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Dart对象 -> JSON字符串无法转换为JSON

jsondart
4

我想将一个对象序列化为JSON格式，然后再将JSON格式的数据反序列化为对象。

以下是可生成正确JSON格式的代码片段:

 LoginRequest req = new LoginRequest();
    req.username = username;
    req.password = password;
    req.created = 123456;
    req.test = "KOTS";
    print(req.toString());

我在控制台上看到的是这样的：

{} (:1)

在 pubspec.yaml 中，我作为一个依赖项导入了 json_object：
environment:
  sdk: '>=1.0.0 <2.0.0'
dependencies:
  ...
  json_object: any

我有一个继承自JsonObject的基本信息类：

import 'package:json_object/json_object.dart';

class Message extends JsonObject {

  int created = new DateTime.now().millisecondsSinceEpoch;

}

然后我有一个LoginRequest，它继承自Message：

import 'Message.dart';

class LoginRequest extends Message {

  String _username;
  String _password;
  String test;

  String get username => _username;
  set username(String username) {
    _username = username.trim();  
  }

  String get password => _password;
  set password(String password) {
    _password = password.trim(); 
  }

}

我认为只有基类需要转换为Json，因此我编写了另一个测试用例：

我在考虑只有基类会转换为Json，所以我写了另一个测试案例：

Message msg = new Message();
msg.created = 123456;
print(msg.toString());

这也是打印：

{} (:1)

调用objectToJson函数也是一样的：

objectToJson(msg).then((jsonStr) => print(jsonStr));
objectToJson(req).then((jsonStr) => print(jsonStr));

输出：

{}
{}

如果删除extends JsonObject，则上述代码会产生堆栈跟踪：

Exception: Uncaught Error: Converting object to an encodable object failed.
Stack Trace:
#0      _JsonStringifier.writeObject (dart:convert/json.dart:660)
#1      _JsonStringStringifier.printOn (dart:convert/json.dart:831)
#2      _JsonStringStringifier.stringify (dart:convert/json.dart:813)
#3      JsonEncoder.convert (dart:convert/json.dart:243)
#4      JsonCodec.encode (dart:convert/json.dart:141)
#5      login (package:falm/login-dialog.dart:47:22)
#6      Function.apply (dart:core-patch/function_patch.dart:28)
#7      GeneratedObjectAccessorService.invoke (package:smoke/static.dart:149:28)
#8      invoke (package:smoke/smoke.dart:43:41)
#9      HtmlElement&Polymer.dispatchMethod (package:polymer/src/instance.dart:1054:19)
#10     BindingDelegate&PolymerEventBindings.getEventHandler.<anonymous closure> (package:polymer/src/events.dart:82:32)
#11     _RootZone.runUnaryGuarded (dart:async/zone.dart:1093)
#12     _RootZone.bindUnaryCallback.<anonymous closure> (dart:async/zone.dart:1122)
#13     BindingDelegate&PolymerEventBindings.prepareEventBinding.<anonymous closure>.<anonymous closure> (package:polymer/src/events.dart:101:67)

Clicked Login (:1)
Exception: Uncaught Error: Class '_LocalClassMirror' has no instance getter 'getters'.

NoSuchMethodError: method not found: 'getters'
Receiver: Instance of '_LocalClassMirror'
Arguments: []
Stack Trace:
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:45)
#1      _serializeObject (package:json_object/src/mirror_based_serializer.dart:127:16)
#2      objectToSerializable (package:json_object/src/mirror_based_serializer.dart:53:21)
#3      objectToJson (package:json_object/src/mirror_based_serializer.dart:22:23)
#4      login (package:falm/login-dialog.dart:43:17)
#5      Function.apply (dart:core-patch/function_patch.dart:28)
#6      GeneratedObjectAccessorService.invoke (package:smoke/static.dart:149:28)
#7      invoke (package:smoke/smoke.dart:43:41)
#8      HtmlElement&Polymer.dispatchMethod (package:polymer/src/instance.dart:1054:19)
#9      BindingDelegate&PolymerEventBindings.getEventHandler.<anonymous closure> (package:polymer/src/events.dart:82:32)
#10     _RootZone.runUnaryGuarded (dart:async/zone.dart:1093)
#11     _RootZone.bindUnaryCallback.<anonymous closure> (dart:async/zone.dart:1122)
#12     BindingDelegate&PolymerEventBindings.prepareEventBinding.<anonymous closure>.<anonymous closure> (package:polymer/src/events.dart:101:67)

