fix nullable function field

Author: schultek

packages/dart_mappable/test/equality/function_fields_test.dart

@@ -0,0 +1,42 @@
+import 'package:dart_mappable/dart_mappable.dart';
+import 'package:test/test.dart';
+
+part 'function_fields_test.mapper.dart';
+
+@MappableClass(generateMethods: GenerateMethods.copy | GenerateMethods.equals |GenerateMethods.stringify)
+class Example with ExampleMappable {
+  const Example({required this.functionField});
+
+  final int Function(int)? functionField;
+}
+
+void main() {
+  
+  group('function fields', () {
+    
+
+    test('should check equality with function field', () {
+      int fn(int x) => x + 1;
+
+      var example1 = Example(functionField: fn);
+      var example2 = Example(functionField:fn);
+      expect(example1, equals(example2));
+    });
+
+    test('should stringify with function field', () {
+      var example = Example(functionField: (x) => x + 1);
+      expect(example.toString(), equals('Example(functionField: Closure: (int) => int)'));
+    });
+
+    test('should check equality with null function field', () {
+      var example1 = Example(functionField: null);
+      var example2 = Example(functionField: null);
+      expect(example1, equals(example2));
+    });
+
+    test('should stringify with null function field', () {
+      var example = Example(functionField: null);
+      expect(example.toString(), equals('Example(functionField: null)'));
+    });
+  });
+}

packages/dart_mappable/test/equality/function_fields_test.mapper.dart

@@ -50,7 +50,7 @@ class ClassMapperFieldElement extends MapperFieldElement {
   @override
   late final String staticGetterType = () {
     if (resolvedType is FunctionType) {
-      return 'Function';
+      return 'Function${resolvedType.isNullable ? '?' : ''}';
     }
     return parent.parent.prefixedType(resolvedType, resolveBounds: true);
   }();
@@ -63,7 +63,7 @@ class ClassMapperFieldElement extends MapperFieldElement {
   @override
   late final String staticArgType = () {
     if (resolvedType is FunctionType) {
-      return 'Function';
+      return 'Function${resolvedType.isNullable ? '?' : ''}';
     }
     return parent.parent.prefixedType(param?.type ?? resolvedType,
         withNullability: false, resolveBounds: true);

packages/dart_mappable_builder/lib/src/elements/field/class_mapper_field_element.dart

@@ -10,8 +10,8 @@ extension FieldsExtension<T extends InterfaceMapperElement>
       if (f.needsGetter) {
         output.write(
             '  static ${f.staticGetterType} _\$${f.name}(${element.prefixedClassName} v) => ');
-        if (f.staticGetterType == 'Function') {
-          output.write('(v as dynamic).${f.name} as Function;\n');
+        if (f.staticGetterType == 'Function' || f.staticGetterType == 'Function?') {
+          output.write('(v as dynamic).${f.name} as ${f.staticGetterType};\n');
         } else {
           output.write('v.${f.name};\n');
         }