Support Function values for JsonKey.default value (google#1216)

Update docs in json_annotation
Prepare to release json_serializable v6.5.0

Closes google#1185

Author: kevmoo

json_annotation/CHANGELOG.md

@@ -1,3 +1,8 @@
+## 4.7.1-dev
+
+- Update `JsonKey` documentation to align with new features in 
+  `package:json_serializable`.
+
 ## 4.7.0
 
 - Added `JsonEnum.valueField` which allows specifying a field in an 

json_annotation/lib/src/json_key.dart

@@ -12,6 +12,10 @@ import 'json_serializable.dart';
 class JsonKey {
   /// The value to use if the source JSON does not contain this key or if the
   /// value is `null`.
+  ///
+  /// Also supported: a top-level or static [Function] or a constructor with no
+  /// required parameters and a return type compatible with the field being
+  /// assigned.
   final Object? defaultValue;
 
   /// If `true`, generated code will throw a [DisallowedNullValueException] if
@@ -30,8 +34,9 @@ class JsonKey {
   /// A [Function] to use when decoding the associated JSON value to the
   /// annotated field.
   ///
-  /// Must be a top-level or static [Function] that takes one argument mapping
-  /// a JSON literal to a value compatible with the type of the annotated field.
+  /// Must be a top-level or static [Function] or a constructor that accepts one
+  /// positional argument mapping a JSON literal to a value compatible with the
+  /// type of the annotated field.
   ///
   /// When creating a class that supports both `toJson` and `fromJson`
   /// (the default), you should also set [toJson] if you set [fromJson].
@@ -94,8 +99,9 @@ class JsonKey {
 
   /// A [Function] to use when encoding the annotated field to JSON.
   ///
-  /// Must be a top-level or static [Function] with one parameter compatible
-  /// with the field being serialized that returns a JSON-compatible value.
+  /// Must be a top-level or static [Function] or a constructor that accepts one
+  /// positional argument compatible with the field being serialized that
+  /// returns a JSON-compatible value.
   ///
   /// When creating a class that supports both `toJson` and `fromJson`
   /// (the default), you should also set [fromJson] if you set [toJson].

json_annotation/pubspec.yaml

@@ -1,5 +1,5 @@
 name: json_annotation
-version: 4.7.0
+version: 4.7.1-dev
 description: >-
   Classes and helper functions that support JSON code generation via the
   `json_serializable` package.

json_serializable/CHANGELOG.md

@@ -1,7 +1,10 @@
-## 6.4.2-dev
+## 6.5.0
 
 - Allow constructors to be passed to `JsonKey` parameters that support
   `Function` types. 
+- Accept `Function` values for `JsonKey.defaultValue`. The provided
+  `Function` will be invoked for the default value if the target JSON element is
+  missing or `null`.
 
 ## 6.4.1
 

json_serializable/lib/src/json_key_utils.dart

@@ -58,7 +58,9 @@ KeyConfig _from(FieldElement element, ClassConfig classAnnotation) {
     } else if (reader.isType) {
       badType = 'Type';
     } else if (dartObject.type is FunctionType) {
-      // TODO: Support calling function for the default value?
+      // Function types at the "root" are already handled. If they occur
+      // here, it's because the function is nested instead of a collection
+      // literal, which is NOT supported!
       badType = 'Function';
     } else if (!reader.isLiteral) {
       badType = dartObject.type!.element2!.name;
@@ -126,11 +128,32 @@ KeyConfig _from(FieldElement element, ClassConfig classAnnotation) {
   /// [fieldName] is not an `enum` value.
   String? createAnnotationValue(String fieldName, {bool mustBeEnum = false}) {
     final annotationValue = obj.read(fieldName);
-    late final DartType annotationType;
 
-    final enumFields = annotationValue.isNull
-        ? null
-        : iterateEnumFields(annotationType = annotationValue.objectValue.type!);
+    if (annotationValue.isNull) {
+      return null;
+    }
+
+    final objectValue = annotationValue.objectValue;
+    final annotationType = objectValue.type!;
+
+    if (annotationType is FunctionType) {
+      // TODO: we could be a LOT more careful here, checking the return type
+      // and the number of parameters. BUT! If any of those things are wrong
+      // the generated code will be invalid, so skipping until we're bored
+      // later
+
+      final functionValue = objectValue.toFunctionValue()!;
+
+      final invokeConst =
+          functionValue is ConstructorElement && functionValue.isConst
+              ? 'const '
+              : '';
+
+      return '$invokeConst${functionValue.qualifiedName}()';
+    }
+
+    final enumFields = iterateEnumFields(annotationType);
+
     if (enumFields != null) {
       if (mustBeEnum) {
         late DartType targetEnumType;
@@ -170,15 +193,12 @@ KeyConfig _from(FieldElement element, ClassConfig classAnnotation) {
       final enumValueNames =
           enumFields.map((p) => p.name).toList(growable: false);
 
