Framework callback for engine memory pressure events (#8778)

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

@@ -179,6 +179,9 @@ class _WidgetsAppState extends State<WidgetsApp> implements WidgetsBindingObserv
   @override
   void didChangeAppLifecycleState(AppLifecycleState state) { }
 
+  @override
+  void didHaveMemoryPressure() { }
+
   @override
   Widget build(BuildContext context) {
     if (config.onLocaleChanged != null && _localeData == null) {

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

@@ -48,6 +48,9 @@ abstract class WidgetsBindingObserver {
   /// Called when the system puts the app in the background or returns
   /// the app to the foreground.
   void didChangeAppLifecycleState(AppLifecycleState state) { }
+
+  /// Called when the system is running low on memory.
+  void didHaveMemoryPressure() { }
 }
 
 /// The glue between the widgets layer and the Flutter engine.
@@ -60,6 +63,7 @@ abstract class WidgetsBinding extends BindingBase implements GestureBinding, Ren
     ui.window.onLocaleChanged = handleLocaleChanged;
     PlatformMessages.setJSONMessageHandler('flutter/navigation', _handleNavigationMessage);
     PlatformMessages.setStringMessageHandler('flutter/lifecycle', _handleLifecycleMessage);
+    PlatformMessages.setJSONMessageHandler('flutter/system', _handleSystemMessage);
   }
 
   /// The current [WidgetsBinding], if one has been created.
@@ -213,6 +217,15 @@ abstract class WidgetsBinding extends BindingBase implements GestureBinding, Ren
     return null;
   }
 
+  Future<dynamic> _handleSystemMessage(Map<String, dynamic> message) async {
+    final String type = message['type'];
+    if (type == 'memoryPressure') {
+      for (WidgetsBindingObserver observer in _observers)
+        observer.didHaveMemoryPressure();
+    }
+    return null;
+  }
+
   bool _needToReportFirstFrame = true;
   bool _thisFrameWasUseful = true;
 

packages/flutter/test/widgets/binding_test.dart

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:typed_data';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter/widgets.dart';
+
+class MemoryPressureObserver implements WidgetsBindingObserver {
+  bool sawMemoryPressure = false;
+
+  @override
+  void didHaveMemoryPressure() {
+    sawMemoryPressure = true;
+  }
+
+  @override
+  Future<bool> didPopRoute() => new Future<bool>.value(false);
+
+  @override
+  void didChangeMetrics() { }
+
+  @override
+  void didChangeLocale(Locale locale) { }
+
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) { }
+}
+
+void main() {
+  setUp(() {
+    WidgetsFlutterBinding.ensureInitialized();
+  });
+
+  testWidgets('didHaveMemoryPressure callback', (WidgetTester tester) async {
+    final MemoryPressureObserver observer = new MemoryPressureObserver();
+    WidgetsBinding.instance.addObserver(observer);
+    final ByteData message = const JSONMessageCodec().encodeMessage(
+      <String, dynamic>{'type': 'memoryPressure'});
+    await PlatformMessages.handlePlatformMessage('flutter/system', message, (_) {});
+    expect(observer.sawMemoryPressure, true);
+    WidgetsBinding.instance.removeObserver(observer);
+  });
+}