wip api [nfc]: Extract and parse UpdateMessageMoveData from UpdateMessageEvent

This data structure encapsulates some checks so that we can make all
fields non-nullable, with reasonable fallback values.  As of writing,
we do not use origStreamId (a.k.a.: 'stream_id') when there was no
message move, even though it is present if there were content edits
This makes dropping 'stream_id' when parsing `moveData` into `null`
acceptable for now.

This is NFC for debug builds because we were using assertions before,
and now start to throw `FormatException`s with the parsing code.

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/api/model/events.dart

@@ -718,16 +718,8 @@ class UpdateMessageEvent extends Event {
 
   // final String? streamName; // ignore
 
-  @JsonKey(name: 'stream_id')
-  final int? origStreamId;
-  final int? newStreamId;
-
-  final PropagateMode? propagateMode;
-
-  @JsonKey(name: 'orig_subject')
-  final TopicName? origTopic;
-  @JsonKey(name: 'subject')
-  final TopicName? newTopic;
+  @JsonKey(readValue: _readMoveData, fromJson: UpdateMessageMoveData.tryParseFromJson, includeToJson: false)
+  final UpdateMessageMoveData? moveData;
 
   // final List<TopicLink> topicLinks; // TODO handle
 
@@ -747,25 +739,103 @@ class UpdateMessageEvent extends Event {
     required this.messageIds,
     required this.flags,
     required this.editTimestamp,
-    required this.origStreamId,
-    required this.newStreamId,
-    required this.propagateMode,
-    required this.origTopic,
-    required this.newTopic,
+    required this.moveData,
     required this.origContent,
     required this.origRenderedContent,
     required this.content,
     required this.renderedContent,
     required this.isMeMessage,
   });
 
+  static Map<String, dynamic> _readMoveData(Map<dynamic, dynamic> json, String key) {
+    // Parsing [UpdateMessageMoveData] requires `json`, not the default `json[key]`.
+    assert(json is Map<String, dynamic>); // value came through `fromJson` with this type
+    return json as Map<String, dynamic>;
+  }
+
   factory UpdateMessageEvent.fromJson(Map<String, dynamic> json) =>
     _$UpdateMessageEventFromJson(json);
 
   @override
   Map<String, dynamic> toJson() => _$UpdateMessageEventToJson(this);
 }
 