-      final enumValueName = enumValueForDartObject<String>(
-          annotationValue.objectValue, enumValueNames, (n) => n);
+      final enumValueName =
+          enumValueForDartObject<String>(objectValue, enumValueNames, (n) => n);
 
-      return '${annotationType.element2!.name}'
-          '.$enumValueName';
+      return '${annotationType.element2!.name}.$enumValueName';
     } else {
-      final defaultValueLiteral = annotationValue.isNull
-          ? null
-          : literalForObject(fieldName, annotationValue.objectValue, []);
+      final defaultValueLiteral = literalForObject(fieldName, objectValue, []);
       if (defaultValueLiteral == null) {
         return null;
       }

json_serializable/pubspec.yaml

@@ -1,5 +1,5 @@
 name: json_serializable
-version: 6.4.2-dev
+version: 6.5.0
 description: >-
   Automatically generate code for converting to and from JSON by annotating
   Dart classes.

json_serializable/test/default_value/default_value.dart

@@ -13,7 +13,8 @@ import 'default_value_interface.dart'
         ConstClass,
         ConstClassConverter,
         constClassFromJson,
-        constClassToJson;
+        constClassToJson,
+        intDefaultValueFunction;
 
 part 'default_value.g.dart';
 
@@ -72,6 +73,15 @@ class DefaultValue implements dvi.DefaultValue {
   @JsonKey(fromJson: constClassFromJson, toJson: constClassToJson)
   ConstClass valueFromFunction;
 
+  @JsonKey(defaultValue: intDefaultValueFunction)
+  int intDefaultValueFromFunction;
+
+  @JsonKey(defaultValue: ConstClass.new)
+  ConstClass valueFromDefaultValueDefaultConstructor;
+
+  @JsonKey(defaultValue: ConstClass.easy)
+  ConstClass valueFromDefaultValueNamedConstructor;
+
   DefaultValue(
     this.fieldBool,
     this.fieldString,
@@ -89,6 +99,9 @@ class DefaultValue implements dvi.DefaultValue {
     this.constClass = const ConstClass('value'),
     this.valueFromConverter = const ConstClass('value'),
     this.valueFromFunction = const ConstClass('value'),
+    required this.intDefaultValueFromFunction,
+    required this.valueFromDefaultValueDefaultConstructor,
+    required this.valueFromDefaultValueNamedConstructor,
   });
 
   factory DefaultValue.fromJson(Map<String, dynamic> json) =>

json_serializable/test/default_value/default_value.g.dart

@@ -13,7 +13,8 @@ import 'default_value_interface.dart'
         ConstClass,
         ConstClassConverter,
         constClassFromJson,
-        constClassToJson;
+        constClassToJson,
+        intDefaultValueFunction;
 
 part 'default_value.g_any_map__checked.g.dart';
 
@@ -75,6 +76,15 @@ class DefaultValue implements dvi.DefaultValue {
   @JsonKey(fromJson: constClassFromJson, toJson: constClassToJson)
   ConstClass valueFromFunction;
 
+  @JsonKey(defaultValue: intDefaultValueFunction)
+  int intDefaultValueFromFunction;
+
+  @JsonKey(defaultValue: ConstClass.new)
+  ConstClass valueFromDefaultValueDefaultConstructor;
+
+  @JsonKey(defaultValue: ConstClass.easy)
+  ConstClass valueFromDefaultValueNamedConstructor;
+
   DefaultValue(
     this.fieldBool,
     this.fieldString,
@@ -92,6 +102,9 @@ class DefaultValue implements dvi.DefaultValue {
     this.constClass = const ConstClass('value'),
     this.valueFromConverter = const ConstClass('value'),
     this.valueFromFunction = const ConstClass('value'),
+    required this.intDefaultValueFromFunction,
+    required this.valueFromDefaultValueDefaultConstructor,
+    required this.valueFromDefaultValueNamedConstructor,
   });
 
   factory DefaultValue.fromJson(Map<String, dynamic> json) =>

json_serializable/test/default_value/default_value.g_any_map__checked.dart

@@ -36,6 +36,12 @@ abstract class DefaultValue {
   ConstClass get valueFromConverter;
 
   ConstClass get valueFromFunction;
+
+  int get intDefaultValueFromFunction;
+
+  ConstClass get valueFromDefaultValueDefaultConstructor;
+
+  ConstClass get valueFromDefaultValueNamedConstructor;
 }
 
 enum Greek { alpha, beta, gamma, delta }
@@ -44,7 +50,9 @@ enum Greek { alpha, beta, gamma, delta }
 class ConstClass {
   final String field;
 
