When testing the return value of method A, we want to mock the return value of method B in method A.

//sample.dart
US>class Sample {
  inta(){
    return b()+4;
  }

  intb(){
    return3;
    // Usually seven.
    // If it's mocking test.
  }
}

//sample_test.dart

import 'package: sellca_pad_3/models/sample.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:mockito/annotations.dart';
import 'package:mockito/mockito.dart';
import'sample_test.mocks.dart';

@GenerateMocks ([Sample])
void main() {
  test('Test Mock',(){
    final mockSample=MockSample();
    when(mockSample.b()) .thenReturn(5);
    final sample=Sample();
    final addResult=sample.a();
    expect(addResult, 9);
  });
}