+/// Data structure representing a message move.
+class UpdateMessageMoveData {
+  final int origStreamId;
+  final int newStreamId;
+
+  final PropagateMode propagateMode;
+
+  final TopicName origTopic;
+  final TopicName newTopic;
+
+  UpdateMessageMoveData({
+    required this.origStreamId,
+    required this.newStreamId,
+    required this.propagateMode,
+    required this.origTopic,
+    required this.newTopic,
+  }) : assert(origStreamId != newStreamId || origTopic != newTopic);
+
+  /// Try to extract [UpdateMessageMoveData] from the JSON object for an
+  /// [UpdateMessageEvent].
+  ///
+  /// Returns `null` if there was no message move.
+  ///
+  /// Throws an error if the data is malformed.
+  // When parsing this, 'stream_id', which is also present when there was only
+  // a content edit, cannot be recovered if this ends up returning `null`.
+  // This may matter if we ever need 'stream_id' when no message move occurred.
+  static UpdateMessageMoveData? tryParseFromJson(Map<String, Object?> json) {
+    final origStreamId = (json['stream_id'] as num?)?.toInt();
+    final newStreamIdRaw = (json['new_stream_id'] as num?)?.toInt();
+    final newStreamId = newStreamIdRaw ?? origStreamId;
+
+    final propagateModeString = json['propagate_mode'] as String?;
+    final propagateMode = propagateModeString == null ? null
+      : PropagateMode.fromRawString(propagateModeString);
+
+    final origTopic = json['orig_subject'] == null ? null
+      : TopicName.fromJson(json['orig_subject'] as String);
+    final newTopicRaw = json['subject'] == null ? null
+      : TopicName.fromJson(json['subject'] as String);
+    final newTopic = newTopicRaw ?? origTopic;
+
+    if (origTopic == newTopic && origStreamId == newStreamId) {
+      if (propagateMode != null) {
+        throw FormatException(
+          'Malformed UpdateMessageEvent: incoherent message-move fields; '
+          'propagate_mode present but no new channel or topic');
+      }
+      return null;
+    }
+
+    if (origStreamId == null || newStreamId == null) {
+      // The `stream_id` field (aka origStreamId) is documented to be present on moves;
+      // newStreamId should not be null either because it falls back to origStreamId.
+      throw FormatException('Malformed UpdateMessageEvent: move but no origStreamId');
+    }
+    if (origTopic == null || newTopic == null) {
+      // The `orig_subject` field (aka origTopic) is documented to be present on moves;
+      // newTopic should not be null either because it falls back to origTopic.
+      throw FormatException('Malformed UpdateMessageEvent: move but no origTopic');
+    }
+    if (propagateMode == null) {
+      // The `propagate_mode` field (aka propagateMode) is documented to be present on moves.
+      throw FormatException('Malformed UpdateMessageEvent: move but no propagateMode');
+    }
+
+    return UpdateMessageMoveData(
+      origStreamId: origStreamId,
+      newStreamId: newStreamId,
+      propagateMode: propagateMode,
+      origTopic: origTopic,
+      newTopic: newTopic,
+    );
+  }
+}
+
 /// A Zulip event of type `delete_message`: https://zulip.com/api/get-events#delete_message
 @JsonSerializable(fieldRename: FieldRename.snake)
 class DeleteMessageEvent extends Event {

lib/api/model/events.g.dart

@@ -169,37 +169,14 @@ class MessageStoreImpl with MessageStore {
   }
 
   void _handleUpdateMessageEventMove(UpdateMessageEvent event) {
-    // The interaction between the fields of these events are a bit tricky.
-    // For reference, see: https://zulip.com/api/get-events#update_message
-
-    final origStreamId = event.origStreamId;
-    final newStreamId = event.newStreamId ?? origStreamId;
-    final origTopic = event.origTopic;
-    final newTopic = event.newTopic ?? origTopic;
-    final propagateMode = event.propagateMode;
-
-    if (newStreamId == origStreamId && newTopic == origTopic) {
+    final messageMove = event.moveData;
+    if (messageMove == null) {
       // There was no move.
       return;
     }
 
-    if (origStreamId == null || newStreamId == null) {
-      // The `stream_id` field (aka origStreamId) is documented to be present on moves.
-      // newStreamId should not be null either because it falls back to origStreamId.
-      assert(debugLog('Malformed UpdateMessageEvent: move but no origStreamId')); // TODO(log)
-      return;
-    }
-    if (origTopic == null || newTopic == null) {
-      // The `orig_subject` field (aka origTopic) is documented to be present on moves.
-      // newTopic should not be null either because it falls back to origTopic.
-      assert(debugLog('Malformed UpdateMessageEvent: move but no origTopic')); // TODO(log)
-      return;
-    }
-    if (propagateMode == null) {
-      // The `propagate_mode` field (aka propagateMode) is documented to be present on moves.
-      assert(debugLog('Malformed UpdateMessageEvent: move but no propagateMode')); // TODO(log)
-      return;
-    }
+    final UpdateMessageMoveData(
+      :origStreamId, :newStreamId, :origTopic, :newTopic) = messageMove;
 
     final wasResolveOrUnresolve = newStreamId == origStreamId
       && MessageEditState.topicMoveWasResolveOrUnresolve(origTopic, newTopic);
@@ -231,14 +208,7 @@ class MessageStoreImpl with MessageStore {
     }
 
