Merge pull request #527 from krille-chan/krille/better-background-fetch-android

feat: Background fetch mode on Android

lib/main.dart

@@ -5,6 +5,7 @@ import 'package:flutter_app_lock/flutter_app_lock.dart';
 import 'package:flutter_secure_storage/flutter_secure_storage.dart';
 import 'package:matrix/matrix.dart';
 
+import 'package:fluffychat/config/app_config.dart';
 import 'package:fluffychat/utils/client_manager.dart';
 import 'package:fluffychat/utils/platform_infos.dart';
 import 'config/setting_keys.dart';
@@ -13,7 +14,7 @@ import 'widgets/fluffy_chat_app.dart';
 import 'widgets/lock_screen.dart';
 
 void main() async {
-  Logs().i('Welcome to FluffyChat');
+  Logs().i('Welcome to ${AppConfig.applicationName} <3');
 
   // Our background push shared isolate accesses flutter-internal things very early in the startup proccess
   // To make sure that the parts of flutter needed are started up already, we need to ensure that the
@@ -28,9 +29,34 @@ void main() async {
   await firstClient?.roomsLoading;
   await firstClient?.accountDataLoading;
 
+  // If the app starts in detached mode, we assume that it is in
+  // background fetch mode for processing push notifications. This is
+  // currently only supported on Android.
+  if (PlatformInfos.isAndroid &&
+      AppLifecycleState.detached == WidgetsBinding.instance.lifecycleState) {
+    // In the background fetch mode we do not want to waste ressources with
+    // starting the Flutter engine but process incoming push notifications.
+    BackgroundPush.clientOnly(clients.first);
+    // To start the flutter engine afterwards we add an custom observer.
+    WidgetsBinding.instance.addObserver(AppStarter(clients));
+    Logs().i(
+      '${AppConfig.applicationName} started in background-fetch mode. No GUI will be created unless the app is no longer detached.',
+    );
+    return;
+  }
+
+  // Started in foreground mode.
+  Logs().i(
+    '${AppConfig.applicationName} started in foreground mode. Rendering GUI...',
+  );
+  await startGui(clients);
+}
+
+/// Fetch the pincode for the applock and start the flutter engine.
+Future<void> startGui(List<Client> clients) async {
+  // Fetch the pin for the applock if existing for mobile applications.
   String? pin;
   if (PlatformInfos.isMobile) {
-    BackgroundPush.clientOnly(clients.first);
     try {
       pin =
           await const FlutterSecureStorage().read(key: SettingKeys.appLockKey);
@@ -39,6 +65,9 @@ void main() async {
     }
   }
 
+  // Start rendering the Flutter app and wrap it in an Applock.
+  // We do this only for mobile applications as we saw routing
+  // problems on other platforms if we wrap it always.
   runApp(
     PlatformInfos.isMobile
         ? AppLock(
@@ -49,3 +78,25 @@ void main() async {
         : FluffyChatApp(clients: clients),
   );
 }
+
+/// Watches the lifecycle changes to start the application when it
+/// is no longer detached.
+class AppStarter with WidgetsBindingObserver {
+  final List<Client> clients;
+  bool guiStarted = false;
+
+  AppStarter(this.clients);
+
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    if (guiStarted) return;
+    if (state == AppLifecycleState.detached) return;
+
+    Logs().i(
+      '${AppConfig.applicationName} switches from the detached background-fetch mode to ${state.name} mode. Rendering GUI...',
+    );
+    startGui(clients);
+    // We must make sure that the GUI is only started once.
+    guiStarted = true;
+  }
+}