Clicked Login (:1)
Exception: Uncaught Error: Converting object to an encodable object failed.
Stack Trace:
#0      _JsonStringifier.writeObject (dart:convert/json.dart:660)
#1      _JsonStringStringifier.printOn (dart:convert/json.dart:831)
#2      _JsonStringStringifier.stringify (dart:convert/json.dart:813)
#3      JsonEncoder.convert (dart:convert/json.dart:243)
#4      JsonCodec.encode (dart:convert/json.dart:141)
#5      login (package:falm/login-dialog.dart:47:22)
#6      Function.apply (dart:core-patch/function_patch.dart:28)
#7      GeneratedObjectAccessorService.invoke (package:smoke/static.dart:149:28)
#8      invoke (package:smoke/smoke.dart:43:41)
#9      HtmlElement&Polymer.dispatchMethod (package:polymer/src/instance.dart:1054:19)
#10     BindingDelegate&PolymerEventBindings.getEventHandler.<anonymous closure> (package:polymer/src/events.dart:82:32)
#11     _RootZone.runUnaryGuarded (dart:async/zone.dart:1093)
#12     _RootZone.bindUnaryCallback.<anonymous closure> (dart:async/zone.dart:1122)
#13     BindingDelegate&PolymerEventBindings.prepareEventBinding.<anonymous closure>.<anonymous closure> (package:polymer/src/events.dart:101:67)

Clicked Login (:1)
Exception: Uncaught Error: Class '_LocalClassMirror' has no instance getter 'getters'.

NoSuchMethodError: method not found: 'getters'
Receiver: Instance of '_LocalClassMirror'
Arguments: []
Stack Trace:
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:45)
#1      _serializeObject (package:json_object/src/mirror_based_serializer.dart:127:16)
#2      objectToSerializable (package:json_object/src/mirror_based_serializer.dart:53:21)
#3      objectToJson (package:json_object/src/mirror_based_serializer.dart:22:23)
#4      login (package:falm/login-dialog.dart:44:17)
#5      Function.apply (dart:core-patch/function_patch.dart:28)
#6      GeneratedObjectAccessorService.invoke (package:smoke/static.dart:149:28)
#7      invoke (package:smoke/smoke.dart:43:41)
#8      HtmlElement&Polymer.dispatchMethod (package:polymer/src/instance.dart:1054:19)
#9      BindingDelegate&PolymerEventBindings.getEventHandler.<anonymous closure> (package:polymer/src/events.dart:82:32)
#10     _RootZone.runUnaryGuarded (dart:async/zone.dart:1093)
#11     _RootZone.bindUnaryCallback.<anonymous closure> (dart:async/zone.dart:1122)
#12     BindingDelegate&PolymerEventBindings.prepareEventBinding.<anonymous closure>.<anonymous closure> (package:polymer/src/events.dart:101:67)

导入dart:convert并使用JSON.encode做的是相同的事情：

import 'dart:convert' show JSON;

...

  print(JSON.encode(msg));
  print(JSON.encode(req));

