json - 将对象转换为可编码对象失败

Question

我收到以下错误:

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<…>

我的“病人”课:

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);
  }

}

Answer 1

您不能只是将任意类实例转换为 JSON。
一种选择是为 JsonEncoder() 构造函数提供自定义函数(通过 toEncodable 参数)。这个自定义函数应该将您的自定义对象映射到 JsonEncoder 已经知道如何处理的类型(即数字、字符串、 bool 值、空值、列表和带有字符串键的映射)。
https://api.dartlang.org/stable/1.24.3/dart-convert/JsonEncoder-class.html
https://pub.dartlang.org/packages/json_serializable是一个为此生成代码的包，因此您无需手动编写它。
另见 https://flutter.io/json/