     for (final view in _messageListViews) {
-      view.messagesMoved(
-        origStreamId: origStreamId,
-        newStreamId: newStreamId,
-        origTopic: origTopic,
-        newTopic: newTopic,
-        messageIds: event.messageIds,
-        propagateMode: propagateMode,
-      );
+      view.messagesMoved(messageMove: messageMove, messageIds: event.messageIds);
     }
   }
 

lib/model/message.dart

@@ -720,13 +720,12 @@ class MessageListView with ChangeNotifier, _MessageSequence {
   }
 
   void messagesMoved({
-    required int origStreamId,
-    required int newStreamId,
-    required TopicName origTopic,
-    required TopicName newTopic,
+    required UpdateMessageMoveData messageMove,
     required List<int> messageIds,
-    required PropagateMode propagateMode,
   }) {
+    final UpdateMessageMoveData(
+      :origStreamId, :newStreamId, :origTopic, :newTopic, :propagateMode,
+    ) = messageMove;
     switch (narrow) {
       case DmNarrow():
         // DMs can't be moved (nor created by moves),

lib/model/message_list.dart

@@ -48,18 +48,22 @@ extension UpdateMessageEventChecks on Subject<UpdateMessageEvent> {
   Subject<List<int>> get messageIds => has((e) => e.messageIds, 'messageIds');
   Subject<List<MessageFlag>> get flags => has((e) => e.flags, 'flags');
   Subject<int?> get editTimestamp => has((e) => e.editTimestamp, 'editTimestamp');
-  Subject<int?> get origStreamId => has((e) => e.origStreamId, 'origStreamId');
-  Subject<int?> get newStreamId => has((e) => e.newStreamId, 'newStreamId');
-  Subject<PropagateMode?> get propagateMode => has((e) => e.propagateMode, 'propagateMode');
-  Subject<TopicName?> get origTopic => has((e) => e.origTopic, 'origTopic');
-  Subject<TopicName?> get newTopic => has((e) => e.newTopic, 'newTopic');
+  Subject<UpdateMessageMoveData?> get moveData => has((e) => e.moveData, 'moveData');
   Subject<String?> get origContent => has((e) => e.origContent, 'origContent');
   Subject<String?> get origRenderedContent => has((e) => e.origRenderedContent, 'origRenderedContent');
   Subject<String?> get content => has((e) => e.content, 'content');
   Subject<String?> get renderedContent => has((e) => e.renderedContent, 'renderedContent');
   Subject<bool?> get isMeMessage => has((e) => e.isMeMessage, 'isMeMessage');
 }
 
+extension UpdateMessageMoveDataChecks on Subject<UpdateMessageMoveData> {
+  Subject<int> get origStreamId => has((e) => e.origStreamId, 'origStreamId');
+  Subject<int> get newStreamId => has((e) => e.newStreamId, 'newStreamId');
+  Subject<PropagateMode> get propagateMode => has((e) => e.propagateMode, 'propagateMode');
+  Subject<TopicName> get origTopic => has((e) => e.origTopic, 'origTopic');
+  Subject<TopicName> get newTopic => has((e) => e.newTopic, 'newTopic');
+}
+
 extension DeleteMessageEventChecks on Subject<DeleteMessageEvent> {
   Subject<MessageType?> get messageType => has((e) => e.messageType, 'messageType');
 }

test/api/model/events_checks.dart

@@ -106,11 +106,26 @@ void main() {
       'propagate_mode': 'change_all',
     };
 
