dart - Add JSON serializer to every model class?

Question

When it comes to JSON encoding in Dart, per Seth Ladd's accouncement the finally approved now official way to go is dart:convert + JSON.Encode.

Let's say we have a bunch of model classes (PODOs) such as:

class Customer
{
  int Id;
  String Name;
}

Now, I'd love to be able to just JSON-encode my domain objects like this:

var customer = new Customer()
  ..Id = 17
  ..Name = "John";
var json = JSON.encode(customer);

Unfortunately, this won't work...

Uncaught Error: Converting object to an encodable object failed.
Stack Trace: 
#0      _JsonStringifier.stringifyValue (dart:convert/json.dart:416)
#1      _JsonStringifier.stringify (dart:convert/json.dart:336)
#2      JsonEncoder.convert (dart:convert/json.dart:177)
....

... unless we explicitly tell dart:convert how to encode:

class Customer
{
  int Id;
  String Name;

  Map toJson() { 
    Map map = new Map();
    map["Id"] = Id;
    map["Name"] = Name;
    return map;
  }  
}

Do I really have to add a toJson method to every single one of my model classes, or is there a better way?

EDIT: this is the simple serialization I'm looking for:

{
    "Id": 17,
    "Name": "John"
}

Compare to ToJson in ServiceStack.Text, for instance.

Dart's serialization library (see Matt B's answer below) seems like a step in the right direction. However, this ...

var serialization = new Serialization()
  ..addRuleFor(Customer); 
var json = JSON.encode(serialization.write(customer, format: new SimpleJsonFormat()));

... produces just an array with the values (no keys):

[17,"John"]

Using the default SimpleMapFormat on the other hand generates this complex representation.

Still haven't found what I'm looking for...

EDIT 2: Adding some context: I'm building a RESTful web service in Dart, and I'm looking for a JSON serialization which can easily be consumed by any client, not just another Dart client. For instance, querying the Stack Exchange API for this very question will create this JSON response. This is the serialization format I'm looking for. - Or, look at typical JSON responses returned by the Twitter REST API or the Facebook Graph API.

EDIT 3: I wrote a small blog post about this. See also the discussion on Hacker News.

Answer 1

IMO this is a major short-coming in Dart, surprising given its Web Application focus. I would've thought that having JSON support in the standard libraries would've meant that serializing classes to and from JSON would work like water, unfortunately the JSON support seems incomplete, where it appears the choices are to work with loosely typed maps or suffer through un-necessary boilerplate to configure your standard (PODO) classes to serialize as expected.

Without Reflection and Mirrors support

As popular Dart platforms like Flutter doesn't support Reflection/Mirrors your only option is to use a code-gen solution. The approach we've taken in ServiceStack's native support for Dart and Flutter lets you generate typed Dart models for all your ServiceStack Services from a remote URL, e.g:

$ npm install -g @servicestack/cli

$ dart-ref https://techstacks.io

Supported in .NET Core and any of .NET's popular hosting options.

The example above generates a Typed API for the .NET TechStacks project using the generated DTOs from techstacks.io/types/dart endpoint. This generates models following Dart's JsonCodec pattern where you can customize serialization for your Dart models by providing a fromJson named constructor and a toJson() instance method, here's an example of one of the generated DTOs:

class UserInfo implements IConvertible
{
    String userName;
    String avatarUrl;
    int stacksCount;

    UserInfo({this.userName,this.avatarUrl,this.stacksCount});
    UserInfo.fromJson(Map<String, dynamic> json) { fromMap(json); }

    fromMap(Map<String, dynamic> json) {
        userName = json['userName'];
        avatarUrl = json['avatarUrl'];
        stacksCount = json['stacksCount'];
        return this;
    }

    Map<String, dynamic> toJson() => {
        'userName': userName,
        'avatarUrl': avatarUrl,
        'stacksCount': stacksCount
    };

    TypeContext context = _ctx;
}

With this model you can use Dart's built-in json:convert APIs to serialize and deserialize your model to JSON, e.g:

//Serialization
var dto = new UserInfo(userName:"foo",avatarUrl:profileUrl,stacksCount:10);
String jsonString = json.encode(dto);

//Deserialization
Map<String,dynamic> jsonObj = json.decode(jsonString);
var fromJson = new UserInfo.fromJson(jsonObj);

The benefit of this approach is that it works in all Dart platforms, including Flutter and AngularDart or Dart Web Apps with and without Dart 2’s Strong Mode.

The generated DTOs can also be used with servicestack's Dart package to enable an end to end typed solution which takes care JSON serialization into and out of your typed DTOs, e.g:

var client = new JsonServiceClient("https://www.techstacks.io");
var response = await client.get(new GetUserInfo(userName:"mythz"));

For more info see docs for ServiceStack's native Dart support.

Dart with Mirrors

If you're using Dart in a platform where Mirrors support is available I've found using a Mixin requires the least effort, e.g:

import 'dart:convert';
import 'dart:mirrors';

abstract class Serializable {

  Map toJson() { 
    Map map = new Map();
    InstanceMirror im = reflect(this);
    ClassMirror cm = im.type;
    var decls = cm.declarations.values.where((dm) => dm is VariableMirror);
    decls.forEach((dm) {
      var key = MirrorSystem.getName(dm.simpleName);
      var val = im.getField(dm.simpleName).reflectee;
      map[key] = val;
    });
    
    return map;
  }  

}

Which you can mixin with your PODO classes with:

class Customer extends Object with Serializable
{
  int Id;
  String Name;
}

Which you can now use with JSON.encode:

var c = new Customer()..Id = 1..Name = "Foo";
  
print(JSON.encode(c));

Result:

{"Id":1,"Name":"Foo"}

Note: see caveats with using Mirrors