Converting object to an encodable object failed

I am getting the following error:

Converting object to an encodable object failed: Instance of 'Patient'
#0      _JsonStringifier.writeObject (dart:convert/json.dart:674)
#1      _JsonStringifier.writeList (dart:convert/json.dart:724)
#2      _JsonStringifier.writeJsonValue (dart:convert/json.dart:706)
#3      _JsonStringifier.writeObject (dart:convert/json.dart:664)
#4      _JsonStringStringifier.printOn (dart:convert/json.dart:873)
#5      _JsonStringStringifier.stringify (dart:convert/json.dart:855)
#6      JsonEncoder.convert (dart:convert/json.dart:256)
#7      JsonCodec.encode (dart:convert/json.dart:155)
#8      Persistence.saveLatestPatients (/Users/dean/Library/Developer/CoreSimulator/Devices/570CC18D-95BF-4062-8523-9C78E106D0CF/data/Containers/Data/Application/70CAEFAA-4AE3-4CBF-A85F-39161E472C83/tmp/flutter_prototypev6jYbr/flutter_prototype/lib/utils/persistence.dart:32:23)
<asynchronous suspension>
#9      _HomeScreenState.fetchData.<anonymous closure> (/Users/dean/Librar<…>

My 'Patient' class:

import 'package:simple_moment/simple_moment.dart';

class Patient {
  String guid;
  String _name;
  String _surname;
  DateTime _updated;

  Patient(String guid) {
    this.guid = guid;
  }

  String get name => _name;
  set name(v) => _name = v;

  String get surname => _surname;
  set surname(v) => _surname = v;

  DateTime get updated => _updated;
  set updated(v) => _updated = v;

  // Helper functions

  String getFullName() => '$_name $_surname';

  String getRelativeLastUpdated() {
    var moment = new Moment.now();
    return moment.from(_updated);
  }

}

Solution

You can't just convert any arbitrary class instance to JSON.

This due to the fact that Flutter does not support reflection. Thus, no dart program in a flutter application can determine, which properties a class features. Therefore, no magic can serialize arbitrarily objects in Flutter.

One option is to provide a custom function to the JsonEncoder() constructor (via the toEncodable argument). This custom function should map your custom objects to types that JsonEncoder already knows how to handle (i.e. numbers, strings, booleans, null, lists and maps with string keys).

Maybe the reflectable plugin might be of use here.

https://api.dartlang.org/stable/1.24.3/dart-convert/JsonEncoder-class.html

https://pub.dartlang.org/packages/json_serializable is a package that generates code for that so that you don't need to write it manually.