Flutter test using Mockito - when API for register stub doesn't work

I'm posting this question because this behavior seems very strange to me!
I ain't an expert on mockito, so this may be only my error.

@GenerateMocks([AuthProvider])
void main() {
  test('If there is not a valid session, the data are clean', () async {
    UserDataProvider provider = UserDataProvider();
    MockAuthProvider mock = MockAuthProvider();

    when(mock.isLoggedIn).thenReturn(false);

    await provider.update(MockAuthProvider());
    expect((await provider.joinedCommunities).isEmpty, true);
  });
}

An exception is thrown

package:mockito/src/mock.dart 191:7                      Mock._noSuchMethod
package:mockito/src/mock.dart 185:45                     Mock.noSuchMethod
test/provider/user_data_provider_test.mocks.dart 59:33   MockAuthProvider.isLoggedIn
package:unimeet/providers/user_data_provider.dart 22:22  UserDataProvider.update
test/provider/user_data_provider_test.dart 17:20         main.<fn>

MissingStubError: 'isLoggedIn'
No stub was found which matches the arguments of this method call:
isLoggedIn

Add a stub for this method using Mockito's 'when' API, or generate the MockAuthProvider mock with the @GenerateNiceMocks annotation (see https://pub.dev/documentation/mockito/latest/annotations/MockSpec-class.html).

As you can see I'm trying to register a return value for a simple getter but an exception is thrown.

Let me know and thank you

What is happening, is you are indeed adding a stub for the mentioned isLoggedIn field/getter:

when(mock.isLoggedIn).thenReturn(false);

but in the next line, you are calling provider.update with a brand new instance of your mock. Not the one that you stubbed. An empty one.

The solution is really easy. Simply replace constructing a new instance with your already stubbed instance, like this:

@GenerateMocks([AuthProvider])
void main() {
  test('If there is not a valid session, the data are clean', () async {
    final provider = UserDataProvider();
    final mock = MockAuthProvider();

    when(mock.isLoggedIn).thenReturn(false);

    await provider.update(mock);
    expect((await provider.joinedCommunities).isEmpty, true);
  });
}

I also changed the definitions of the variables from using type on the left-hand side to a final keyword, because type is redundant here - you are already showing the type to the developer that reads the code on the right-hand side. By using only a type on the left side, you mark it as a var, a mutable variable. Use final as a default for your variables, except for cases where you indeed want to mutate it. As a rule of thumb. You still can mutate the object (like add() to a List) in a final. Just not redefine the variable itself.

> If you never intend to change a variable, use final or const, either instead of var or in addition to a type. A final variable can be set only once; a const variable is a compile-time constant. (Const variables are implicitly final.)

• Final and const | Language tour | Dart