+    test('smoke moveData', () {
+      check(Event.fromJson({ ...baseMoveJson,
+        'stream_id': 1,
+        'new_stream_id': 2,
+        'orig_subject': 'foo',
+        'subject': 'bar',
+        'propagate_mode': 'change_all',
+      })).isA<UpdateMessageEvent>().moveData.isNotNull()
+        ..origStreamId.equals(1)
+        ..newStreamId.equals(2)
+        ..origTopic.equals(const TopicName('foo'))
+        ..newTopic.equals(const TopicName('bar'))
+        ..propagateMode.equals(PropagateMode.changeAll);
+    });
+
     test('stream_id -> origStreamId', () {
       check(Event.fromJson({ ...baseMoveJson,
         'stream_id': 1,
         'new_stream_id': 2,
-      })).isA<UpdateMessageEvent>()
+      })).isA<UpdateMessageEvent>().moveData.isNotNull()
         ..origStreamId.equals(1)
         ..newStreamId.equals(2);
     });
@@ -119,10 +134,34 @@ void main() {
       check(Event.fromJson({ ...baseMoveJson,
         'orig_subject': 'foo',
         'subject': 'bar',
-      })).isA<UpdateMessageEvent>()
+      })).isA<UpdateMessageEvent>().moveData.isNotNull()
         ..origTopic.equals(const TopicName('foo'))
         ..newTopic.equals(const TopicName('bar'));
     });
+
+    test('new channel, same topic: fill in newTopic', () {
+      // The server omits 'subject' in this situation.
+      check(Event.fromJson({ ...baseMoveJson,
+        'orig_subject': 'foo',
+        'subject': null,
+        'stream_id': 1,
+        'new_stream_id': 2,
+      })).isA<UpdateMessageEvent>().moveData.isNotNull()
+        ..origTopic.equals(const TopicName('foo'))
+        ..newTopic.equals(const TopicName('foo'));
+    });
+
+    test('same channel, new topic; fill in newStreamId', () {
+      // The server omits 'new_stream_id' in this situation.
+      check(Event.fromJson({ ...baseMoveJson,
+        'orig_subject': 'foo',
+        'subject': 'bar',
+        'stream_id': 1,
+        'new_stream_id': null,
+      })).isA<UpdateMessageEvent>().moveData.isNotNull()
+        ..origStreamId.equals(1)
+        ..newStreamId.equals(1);
+    });
   });
 
   test('delete_message: require streamId and topic for stream messages', () {

test/api/model/events_test.dart

@@ -53,6 +53,7 @@ extension TopicNameChecks on Subject<TopicName> {
 
 extension StreamMessageChecks on Subject<StreamMessage> {
   Subject<String?> get displayRecipient => has((e) => e.displayRecipient, 'displayRecipient');
+  Subject<int> get streamId => has((e) => e.streamId, 'streamId');
   Subject<TopicName> get topic => has((e) => e.topic, 'topic');
 }
 

test/api/model/model_checks.dart

@@ -655,11 +655,7 @@ UpdateMessageEvent updateMessageEditEvent(
     messageIds: [messageId],
     flags: flags ?? origMessage.flags,
     editTimestamp: editTimestamp ?? 1234567890, // TODO generate timestamp
-    origStreamId: origMessage is StreamMessage ? origMessage.streamId : null,
-    newStreamId: null,
-    propagateMode: null,
-    origTopic: null,
-    newTopic: null,
+    moveData: null,
     origContent: 'some probably-mismatched old Markdown',
     origRenderedContent: origMessage.content,
     content: 'some probably-mismatched new Markdown',
@@ -692,11 +688,13 @@ UpdateMessageEvent _updateMessageMoveEvent(
     messageIds: messageIds,
     flags: flags,
     editTimestamp: 1234567890, // TODO generate timestamp
-    origStreamId: origStreamId,
-    newStreamId: newStreamId,
-    propagateMode: propagateMode,
-    origTopic: origTopic,
-    newTopic: newTopic,
+    moveData: UpdateMessageMoveData(
+      origStreamId: origStreamId,
+      newStreamId: newStreamId ?? origStreamId,
+      propagateMode: propagateMode,
+      origTopic: origTopic,
+      newTopic: newTopic ?? origTopic,
+    ),
     origContent: origContent,
     origRenderedContent: origContent,
     content: newContent,