Mocking a DateTime parameter input - flutter

Question

Has anyone had issues supplying the any Mockito method to a DateTime mock method parameter? Basically I have this method implemented on the mock class Future<int> testMethod(DateTime datetime).
So when I tried to stub the response on the test scenario through when(mockClass.testMethod(any)).thenAnswer((_) async => 1), this shows always on IntelliJ line editor - error: The argument type 'Null' can't be assigned to the parameter type 'DateTime'. (argument_type_not_assignable).
import 'package:flutter_test/flutter_test.dart';
import 'package:mockito/annotations.dart';
import 'package:mockito/mockito.dart';
abstract class TestClass {
Future<int> testMethod(DateTime dateTime);
}
class MockTestClass extends Mock implements TestClass {}
class InvokeClass {
final TestClass testClass;
InvokeClass(this.testClass);
}
// #GenerateMocks([InvokeClass, TestClass])
void main() {
TestClass testClass = MockTestClass();
InvokeClass invokeClass;
setUp(() {
testClass = MockTestClass();
invokeClass = InvokeClass(testClass);
});
test('', () async {
when(testClass.testMethod(any)).thenAnswer((_) => 1);
});
}

Answer 1

Have you tried making the parameter Nullable so that the method becomes
testMethod(DateTime? datetime)

Answer 2

The easiest way is to use the code generator from mockito 5.0.0: https://pub.dev/packages/mockito#lets-create-mocks
basically you would use:
abstract class MyClass {
Future<int> testMethod(DateTime dateTime);
}
#GenerateMocks([MyClass])
void main() {
final mock = MockMyClass();
when(mock.testMethod(any)).....
}
the generator will create a method which takes a nullable DateTime, so any will work. see the mockito documentation for more details.
Update: giving your example it should look like:
import 'package:flutter_test/flutter_test.dart';
import 'package:mockito/annotations.dart';
import 'package:mockito/mockito.dart';
import 'testclass.mocks.dart'; // this will be generated by `build_runner build`
abstract class TestClass {
Future<int> testMethod(DateTime dateTime);
}
class InvokeClass {
final TestClass testClass;
InvokeClass(this.testClass);
}
#GenerateMocks([TestClass])
void main() {
MockTestClass testClass = MockTestClass();
InvokeClass invokeClass;
setUp(() {
testClass = MockTestClass();
invokeClass = InvokeClass(testClass);
});
test('', () async {
when(testClass.testMethod(any)).thenAnswer((_) => 1);
});
}
after those changes run flutter run build_runner build or dart run build_runner build.