Support zero argument function in custom mapping

packages/auto_mappr/README.md

@@ -280,9 +280,11 @@ class Mappr extends $Mappr {}
 When you need to assign a custom function or a const value as a value for given target field,
 you can use the `custom` argument in a `Field` mapping.
 Alternatively, you can use the `Field.custom()` constructor
-which hides other then-invalid parameters.
+which hides other then-invalid parameters. Provide `const Target` value or custom mapping function.
 
-You can set up `Target Function(Source dto)` function or `const Target` value.
+Custom function has to follow one of these formals:
+-  has `Source` model argument -  `Target Function(Source dto)`
+-  has exactly zero arguments and returns `Target`
 
 ```dart
 @AutoMappr([
@@ -298,7 +300,8 @@ class Mappr extends $Mappr {
   static String mapName(UserDto dto) => dto.name.toUpperCase();
 }
 
-int mapAge(UserDto _) => 42;
+/// Return always 42
+int mapAge() => 42;
 ```
 
 ### Ignore mapping

packages/auto_mappr/example/lib/mappr.dart

@@ -1,11 +1,23 @@
 import 'package:auto_mappr_annotation/auto_mappr_annotation.dart';
-
 import 'package:auto_mappr_example/mappr.auto_mappr.dart';
 
+class Utils {
+  static DateTime mapDateTime() {
+    return DateTime.now();
+  }
+
+  static String? mapSpecial(UserDto model) => '${model.xname}?!';
+}
+
 @AutoMappr([
   MapType<UserDto, User>(
+    fields: [Field('name', from: 'xname'), Field('born', custom: Utils.mapDateTime)],
+  ),
+  MapType<UserDto, AnotherUser>(
     fields: [
       Field('name', from: 'xname'),
+      Field('born', custom: Utils.mapDateTime),
+      Field('special', custom: Utils.mapSpecial)
     ],
   ),
 ])
@@ -14,10 +26,26 @@ class Mappr extends $Mappr {}
 class User {
   final int id;
   final String name;
+  final DateTime born;
 
   const User({
     required this.id,
     required this.name,
+    required this.born,
+  });
+}
+
+class AnotherUser {
+  final int id;
+  final String name;
+  final DateTime born;
+  final String? special;
+
+  AnotherUser({
+    required this.id,
+    required this.name,
+    required this.born,
+    this.special,
   });
 }
 

packages/auto_mappr/lib/src/extensions/dart_object_extension.dart

@@ -20,7 +20,7 @@ extension DartObjectExtension on DartObject {
     final asFunction = toFunctionValue();
     if (asFunction != null) {
       return EmitterHelper.current.refer(asFunction.referCallString, asFunction.library.identifier).call([
-        if (passModelArgument) refer('model'),
+        if (passModelArgument && asFunction.parameters.isNotEmpty) refer('model'),
       ]);
     }
 

packages/auto_mappr/test/integration/custom_mapping_test.dart

@@ -81,19 +81,20 @@ void main() {
       expect(
         converted,
         equals(
-          const fixture.CustomFunctionFromEmpty(
+          fixture.CustomFunctionFromEmpty(
             1.2,
             2,
             74.58,
             'some test',
             true,
-            [null, true, 3, 8.6],
-            [
+            const [null, true, 3, 8.6],
+            const [
               [null, 'xx'],
               [true, 3, 8.6],
             ],
-            {1, 2, 3, 4, 5},
-            {'one': 11, 'two': 22, 'three': 33},
+            const {1, 2, 3, 4, 5},
+            const {'one': 11, 'two': 22, 'three': 33},
+            DateTime(2023),
           ),
         ),
       );

packages/auto_mappr/test/integration/fixture/custom_mapping.dart

@@ -59,6 +59,7 @@ import 'custom_mapping.auto_mappr.dart';
       Field.custom('listListValue', custom: Mappr.emptyToListListValue),
       Field.custom('setValue', custom: Mappr.emptyToSetValue),
       Field.custom('mapValue', custom: Mappr.emptyToMapValue),
+      Field.custom('dateValue', custom: Mappr.dateTimeFixed),
     ],
   ),
   // from value
@@ -107,6 +108,8 @@ class Mappr extends $Mappr {
   static String convertToNameAndIdPositional(CustomFunctionPositionalDto? dto) => '${dto?.name} #${dto?.id}';
 
   static String convertToNameAndIdNamed(CustomFunctionNamedDto? dto) => '${dto?.name} #${dto?.id}';
+
+  static DateTime dateTimeFixed() => DateTime(2023);
 }
 
 // custom type
@@ -203,6 +206,7 @@ class CustomFunctionFromEmpty extends Equatable {
   final List<List<Object?>> listListValue;
   final Set<int> setValue;
   final Map<String, int> mapValue;
+  final DateTime dateValue;
 
   @override
   List<Object?> get props => [
@@ -215,6 +219,7 @@ class CustomFunctionFromEmpty extends Equatable {
         listListValue,
         setValue,
         mapValue,
+        dateValue,
       ];
 
   const CustomFunctionFromEmpty(
@@ -227,6 +232,7 @@ class CustomFunctionFromEmpty extends Equatable {
     this.listListValue,
     this.setValue,
     this.mapValue,
+    this.dateValue,
   );
 }