Use WebSocket to update likes

lib/models/ws_message.dart

@@ -1,3 +1,5 @@
+import 'dart:convert';
+
 import 'package:flutter/widgets.dart';
 
 /// WebSocket message
@@ -14,12 +16,21 @@ class WsMessage {
     @required this.title,
     @required this.data,
   })  : assert(id != null),
-        assert(title != null);
+        assert(title != null),
+        assert(title.length > 0);
 
   /// Construct a message from a JSON document (messages coming from the server)
   static WsMessage fromJSON(final Map<dynamic, dynamic> m) {
     return WsMessage(id: m["id"], title: m["title"], data: m["data"]);
   }
 
+  /// Turn a message into a JSON object to send it to the API
+  String toJSON() =>
+      jsonEncode({
+        "id": id,
+        "title": title,
+        "data": data,
+      });
+
   bool get hasId => this.id != null && this.id.isNotEmpty;
 }