start trying to handle custom key converters to json string

Author: TimWhiting

json_serializable/example/example.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+import 'dart:convert';
+
 import 'package:json_annotation/json_annotation.dart';
 
 part 'example.g.dart';
@@ -15,7 +17,15 @@ class Person {
   /// exist or is empty.
   final DateTime? dateOfBirth;
 
-  Person({required this.firstName, required this.lastName, this.dateOfBirth});
+  /// An example of a custom map like type with non-string key
+  final Person2<int, String> person;
+
+  Person({
+    required this.firstName,
+    required this.lastName,
+    this.dateOfBirth,
+    required this.person,
+  });
 
   /// Connect the generated [_$PersonFromJson] function to the `fromJson`
   /// factory.
@@ -24,3 +34,18 @@ class Person {
   /// Connect the generated [_$PersonToJson] function to the `toJson` method.
   Map<String, dynamic> toJson() => _$PersonToJson(this);
 }
+
+const jsonKeyType = 'jsonKeyType';
+
+class Person2<@jsonKeyType K, V> {
+  Person2({required this.name, required this.id});
+  final V name;
+  final K id;
+
+  factory Person2.fromJson(Map<String, dynamic> json,
+          K Function(Object?) fromJsonK, V Function(Object?) fromJsonV) =>
+      Person2(id: fromJsonK(json), name: fromJsonV(json));
+  Object toJson(Object? Function(K) toJsonK, Object? Function(V) toJsonV) => {
+        toJsonK(id): toJsonV(name),
+      };
+}

json_serializable/example/example.g.dart

@@ -157,7 +157,12 @@ List<String> _helperParams(
     // TODO: throw here if `typeParamIndex` is -1 ?
     final typeArg = type.typeArguments[typeParamIndex];
     final body = execute(typeArg, _helperLambdaParam);
-    args.add('($_helperLambdaParam) => $body');
+    if (helperArg.element.declaration.metadata.any((md) => md.element?.displayName == 'jsonKeyType')){
+      final newBody = _helperParams(execute, paramMapper, type, positionalParams, targetElement);
+      args.add('($_helperLambdaParam) => $newBody /* todo json key */');
+    } else {
+      args.add('($_helperLambdaParam) => $body');
+    }
   }
 
   return args;