Bad state: Mock method was not called within `when()`. Was a real method called?

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This issue may happen when you implement a method you want to mock instead of letting Mockito do that.

This code below will return Bad state: Mock method was not called within when(). Was a real method called?:

class MockFirebaseAuth extends Mock implements FirebaseAuth {
  FirebaseUser _currentUser;

  MockFirebaseAuth(this._currentUser);

  // This method causes the issue.
  Future<FirebaseUser> currentUser() async {
    return _currentUser;
  }
}

final user = MockFirebaseUser();
final mockFirebaseAuth = MockFirebaseAuth(user);

// Will throw `Bad state: Mock method was not called within `when()`. Was a real method called?`
when(mockFirebaseAuth.currentUser())
    .thenAnswer((_) => Future.value(user));

What do you want instead is:

class MockFirebaseAuth extends Mock implements FirebaseAuth {}

final user = MockFirebaseUser();
final mockFirebaseAuth = MockFirebaseAuth();

// Will work as expected
when(mockFirebaseAuth.currentUser())
    .thenAnswer((_) => Future.value(user));

Also this issue happens when you try to call when() on a non-mock sublass:

class MyClass {
  String doSomething() {
    return 'test';
  }
}

final myClassInstance = MyClass();

// Will throw `Bad state: Mock method was not called within `when()`. Was a real method called?`
when(myClassInstance.doSomething())
    .thenReturn((_) => 'mockedValue');

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