JSON serialization emitDefaults option

protobuf/lib/protobuf.dart

@@ -13,6 +13,7 @@ import 'dart:typed_data' show TypedData, Uint8List, ByteData, Endian;
 import 'package:fixnum/fixnum.dart' show Int64;
 
 import 'src/protobuf/json_parsing_context.dart';
+import 'src/protobuf/json_serialization_context.dart';
 import 'src/protobuf/permissive_compare.dart';
 import 'src/protobuf/type_registry.dart';
 export 'src/protobuf/type_registry.dart' show TypeRegistry;

protobuf/lib/src/protobuf/generated_message.dart

@@ -233,8 +233,9 @@ abstract class GeneratedMessage {
   /// message encoding a type not in [typeRegistry] is encountered, an
   /// error is thrown.
   Object? toProto3Json(
-          {TypeRegistry typeRegistry = const TypeRegistry.empty()}) =>
-      _writeToProto3Json(_fieldSet, typeRegistry);
+          {TypeRegistry typeRegistry = const TypeRegistry.empty(),
+          bool emitDefaults = false}) =>
+      _writeToProto3Json(_fieldSet, typeRegistry, emitDefaults);
 
   /// Merges field values from [json], a JSON object using proto3 encoding.
   ///

protobuf/lib/src/protobuf/json_serialization_context.dart

@@ -0,0 +1,9 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+class JsonSerializationContext {
+  final bool emitDefaults;
+
+  JsonSerializationContext(this.emitDefaults);
+}

protobuf/lib/src/protobuf/proto3_json.dart

@@ -4,7 +4,10 @@
 
 part of protobuf;
 
-Object? _writeToProto3Json(_FieldSet fs, TypeRegistry typeRegistry) {
+Object? _writeToProto3Json(
+    _FieldSet fs, TypeRegistry typeRegistry, bool emitDefaults) {
+  var context = JsonSerializationContext(emitDefaults);
+
   String? convertToMapKey(dynamic key, int keyType) {
     var baseType = PbFieldType._baseType(keyType);
 
@@ -36,8 +39,8 @@ Object? _writeToProto3Json(_FieldSet fs, TypeRegistry typeRegistry) {
     if (fieldValue == null) return null;
 
     if (_isGroupOrMessage(fieldType!)) {
-      return _writeToProto3Json(
-          (fieldValue as GeneratedMessage)._fieldSet, typeRegistry);
+      return _writeToProto3Json((fieldValue as GeneratedMessage)._fieldSet,
+          typeRegistry, context.emitDefaults);
     } else if (_isEnum(fieldType)) {
       return (fieldValue as ProtobufEnum).name;
     } else {
@@ -81,6 +84,14 @@ Object? _writeToProto3Json(_FieldSet fs, TypeRegistry typeRegistry) {
     }
   }
 
+  bool isNullOrEmptyList(dynamic value) {
+    return value == null || (value is List && value.isEmpty);
+  }
+
+  bool isNullOrEmptyMap(dynamic value) {
+    return value == null || (value is Map && value.isEmpty);
+  }
+
   final meta = fs._meta;
   if (meta.toProto3Json != null) {
     return meta.toProto3Json!(fs._message!, typeRegistry);
@@ -89,11 +100,32 @@ Object? _writeToProto3Json(_FieldSet fs, TypeRegistry typeRegistry) {
   var result = <String, dynamic>{};
   for (var fieldInfo in fs._infosSortedByTag) {
     var value = fs._values[fieldInfo.index!];
-    if (value == null || (value is List && value.isEmpty)) {
+    var overrideForEmitsDefaults = false;
+    dynamic overrideForEmitsDefaultsValue;
+    if (context.emitDefaults) {
+      if (fieldInfo.isRepeated && isNullOrEmptyList(value)) {
+        overrideForEmitsDefaults = true;
+        overrideForEmitsDefaultsValue = [];
+      } else if (fieldInfo.isMapField && isNullOrEmptyMap(value)) {
+        overrideForEmitsDefaults = true;
+        overrideForEmitsDefaultsValue = {};
+      } else if (_isBytes(fieldInfo.type) && isNullOrEmptyList(value)) {
+        overrideForEmitsDefaults = true;
+        overrideForEmitsDefaultsValue = null;
+      } else if (_isGroupOrMessage(fieldInfo.type) && value == null) {
+        overrideForEmitsDefaults = true;
+        overrideForEmitsDefaultsValue = null;
+      } else {
+        value ??= fieldInfo.makeDefault!();
+      }
+    }
+    if (isNullOrEmptyList(value) && !overrideForEmitsDefaults) {
       continue; // It's missing, repeated, or an empty byte array.
     }
     dynamic jsonValue;
-    if (fieldInfo.isMapField) {
+    if (overrideForEmitsDefaults) {
+      jsonValue = overrideForEmitsDefaultsValue;
+    } else if (fieldInfo.isMapField) {
       jsonValue = (value as PbMap).map((key, entryValue) {
         var mapEntryInfo = fieldInfo as MapFieldInfo;
         return MapEntry(convertToMapKey(key, mapEntryInfo.keyFieldType!),

protobuf/test/json_test.dart

@@ -115,6 +115,64 @@ void main() {
     final decoded = T()..mergeFromJsonMap(encoded);
     expect(decoded.int64, value);
   });
+
+  test('testToProto3Json', () {
+    var json = jsonEncode(example.toProto3Json());
+    checkProto3JsonMap(jsonDecode(json), 3);
+  });
+
+  test('testToProto3JsonEmitDefaults', () {
+    var json = jsonEncode(example.toProto3Json(emitDefaults: true));
+    checkProto3JsonMap(jsonDecode(json), 6);
+    expect(json.contains('"child":null'), isTrue);
+  });
+
+  test('testToProto3JsonEmitDefaultsNoValues', () {
+    final exampleAllDefaults = T();
+    var json = jsonEncode(exampleAllDefaults.toProto3Json(emitDefaults: true));
+    Map m = jsonDecode(json);
+    expect(m.length, 6);
+  });
+
+  test('testToProto3JsonEmitDefaultsWithChild', () {
+    var child = example;
+
+    var parent = T()
+      ..val = 123
+      ..str = 'hello'
+      ..int32s.addAll(<int>[1, 2, 3])
+      ..child = example;
+    var parentJson = jsonEncode(parent.toProto3Json(emitDefaults: true));
+    var childJson = jsonEncode(child.toProto3Json(emitDefaults: true));
+    checkProto3JsonMap(jsonDecode(parentJson), 6);
+    expect(parentJson.contains(childJson), isTrue);
+  });
+
+  test('testToProto3JsonEmitDefaultsWithNullList', () {
+    var exampleEmptyList = T()
+      ..val = example.val
+      ..str = example.str;
+
+    var json = jsonEncode(exampleEmptyList.toProto3Json(emitDefaults: true));
+    expect(json.contains('"int32s":[]'), isTrue);
+  });
+
+  test('testToProto3JsonEmitDefaultsWithEmptyList', () {
+    var exampleEmptyList = T()
+      ..val = example.val
+      ..str = example.str
+      ..int32s.addAll(<int>[]);
+
+    var json = jsonEncode(exampleEmptyList.toProto3Json(emitDefaults: true));
+    expect(json.contains('"int32s":[]'), isTrue);
+  });
+}
+
+void checkProto3JsonMap(Map m, int expectedLength) {
+  expect(m.length, expectedLength);
+  expect(m['val'], 123);
+  expect(m['str'], 'hello');
+  expect(m['int32s'], [1, 2, 3]);
 }
 
 void checkJsonMap(Map m) {