notif: Handle when app in background, too (on Android)

This completes the core functionality of showing notifications on
Android, #320.

Sadly this does not work very well if the app isn't running at all:
e.g., if you terminate the app by swiping it away in the app switcher.
In that case the notification can be quite a bit delayed.

But fixing that seems likely to require some deeper debugging, and
getting our hands into Java or Kotlin code for I think the first time
in zulip-flutter.  So we'll deal with that as a followup issue, #342.
Details there.

Fixes: #320

lib/notifications.dart

@@ -19,8 +19,22 @@ class NotificationService {
   static void debugReset() {
     instance.token.dispose();
     _instance = null;
+    assert(debugBackgroundIsolateIsLive = true);
   }
 
+  /// Whether a background isolate should initialize [LiveZulipBinding].
+  ///
+  /// Ordinarily a [ZulipBinding.firebaseMessagingOnBackgroundMessage] callback
+  /// will be invoked in a background isolate where it must set up its
+  /// [ZulipBinding], just as the `main` function does for most of the app.
+  /// Consequently, by default we have that callback initialize
+  /// [LiveZulipBinding], just like `main` does.
+  ///
+  /// In a test that behavior is undesirable.  Tests that will cause
+  /// [ZulipBinding.firebaseMessagingOnBackgroundMessage] callbacks
+  /// to get invoked should therefore set this to false.
+  static bool debugBackgroundIsolateIsLive = true;
+
   /// The FCM registration token for this install of the app.
   ///
   /// This is unique to the (app, device) pair, but not permanent.
@@ -41,7 +55,8 @@ class NotificationService {
     //   (in order to avoid calling for permissions)
 
     await NotificationDisplayManager._init();
-    ZulipBinding.instance.firebaseMessagingOnMessage.listen(_onRemoteMessage);
+    ZulipBinding.instance.firebaseMessagingOnMessage.listen(_onForegroundMessage);
+    ZulipBinding.instance.firebaseMessagingOnBackgroundMessage(_onBackgroundMessage);
 
     // Get the FCM registration token, now and upon changes.  See FCM API docs:
     //   https://firebase.google.com/docs/cloud-messaging/android/client#sample-register
@@ -71,8 +86,41 @@ class NotificationService {
     token.value = value;
   }
 
-  static void _onRemoteMessage(FirebaseRemoteMessage message) {
+  static void _onForegroundMessage(FirebaseRemoteMessage message) {
     assert(debugLog("notif message: ${message.data}"));
+    _onRemoteMessage(message);
+  }
+
+  static Future<void> _onBackgroundMessage(FirebaseRemoteMessage message) async {
+    // This callback will run in a separate isolate from the rest of the app.
+    // See docs:
+    //   https://firebase.flutter.dev/docs/messaging/usage/#background-messages
+    _initBackgroundIsolate();
+
+    assert(debugLog("notif message in background: ${message.data}"));
+    _onRemoteMessage(message);
+  }
+
+  static void _initBackgroundIsolate() {
+    bool isolateIsLive = true;
+    assert(() {
+      isolateIsLive = debugBackgroundIsolateIsLive;
+      return true;
+    }());
+    if (!isolateIsLive) {
+      return;
+    }
+
+    // Compare these setup steps to the ones in `main` in lib/main.dart .
+    assert(() {
+      debugLogEnabled = true;
+      return true;
+    }());
+    LiveZulipBinding.ensureInitialized();
+    NotificationDisplayManager._init(); // TODO call this just once per isolate
+  }
+
+  static void _onRemoteMessage(FirebaseRemoteMessage message) {
     final data = FcmMessage.fromJson(message.data);
     switch (data) {
       case MessageFcmMessage(): NotificationDisplayManager._onMessageFcmMessage(data, message.data);

test/notifications_test.dart

@@ -68,6 +68,7 @@ void main() {
     addTearDown(testBinding.reset);
     testBinding.firebaseMessagingInitialToken = '012abc';
     addTearDown(NotificationService.debugReset);
+    NotificationService.debugBackgroundIsolateIsLive = false;
     await NotificationService.instance.start();
   }
 
@@ -93,13 +94,10 @@ void main() {
   });
 
   group('NotificationDisplayManager', () {
-    Future<void> checkNotification(MessageFcmMessage data, {
+    void checkNotification(MessageFcmMessage data, {
       required String expectedTitle,
       required String expectedTagComponent,
-    }) async {
-      testBinding.firebaseMessaging.onMessage.add(
-        RemoteMessage(data: data.toJson()));
-      await null;
+    }) {
       check(testBinding.notifications.takeShowCalls()).single
         ..id.equals(NotificationDisplayManager.kNotificationId)
         ..title.equals(expectedTitle)
@@ -112,11 +110,28 @@ void main() {
         );
     }
 
+    Future<void> checkNotifications(MessageFcmMessage data, {
+      required String expectedTitle,
+      required String expectedTagComponent,
+    }) async {
+      testBinding.firebaseMessaging.onMessage.add(
+        RemoteMessage(data: data.toJson()));
+      await null;
+      checkNotification(data, expectedTitle: expectedTitle,
+        expectedTagComponent: expectedTagComponent);
+
+      testBinding.firebaseMessaging.onBackgroundMessage.add(
+        RemoteMessage(data: data.toJson()));
+      await null;
+      checkNotification(data, expectedTitle: expectedTitle,
+        expectedTagComponent: expectedTagComponent);
+    }
+
     test('stream message', () async {
       await init();
       final stream = eg.stream();
       final message = eg.streamMessage(stream: stream);
-      await checkNotification(messageFcmMessage(message, streamName: stream.name),
+      await checkNotifications(messageFcmMessage(message, streamName: stream.name),
         expectedTitle: '${stream.name} > ${message.subject}',
         expectedTagComponent: 'stream:${message.streamId}:${message.subject}');
     });
@@ -125,31 +140,31 @@ void main() {
       await init();
       final stream = eg.stream();
       final message = eg.streamMessage(stream: stream);
-      await checkNotification(messageFcmMessage(message, streamName: null),
+      await checkNotifications(messageFcmMessage(message, streamName: null),
         expectedTitle: '(unknown stream) > ${message.subject}',
         expectedTagComponent: 'stream:${message.streamId}:${message.subject}');
     });
 
     test('group DM', () async {
       await init();
       final message = eg.dmMessage(from: eg.thirdUser, to: [eg.otherUser, eg.selfUser]);
-      await checkNotification(messageFcmMessage(message),
+      await checkNotifications(messageFcmMessage(message),
         expectedTitle: "${eg.thirdUser.fullName} to you and 1 others",
         expectedTagComponent: 'dm:${message.allRecipientIds.join(",")}');
     });
 
     test('1:1 DM', () async {
       await init();
       final message = eg.dmMessage(from: eg.otherUser, to: [eg.selfUser]);
-      await checkNotification(messageFcmMessage(message),
+      await checkNotifications(messageFcmMessage(message),
         expectedTitle: eg.otherUser.fullName,
         expectedTagComponent: 'dm:${message.allRecipientIds.join(",")}');
     });
 
     test('self-DM', () async {
       await init();
       final message = eg.dmMessage(from: eg.selfUser, to: []);
-      await checkNotification(messageFcmMessage(message),
+      await checkNotifications(messageFcmMessage(message),
         expectedTitle: eg.selfUser.fullName,
         expectedTagComponent: 'dm:${message.allRecipientIds.join(",")}');
     });