How to use private constructor in json_serializable - flutter

Question

I'm using a private constructor in my class, but the code generation fails with
The class Foo has no default constructor.
I'm using latest json_serializable: version i.e. 6.1.5:
#JsonSerializable()
class Foo {
final int count;
Foo._(this.count);
factory Foo.fromJson(Map<String, dynamic> json) => _$Foo._FromJson(json);
}
What am I doing wrong?

Answer 1

You can use #JsonSerializable(constructor: '_') which is introduced in the 4.2.0-dev of the JsonSerializable.
This will allow you to field to specify an alternative constructor to invoke when creating a fromJson helper.
For example:
import 'package:json_annotation/json_annotation.dart';
part 'foo.g.dart';
#JsonSerializable(constructor: '_')
class Foo {
final int count;
Foo._(this.count);
factory Foo.fromJson(Map<String, dynamic> json) => _$FooFromJson(json);
}
Now here, instead of using fromJson like this _$Foo._FromJson(json), use it as _$FooFromJson(json)

Answer 2

Since you declared your constructor with _, there is no default constructor for your class. To fix your issue remove the _.
#JsonSerializable()
class Foo {
final int count;
const Foo(this.count);
factory Foo.fromJson(Map<String, dynamic> json) => _$Foo._FromJson(json);
}

Answer 3

if you need to json_serializable: generate is for you, You have to define the default constructor. but you can mack this trick:
first do this and run build_runner:
#JsonSerializable()
class Foo {
final int count;
Foo(this.count);
factory Foo.fromJson(Map<String, dynamic> json) => _$FooFromJson(json);
}
then change it to this:
#JsonSerializable()
class Foo {
final int count;
Foo._(this.count);
factory Foo.fromJson(Map<String, dynamic> json) => _$FooFromJson(json);
}
and go to _.g.dart and mack call the Private constructor:
Foo _$FooFromJson(Map<String, dynamic> json) => Foo._(
json['count'] as int,
);
Map<String, dynamic> _$FooToJson(Foo instance) => <String, dynamic>{
'count': instance.count,
};