Allow extra non-serializable fields to be passed into fromJson and ignored in toJson

json_annotation/lib/src/json_key.dart

@@ -112,6 +112,15 @@ class JsonKey {
   /// valid on a nullable enum field.
   final Enum? unknownEnumValue;
 
+  /// `true` if this field should not be serialized.
+  ///
+  /// Fields annotated with [extra] set to true will not be included in JSON
+  /// output, and their values may be passed in as additional arguments
+  /// alongside JSON data in the deserialization function.
+  ///
+  /// If `null` (the default) or false, this argument has no effect.
+  final bool? extra;
+
   /// Creates a new [JsonKey] instance.
   ///
   /// Only required when the default behavior is not desired.
@@ -127,6 +136,7 @@ class JsonKey {
     this.required,
     this.toJson,
     this.unknownEnumValue,
+    this.extra,
   });
 
   /// Sentinel value for use with [unknownEnumValue].

json_serializable/lib/src/decode_helper.dart

@@ -25,6 +25,7 @@ abstract class DecodeHelper implements HelperCore {
   CreateFactoryResult createFactory(
     Map<String, FieldElement> accessibleFields,
     Map<String, String> unavailableReasons,
+    List<FieldElement> extras,
   ) {
     assert(config.createFactory);
     final buffer = StringBuffer();
@@ -47,6 +48,16 @@ abstract class DecodeHelper implements HelperCore {
       }
     }
 
+    if (extras.isNotEmpty) {
+      buffer.writeln(', {');
+      for (final extra in extras) {
+        if (!extra.type.isNullableType) buffer.write('required ');
+        buffer.writeln(
+            '${extra.type.getDisplayString(withNullability: true)} ${extra.name},');
+      }
+      buffer.write('}');
+    }
+
     buffer.write(')');
 
     final fromJsonLines = <String>[];
@@ -56,15 +67,17 @@ abstract class DecodeHelper implements HelperCore {
         _deserializeForField(accessibleFields[paramOrFieldName]!,
             ctorParam: ctorParam);
 
+    final extraNames = [for (final extra in extras) extra.name];
     final data = _writeConstructorInvocation(
       element,
       config.constructor,
-      accessibleFields.keys,
+      [...accessibleFields.keys, ...extraNames],
       accessibleFields.values
           .where((fe) => element.lookUpSetter(fe.name, element.library) != null)
           .map((fe) => fe.name)
           .toList(),
       unavailableReasons,
+      extraNames,
       deserializeFun,
     );
 
@@ -265,6 +278,7 @@ _ConstructorData _writeConstructorInvocation(
   Iterable<String> availableConstructorParameters,
   Iterable<String> writableFields,
   Map<String, String> unavailableReasons,
+  List<String> extras,
   String Function(String paramOrFieldName, {ParameterElement ctorParam})
       deserializeForField,
 ) {
@@ -301,7 +315,7 @@ _ConstructorData _writeConstructorInvocation(
     } else {
       constructorArguments.add(arg);
     }
-    usedCtorParamsAndFields.add(arg.name);
+    if (!extras.contains(arg.name)) usedCtorParamsAndFields.add(arg.name);
   }
 
   // fields that aren't already set by the constructor and that aren't final
@@ -320,17 +334,19 @@ _ConstructorData _writeConstructorInvocation(
     buffer
       ..writeln()
       ..writeAll(constructorArguments.map((paramElement) {
-        final content =
-            deserializeForField(paramElement.name, ctorParam: paramElement);
+        final content = extras.contains(paramElement.name)
+            ? paramElement.name
+            : deserializeForField(paramElement.name, ctorParam: paramElement);
         return '      $content,\n';
       }));
   }
   if (namedConstructorArguments.isNotEmpty) {
     buffer
       ..writeln()
       ..writeAll(namedConstructorArguments.map((paramElement) {
-        final value =
-            deserializeForField(paramElement.name, ctorParam: paramElement);
+        final value = extras.contains(paramElement.name)
+            ? paramElement.name
+            : deserializeForField(paramElement.name, ctorParam: paramElement);
         return '      ${paramElement.name}: $value,\n';
       }));
   }

json_serializable/lib/src/generator_helper.dart

@@ -58,6 +58,8 @@ class GeneratorHelper extends HelperCore with EncodeHelper, DecodeHelper {
     // these fields.
     final unavailableReasons = <String, String>{};
 
+    final extras = <FieldElement>[];
+
     final accessibleFields = sortedFields.fold<Map<String, FieldElement>>(
       <String, FieldElement>{},
       (map, field) {
@@ -68,21 +70,26 @@ class GeneratorHelper extends HelperCore with EncodeHelper, DecodeHelper {
           unavailableReasons[field.name] =
               'Setter-only properties are not supported.';
           log.warning('Setters are ignored: ${element.name}.${field.name}');
-        } else if (jsonKeyFor(field).ignore) {
-          unavailableReasons[field.name] =
-              'It is assigned to an ignored field.';
         } else {
-          assert(!map.containsKey(field.name));
-          map[field.name] = field;
+          final jsonKey = jsonKeyFor(field);
+          if (jsonKey.ignore) {
+            unavailableReasons[field.name] =
+                'It is assigned to an ignored field.';
+          } else if (jsonKey.extra) {
+            extras.add(field);
+          } else {
+            assert(!map.containsKey(field.name));
+            map[field.name] = field;
+          }
         }
-
         return map;
       },
     );
 
     var accessibleFieldSet = accessibleFields.values.toSet();
     if (config.createFactory) {
-      final createResult = createFactory(accessibleFields, unavailableReasons);
+      final createResult =
+          createFactory(accessibleFields, unavailableReasons, extras);
       yield createResult.output;
 
       accessibleFieldSet = accessibleFields.entries

json_serializable/lib/src/json_key_utils.dart

@@ -227,6 +227,7 @@ KeyConfig _from(FieldElement element, ClassConfig classAnnotation) {
     readValueFunctionName: readValueFunctionName,
     required: obj.read('required').literalValue as bool?,
     unknownEnumValue: _annotationValue('unknownEnumValue', mustBeEnum: true),
+    extra: obj.read('extra').literalValue as bool?,
   );
 }
 
@@ -241,6 +242,7 @@ KeyConfig _populateJsonKey(
   String? readValueFunctionName,
   bool? required,
   String? unknownEnumValue,
+  bool? extra,
 }) {
   if (disallowNullValue == true) {
     if (includeIfNull == true) {
@@ -261,6 +263,7 @@ KeyConfig _populateJsonKey(
     readValueFunctionName: readValueFunctionName,
     required: required ?? false,
     unknownEnumValue: unknownEnumValue,
+    extra: extra ?? false,
   );
 }
 

json_serializable/lib/src/type_helpers/config_types.dart

@@ -23,6 +23,8 @@ class KeyConfig {
 
   final String? readValueFunctionName;
 
+  final bool extra;
+
   KeyConfig({
     required this.defaultValue,
     required this.disallowNullValue,
@@ -32,6 +34,7 @@ class KeyConfig {
     required this.readValueFunctionName,
     required this.required,
     required this.unknownEnumValue,
+    required this.extra,
   });
 }