api: Add reaction/add and reaction/remove events (not yet handled)

Ongoing discussion about how to handle these:
  https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/flutter.3A.20reaction.20events/near/1619282

But for now, at least we can represent the events in the API model,
before we handle them.

Related: #121

lib/api/model/events.dart

@@ -33,6 +33,12 @@ sealed class Event {
       case 'message': return MessageEvent.fromJson(json);
       case 'update_message': return UpdateMessageEvent.fromJson(json);
       case 'delete_message': return DeleteMessageEvent.fromJson(json);
+      case 'reaction':
+        switch (json['op'] as String) {
+          case 'add': return ReactionAddEvent.fromJson(json);
+          case 'remove': return ReactionRemoveEvent.fromJson(json);
+          default: return UnexpectedEvent.fromJson(json);
+        }
       case 'heartbeat': return HeartbeatEvent.fromJson(json);
       // TODO add many more event types
       default: return UnexpectedEvent.fromJson(json);
@@ -370,6 +376,75 @@ enum MessageType {
   private;
 }
 
+/// A Zulip event of type `reaction`.
+abstract class ReactionEvent extends Event {
+  @override
+  @JsonKey(includeToJson: true)
+  String get type => 'reaction';
+
+  String get op;
+
+  ReactionEvent({required super.id});
+}
+
+/// A [ReactionEvent] with op `add`: https://zulip.com/api/get-events#reaction-add
+@JsonSerializable(fieldRename: FieldRename.snake)
+class ReactionAddEvent extends ReactionEvent {
+  @override
+  String get op => 'add';
+
+  final String emojiName;
+  final String emojiCode;
+  final ReactionType reactionType;
+  final int userId;
+  // final Map<String, dynamic> user; // deprecated; ignore
+  final int messageId;
+
+  ReactionAddEvent({
+    required super.id,
+    required this.emojiName,
+    required this.emojiCode,
+    required this.reactionType,
+    required this.userId,
+    required this.messageId,
+  });
+
+  factory ReactionAddEvent.fromJson(Map<String, dynamic> json) =>
+    _$ReactionAddEventFromJson(json);
+
+  @override
+  Map<String, dynamic> toJson() => _$ReactionAddEventToJson(this);
+}
+
+/// A [ReactionEvent] with op `remove`: https://zulip.com/api/get-events#reaction-remove
+@JsonSerializable(fieldRename: FieldRename.snake)
+class ReactionRemoveEvent extends ReactionEvent {
+  @override
+  String get op => 'remove';
+
+  final String emojiName;
+  final String emojiCode;
+  final ReactionType reactionType;
+  final int userId;
+  // final Map<String, dynamic> user; // deprecated; ignore
+  final int messageId;
+
+  ReactionRemoveEvent({
+    required super.id,
+    required this.emojiName,
+    required this.emojiCode,
+    required this.reactionType,
+    required this.userId,
+    required this.messageId,
+  });
+
+  factory ReactionRemoveEvent.fromJson(Map<String, dynamic> json) =>
+    _$ReactionRemoveEventFromJson(json);
+
+  @override
+  Map<String, dynamic> toJson() => _$ReactionRemoveEventToJson(this);
+}
+
 /// A Zulip event of type `heartbeat`: https://zulip.com/api/get-events#heartbeat
 @JsonSerializable(fieldRename: FieldRename.snake)
 class HeartbeatEvent extends Event {

lib/model/store.dart

@@ -278,6 +278,12 @@ class PerAccountStore extends ChangeNotifier {
     } else if (event is DeleteMessageEvent) {
       assert(debugLog("server event: delete_message ${event.messageIds}"));
       // TODO handle
+    } else if (event is ReactionAddEvent) {
+      assert(debugLog("server event: reaction/add"));
+      // TODO handle
+    } else if (event is ReactionRemoveEvent) {
+      assert(debugLog("server event: reaction/remove"));
+      // TODO handle
     } else if (event is UnexpectedEvent) {
       assert(debugLog("server event: ${jsonEncode(event.toJson())}")); // TODO log better
     } else {