanged(ui.AppLifecycleState.detached);
+    await tester.idle();
+    SystemChrome.handleAppLifecycleStateChanged(ui.AppLifecycleState.resumed);
+    SystemChrome.setSystemUIOverlayStyle(systemUiOverlayStyle);
+    await tester.idle();
+    expect(log.length, equals(2));
+  });
 }