notif: Support migration of Android notification channels

Needed for zulip#340, when updating notification channels to use a custom
notification sound.

lib/notifications/display.dart

@@ -25,6 +25,10 @@ AndroidNotificationHostApi get _androidHost => ZulipBinding.instance.androidNoti
 
 /// Service for configuring our Android "notification channel".
 class NotificationChannelManager {
+  /// The channel ID we use for our one notification channel, which we use for
+  /// all notifications.
+  // TODO(launch) check this doesn't match zulip-mobile's current or previous
+  // channel IDs
   @visibleForTesting
   static const kChannelId = 'messages-1';
 
@@ -36,6 +40,8 @@ class NotificationChannelManager {
   static final kVibrationPattern = Int64List.fromList([0, 125, 100, 450]);
 
   /// Create our notification channel, if it doesn't already exist.
+  ///
+  /// Deletes obsolete channels, if present, from old versions of the app.
   //
   // NOTE when changing anything here: the changes will not take effect
   // for existing installs of the app!  That's because we'll have already
@@ -52,11 +58,28 @@ class NotificationChannelManager {
   //    settings for the channel -- like "override Do Not Disturb", or "use
   //    a different sound", or "don't pop on screen" -- their changes get
   //    reset.  So this has to be done sparingly.
-  //
-  //    If we do this, we should also look for any channel with the old
-  //    channel ID and delete it.  See zulip-mobile's `createNotificationChannel`
-  //    in android/app/src/main/java/com/zulipmobile/notifications/NotificationChannelManager.kt .
-  static Future<void> _ensureChannel() async {
+  @visibleForTesting
+  static Future<void> ensureChannel() async {
+    // See if our current-version channel already exists; delete any obsolete
+    // previous channels.
+    var found = false;
+    final channels = await _androidHost.getNotificationChannels();
+    for (final channel in channels) {
+      assert(channel != null); // TODO(#942)
+      if (channel!.id == kChannelId) {
+        found = true;
+      } else {
+        await _androidHost.deleteNotificationChannel(channel.id);
+      }
+    }
+
+    if (found) {
+      // The channel already exists; nothing to do.
+      return;
+    }
+
+    // The channel doesn't exist. Create it.
+
     await _androidHost.createNotificationChannel(NotificationChannel(
       id: kChannelId,
       name: 'Messages', // TODO(i18n)
@@ -81,7 +104,7 @@ class NotificationDisplayManager {
     if (launchDetails?.didNotificationLaunchApp ?? false) {
       _handleNotificationAppLaunch(launchDetails!.notificationResponse);
     }
-    await NotificationChannelManager._ensureChannel();
+    await NotificationChannelManager.ensureChannel();
   }
 
   static void onFcmMessage(FcmMessage data, Map<String, dynamic> dataJson) {

test/model/binding.dart

@@ -554,20 +554,32 @@ class FakeAndroidNotificationHostApi implements AndroidNotificationHostApi {
   }
   List<NotificationChannel> _createdChannels = [];
 
+  /// Consumes the log of calls made to [getNotificationChannels],
+  /// [deleteNotificationChannel] and [createNotificationChannel].
+  ///
+  /// Returns a list of function names in the order they were invoked.
+  List<String> takeChannelMethodCallLogs() {
+    final result = _channelMethodCallLogs;
+    _channelMethodCallLogs = [];
+    return result;
+  }
+  List<String> _channelMethodCallLogs = [];
+
   @override
-  Future<List<NotificationChannel?>> getNotificationChannels() {
-    // TODO: implement getNotificationChannels
-    throw UnimplementedError();
+  Future<List<NotificationChannel?>> getNotificationChannels() async {
+    _channelMethodCallLogs.add('getNotificationChannels');
+    return _createdChannels.toList(growable: false);
   }
 
   @override
-  Future<void> deleteNotificationChannel(String channelId) {
-    // TODO: implement deleteNotificationChannel
-    throw UnimplementedError();
+  Future<void> deleteNotificationChannel(String channelId) async {
+    _channelMethodCallLogs.add('deleteNotificationChannel');
+    _createdChannels.removeWhere((e) => e.id == channelId);
   }
 
   @override
   Future<void> createNotificationChannel(NotificationChannel channel) async {
+    _channelMethodCallLogs.add('createNotificationChannel');
     _createdChannels.add(channel);
   }
 

test/notifications/display_test.dart

@@ -131,6 +131,59 @@ void main() {
             NotificationChannelManager.kVibrationPattern)
       ;
     });
+
+    test('channel is not recreated if one with same id already exists', () async {
+      await NotificationChannelManager.ensureChannel();
+      check(testBinding.androidNotificationHost.takeChannelMethodCallLogs())
+        .deepEquals(['getNotificationChannels', 'createNotificationChannel']);
+
+      await NotificationChannelManager.ensureChannel();
+      check(testBinding.androidNotificationHost.takeChannelMethodCallLogs())
+        .deepEquals(['getNotificationChannels']);
+
+      check(testBinding.androidNotificationHost.takeCreatedChannels()).single
+        ..id.equals(NotificationChannelManager.kChannelId)
+        ..name.equals('Messages')
+        ..importance.equals(NotificationImportance.high)
+        ..lightsEnabled.equals(true)
+        ..vibrationPattern.isNotNull().deepEquals(
+            NotificationChannelManager.kVibrationPattern);
+    });
+
+    test('obsolete channels are removed', () async {
+      await NotificationChannelManager.ensureChannel();
+      check(testBinding.androidNotificationHost.takeChannelMethodCallLogs())
+        .deepEquals(['getNotificationChannels', 'createNotificationChannel']);
+
+      await testBinding.androidNotificationHost.createNotificationChannel(NotificationChannel(
+        id: 'obsolete-1',
+        name: 'Obsolete 1',
+        importance: NotificationImportance.high,
+        lightsEnabled: true,
+        vibrationPattern: NotificationChannelManager.kVibrationPattern));
+      await testBinding.androidNotificationHost.createNotificationChannel(NotificationChannel(
+        id: 'obsolete-2',
+        name: 'Obsolete 2',
+        importance: NotificationImportance.high,
+        lightsEnabled: true,
+        vibrationPattern: NotificationChannelManager.kVibrationPattern));
+      check(testBinding.androidNotificationHost.takeChannelMethodCallLogs())
+        .deepEquals(['createNotificationChannel', 'createNotificationChannel']);
+
+      await NotificationChannelManager.ensureChannel();
+      check(testBinding.androidNotificationHost.takeChannelMethodCallLogs())
+        .deepEquals([
+          'getNotificationChannels',
+          'deleteNotificationChannel',
+          'deleteNotificationChannel']);
+      check(testBinding.androidNotificationHost.takeCreatedChannels()).single
+        ..id.equals(NotificationChannelManager.kChannelId)
+        ..name.equals('Messages')
+        ..importance.equals(NotificationImportance.high)
+        ..lightsEnabled.equals(true)
+        ..vibrationPattern.isNotNull().deepEquals(
+            NotificationChannelManager.kVibrationPattern);
+    });
   });
 
   group('NotificationDisplayManager show', () {