输出：

{}
{}

如果我删除extends JsonObject，那么它会抛出一个堆栈跟踪：
Exception: Uncaught Error: Converting object to an encodable object failed.
Stack Trace:
#0      _JsonStringifier.writeObject (dart:convert/json.dart:660)
#1      _JsonStringStringifier.printOn (dart:convert/json.dart:831)
#2      _JsonStringStringifier.stringify (dart:convert/json.dart:813)
#3      JsonEncoder.convert (dart:convert/json.dart:243)
#4      JsonCodec.encode (dart:convert/json.dart:141)
#5      login (package:falm/login-dialog.dart:47:22)
#6      Function.apply (dart:core-patch/function_patch.dart:28)
#7      GeneratedObjectAccessorService.invoke (package:smoke/static.dart:149:28)
#8      invoke (package:smoke/smoke.dart:43:41)
#9      HtmlElement&Polymer.dispatchMethod (package:polymer/src/instance.dart:1054:19)
#10     BindingDelegate&PolymerEventBindings.getEventHandler.<anonymous closure> (package:polymer/src/events.dart:82:32)
#11     _RootZone.runUnaryGuarded (dart:async/zone.dart:1093)
#12     _RootZone.bindUnaryCallback.<anonymous closure> (dart:async/zone.dart:1122)
#13     BindingDelegate&PolymerEventBindings.prepareEventBinding.<anonymous closure>.<anonymous closure> (package:polymer/src/events.dart:101:67)

Clicked Login (:1)
Exception: Uncaught Error: Class '_LocalClassMirror' has no instance getter 'getters'.

NoSuchMethodError: method not found: 'getters'
Receiver: Instance of '_LocalClassMirror'
Arguments: []
Stack Trace:
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:45)
#1      _serializeObject (package:json_object/src/mirror_based_serializer.dart:127:16)
#2      objectToSerializable (package:json_object/src/mirror_based_serializer.dart:53:21)
#3      objectToJson (package:json_object/src/mirror_based_serializer.dart:22:23)
#4      login (package:falm/login-dialog.dart:43:17)
#5      Function.apply (dart:core-patch/function_patch.dart:28)
#6      GeneratedObjectAccessorService.invoke (package:smoke/static.dart:149:28)
#7      invoke (package:smoke/smoke.dart:43:41)
#8      HtmlElement&Polymer.dispatchMethod (package:polymer/src/instance.dart:1054:19)
#9      BindingDelegate&PolymerEventBindings.getEventHandler.<anonymous closure> (package:polymer/src/events.dart:82:32)
#10     _RootZone.runUnaryGuarded (dart:async/zone.dart:1093)
#11     _RootZone.bindUnaryCallback.<anonymous closure> (dart:async/zone.dart:1122)
#12     BindingDelegate&PolymerEventBindings.prepareEventBinding.<anonymous closure>.<anonymous closure> (package:polymer/src/events.dart:101:67)

Clicked Login (:1)
Exception: Uncaught Error: Converting object to an encodable object failed.
Stack Trace:
#0      _JsonStringifier.writeObject (dart:convert/json.dart:660)
#1      _JsonStringStringifier.printOn (dart:convert/json.dart:831)
#2      _JsonStringStringifier.stringify (dart:convert/json.dart:813)
#3      JsonEncoder.convert (dart:convert/json.dart:243)
#4      JsonCodec.encode (dart:convert/json.dart:141)
#5      login (package:falm/login-dialog.dart:47:22)
#6      Function.apply (dart:core-patch/function_patch.dart:28)
#7      GeneratedObjectAccessorService.invoke (package:smoke/static.dart:149:28)
#8      invoke (package:smoke/smoke.dart:43:41)
#9      HtmlElement&Polymer.dispatchMethod (package:polymer/src/instance.dart:1054:19)
#10     BindingDelegate&PolymerEventBindings.getEventHandler.<anonymous closure> (package:polymer/src/events.dart:82:32)
#11     _RootZone.runUnaryGuarded (dart:async/zone.dart:1093)
#12     _RootZone.bindUnaryCallback.<anonymous closure> (dart:async/zone.dart:1122)
#13     BindingDelegate&PolymerEventBindings.prepareEventBinding.<anonymous closure>.<anonymous closure> (package:polymer/src/events.dart:101:67)

