I figured it out how to use your implementation In null-safety and with the code generation of Mockito.

Code gen:

Map<String, bool> getData() {
  return {}; //this will be overriden
}

@GenerateMocks(
  [
    FirebaseFirestore,
    CollectionReference,
    DocumentReference,
    QuerySnapshot,
  ],
  customMocks: [
    MockSpec<QueryDocumentSnapshot>(
        as: #MockQueryDocumentSnapshot, fallbackGenerators: {#data: getData}),
  ],
)

Then in your declaration of your mocked vars you need to tell the compiler what you are actually extending:

late MockFirebaseFirestore firestore;
late MockCollectionReference<Map<String, dynamic>> userCollection;
late MockDocumentReference<Map<String, dynamic>> doc;
late MockCollectionReference<Map<String, dynamic>> notificationCollection;
late MockQuerySnapshot<Map<String, dynamic>> querySnapshot;
late MockQueryDocumentSnapshot<Map<String, dynamic>> queryDocumentSnapshot;
...
setState 
...

Here is a usage of my mocked methods:

when(firestore.collection("users")).thenReturn(userCollection);
when(userCollection.doc("1")).thenReturn(doc);
when(doc.collection("notifications")).thenReturn(notificationCollection);
when(notificationCollection.get()).thenAnswer((_) async => querySnapshot);
when(querySnapshot.docs)
          .thenAnswer((realInvocation) => [queryDocumentSnapshot]);
when(queryDocumentSnapshot.data())
          .thenAnswer((realInvocation) => {"message": true});

To understand why the custom mock is needed read this issue and the readme of the Mockito repo.

Hope that helps anyone.

when(instance.collection("sold")).thenReturn(mockCollectionReferenceSold);
    when(mockCollectionReferenceSold.doc("abc"))
        .thenReturn(mockDocumentReferenceSold);
   
    when(mockDocumentReferenceSold.set({
      'appointmentID': "tAppointmentID",
      'date': DateTime.parse("2020-12-05 20:18:04Z"),
      'description': "test description",
    })).thenAnswer((_) async => mockDocumentSnapshotSold);

    print(mockDocumentSnapshotSold.id);

prints null

any idea?
why data hasnt inserted into firestore?

If what You write is your whole code, then you are refering to mockDocumentSnapshotSold.id, but didn't call mockDocumentReferenceSold

@Babwenbiber could u show in code how and where it shoudl be called? i am not getting you.

I am also stuck in the same as @sath26.
prints null

sath26 commented on Sep 16 •
when(instance.collection("sold")).thenReturn(mockCollectionReferenceSold);
when(mockCollectionReferenceSold.doc("abc"))
.thenReturn(mockDocumentReferenceSold);

when(mockDocumentReferenceSold.set({
  'appointmentID': "tAppointmentID",
  'date': DateTime.parse("2020-12-05 20:18:04Z"),
  'description': "test description",
})).thenAnswer((_) async => mockDocumentSnapshotSold);

print(mockDocumentSnapshotSold.id);

prints null

any idea?
why data hasnt inserted into firestore?

How can we test streams in firestore and model class.
//arrange
when(instance.collection("todos")).thenReturn(mockCollectionReference);
when(mockCollectionReference.doc(any)).thenReturn(mockDocumentReference);
when(mockDocumentReference.collection("todos"))
.thenReturn(mockCollectionReference);
when(mockCollectionReference.snapshots())
.thenReturn(mockQuerySnapshotStream);
when(await mockQuerySnapshotStream.toList())
.thenReturn(mockQuerySnapshotList);
when(mockQuerySnapshotList.map((e) {
final List retVal = [];
for (final DocumentSnapshot doc in e.docs) {
retVal.add(TodoModel.fromDocumentSnapshot(documentSnapshot: doc));
}
return retVal;
}));

  //test
  int i = 0;
  stream.listen(
    expectAsync1((event) {
      expect(event.content, actual[i].content);
      i++;
    }, count: take),
  );