-  const ConstClass(this.field);
+  const ConstClass([this.field = 'default']);
+
+  ConstClass.easy() : field = 'easy';
 
   factory ConstClass.fromJson(Map<String, dynamic> json) => ConstClass(
         json['field'] as String,
@@ -68,3 +76,5 @@ class ConstClassConverter extends JsonConverter<ConstClass, String> {
   @override
   String toJson(ConstClass object) => object.field;
 }
+
+int intDefaultValueFunction() => 43;

json_serializable/test/default_value/default_value.g_any_map__checked.g.dart

@@ -29,6 +29,9 @@ const _defaultInstance = {
   'constClass': {'field': 'value'},
   'valueFromConverter': 'value',
   'valueFromFunction': 'value',
+  'intDefaultValueFromFunction': 43,
+  'valueFromDefaultValueDefaultConstructor': {'field': 'default'},
+  'valueFromDefaultValueNamedConstructor': {'field': 'easy'},
 };
 
 const _otherValues = {
@@ -50,6 +53,9 @@ const _otherValues = {
   'constClass': {'field': 'otherValue'},
   'valueFromConverter': 'otherValue',
   'valueFromFunction': 'otherValue',
+  'intDefaultValueFromFunction': 44,
+  'valueFromDefaultValueDefaultConstructor': {'field': 'other'},
+  'valueFromDefaultValueNamedConstructor': {'field': 'other'},
 };
 
 void main() {

json_serializable/test/default_value/default_value_interface.dart

@@ -46,6 +46,12 @@ class DefaultValueImplicit implements dvi.DefaultValue {
   @JsonKey(fromJson: constClassFromJson, toJson: constClassToJson)
   ConstClass valueFromFunction;
 
+  int intDefaultValueFromFunction;
+
+  ConstClass valueFromDefaultValueDefaultConstructor;
+
+  ConstClass valueFromDefaultValueNamedConstructor;
+
   DefaultValueImplicit({
     this.fieldBool = true,
     this.fieldString = 'string',
@@ -65,6 +71,9 @@ class DefaultValueImplicit implements dvi.DefaultValue {
     this.constClass = const ConstClass('value'),
     this.valueFromConverter = const ConstClass('value'),
     this.valueFromFunction = const ConstClass('value'),
+    this.intDefaultValueFromFunction = 43,
+    this.valueFromDefaultValueDefaultConstructor = const ConstClass(),
+    this.valueFromDefaultValueNamedConstructor = const ConstClass('easy'),
   });
 
   factory DefaultValueImplicit.fromJson(Map<String, dynamic> json) =>

json_serializable/test/default_value/default_value_test.dart

@@ -55,6 +55,7 @@ const _expectedAnnotatedTests = {
   'DefaultWithConstObject',
   'DefaultWithDisallowNullRequiredClass',
   'DefaultWithFunction',
+  'DefaultWithFunctionInList',
   'DefaultWithNestedEnum',
   'DefaultWithSymbol',
   'DefaultWithToJsonClass',

json_serializable/test/default_value/implicit_default_value.dart

@@ -19,10 +19,17 @@ class DefaultWithSymbol {
 
 int _function() => 42;
 
-@ShouldThrow(
-  'Error with `@JsonKey` on the `field` field. '
-  '`defaultValue` is `Function`, it must be a literal.',
-  element: 'field',
+@ShouldGenerate(
+  r'''
+DefaultWithFunction _$DefaultWithFunctionFromJson(Map<String, dynamic> json) =>
+    DefaultWithFunction()..field = json['field'] ?? _function();
+
+Map<String, dynamic> _$DefaultWithFunctionToJson(
+        DefaultWithFunction instance) =>
+    <String, dynamic>{
+      'field': instance.field,
+    };
+''',
 )
 @JsonSerializable()
 class DefaultWithFunction {
@@ -32,6 +39,19 @@ class DefaultWithFunction {
   DefaultWithFunction();
 }
 
+@ShouldThrow(
+  'Error with `@JsonKey` on the `field` field. '
+  '`defaultValue` is `List > Function`, it must be a literal.',
+  element: 'field',
+)
+@JsonSerializable()
+class DefaultWithFunctionInList {
+  @JsonKey(defaultValue: [_function])
+  Object? field;
+
+  DefaultWithFunctionInList();
+}
+
 @ShouldThrow(
   'Error with `@JsonKey` on the `field` field. '
   '`defaultValue` is `Type`, it must be a literal.',