JsonObject仍然是将对象序列化为JSON并将JSON反序列化为对象的正确方法吗？（我看到代码版权为2013年，现在已经古老了）。如果是这样，那么我的类中是否有遗漏的内容？
在dart cookbook中，有例子展示每个类都有自己的toJson方法，并且所有值都手动复制到一个map中，这很繁琐。如果我的整个应用程序都受JSON驱动，那么我会花费大部分时间编写样板toJson/fromJson方法，这正是我想要摆脱的原因，因此选择了dart。
此外，我在StackOverflow上看到了使用mirrors的示例，随后是注释说mirrors在dart2js中不完全支持——由于这是一个基于浏览器的应用程序，它能否编译成JavaScript至关重要。
更新：
根据Robert的回答，似乎避免编写样板是不可避免的：
Message.dart
import 'dart:convert' show JSON;

class Message {

  int created = new DateTime.now().millisecondsSinceEpoch;

  Map toJson() { 
    Map map = new Map();
    map["created"] = this.created;
    return map;
  }  

  String toString(){
    return JSON.encode(this);
  }

}

LoginRequest.dart

import 'Message.dart';

class LoginRequest extends Message {

  String _username;
  String _password;
  String test;

  String get username => _username;
  set username(String username) {
    _username = username.trim();  
  }

  String get password => _password;
  set password(String password) {
    _password = password.trim(); 
  }

  Map toJson() { 
    Map map = super.toJson();
    map["username"] = this.username;
    map["password"] = this.password;
    return map;
  }  

}

测试代码：

LoginRequest req = new LoginRequest();
req.username = username;
req.password = password;
req.created = 123456;
req.test = "KOTS";
print(req);
// outputs: {"created":123456,"username":"asdfasdf","password":"adfasdf"} (:1)

Message msg = new Message();
msg.created = 123456;
print(msg);
// outputs: {"created":123456} (:1)

toString方法只需要在Message类中实现一次，toJson方法则需要在每个类中实现。

使用JsonObject/json_object似乎开销很大，最好编写自己的函数或基于镜像的序列化。 - Robert
镜像在dart2js中受支持吗？上次我检查时，有人评论说它在dart2js中没有完全实现，这意味着我无法在前端中使用它。 - Jan Vladimir Mostert
还可以参考这个链接：https://dev59.com/smIj5IYBdhLWcg3wpmsH - Robert
我看到过那篇文章，但是那是在2013年发布的，已经一年多了，所以我再次提出这个问题，希望早已得到解决。 - Jan Vladimir Mostert
1个回答
2
我认为你的问题在于使用了print(req.toString());。你尝试过这样做吗： objectToJson(req).then((jsonStr) => print(jsonStr));也许这会给你一个json字符串。
我个人认为你实际上应该提供一个toJson/toObject方法，因为你完全控制序列化的内容和方式（例如，排除私有字段、空值等）。
// EDIT
由于您使用了extends JsonObjecttoString()返回了一个空映射。
// EDIT https://code.google.com/p/dart/issues/detail?id=6490看起来并不被很好地支持。但是，您无论如何都不能使用json_object包。
异常：Uncaught Error: Class '_LocalClassMirror'没有实例getter 'getters'。这应该是因为该软件包需要实验性镜像功能。
使用JSON.encode，您不能将一个对象放入其中。请参见https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/dart:convert.JsonCodec#id_encode：
要么您指定第二个参数，要么实现.toJson()。
我已经尝试了更新后问题中显示的方法，但仍然输出{}。 - Jan Vladimir Mostert
请尝试移除 extends JsonObject。 - Robert
这样做会抛出一个堆栈跟踪，我已经更新了我的问题，并包含了堆栈跟踪。 - Jan Vladimir Mostert
实现toJson似乎是最简洁的解决方案，尽管它需要一些模板代码。我已经更新了我的问题，并提供了可工作的toJson实现，谢谢Robert！ - Jan Vladimir Mostert
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