Chain multiple calls with same arguments to return different results

Question

I'm in the process of writing a Flutter app with some extensive unit test coverage.
I'm using Mockito to mock my classes.
Coming from a Java (Android) world where I can use Mockito to chain calls to return different values on subsequent calls.
I would expect this to work.

import 'package:test/test.dart';
import 'package:mockito/mockito.dart';

void main() {
  test("some string test", () {
    StringProvider strProvider = MockStringProvider();
    when(strProvider.randomStr()).thenReturn("hello");
    when(strProvider.randomStr()).thenReturn("world");

    expect(strProvider.randomStr(), "hello");
    expect(strProvider.randomStr(), "world");
  });
}

class StringProvider {
  String randomStr() => "real implementation";
}

class MockStringProvider extends Mock implements StringProvider {}

However it throws:

Expected: 'hello'
Actual:   'world'
  Which: is different.

The only working way I found that works is by keeping track myself.

void main() {
  test("some string test", () {
    StringProvider strProvider = MockStringProvider();

    var invocations = 0;
    when(strProvider.randomStr()).thenAnswer((_) {
      var a = '';
      if (invocations == 0) {
        a = 'hello';
      } else {
        a = 'world';
      }
      invocations++;
      return a;
    });

    expect(strProvider.randomStr(), "hello");
    expect(strProvider.randomStr(), "world");
  });
}

00:01 +1: All tests passed!

Is there a better way?

Solution 1

Use a list and return the answers with removeAt:

import 'package:test/test.dart';
import 'package:mockito/mockito.dart';

void main() {
  test("some string test", () {
    StringProvider strProvider = MockStringProvider();
    var answers = ["hello", "world"];

    when(strProvider.randomStr()).thenAnswer((_) => answers.removeAt(0));

    expect(strProvider.randomStr(), "hello");
    expect(strProvider.randomStr(), "world");
  });
}

class StringProvider {
  String randomStr() => "real implementation";
}

class MockStringProvider extends Mock implements StringProvider {}

Solution 2

You're not forced to call when in the start of the test:

StringProvider strProvider = MockStringProvider();
when(strProvider.randomStr()).thenReturn("hello");
expect(strProvider.randomStr(), "hello");

when(strProvider.randomStr()).thenReturn("world");
expect(strProvider.randomStr(), "world");

Mockito is dart has a different behavior. Subsequent calls override the value.

Comments

[timr]

This seems like a reasonable alternative. Thanks