How to mock and verify Future return value function with Mockito Flutter?

Question

I'm trying to mock a function which is having Future<bool> as return with String` paramater. I have defined the following test:

      final Future<bool> Function(String) _handleFilterSelect = MockFunction();

      when(_handleFilterSelect.call('/view/movie_1234'))
          .thenAnswer((_) => Future.value(true));
      FilterList filterList = new FilterList(
        '/view/movie',
        onFilterSelect: _handleFilterSelect,
      );
      await widgetTester.pumpWidget(MaterialApp(
        localizationsDelegates: delegate,
        home: Scaffold(
          body: filterList,
        ),
        theme: ThemeUtil().getCurrentTheme()!.defaultThemeData,
      ));
      await widgetTester.pumpAndSettle();
      final filtersFinder = find.byType(ListTile);
      await widgetTester.tap(filtersFinder);
      await widgetTester.pumpAndSettle();
      verify(_handleFilterSelect('/view/movie_1234')).called(1);

## Outside these tests:

class MockFunction extends Mock {
  Future<bool> call(String param);
}

But I'm receiving following error:

The following _TypeError was thrown running a test: type 'Null' is not a subtype of type 'Future'

What is correct way of implementing _handleFilterSelect for Future return values? The test works if the function is non-future function.

Answer 1

Another solution to this problem is as below:

      final Future<bool> Function(String) _handleFilterSelect =
          MockFunction().call;
      when(_handleFilterSelect('/view/movie_1234'))
          .thenAnswer((_) => Future.value(true));
      FilterList filterList = new FilterList(
        '/type/movie',
        onFilterSelect: _handleFilterSelect,
      );
    ....
    # Mock function will look like:
    class MockFunction extends Mock {
      Future<bool> call(String param) {
        return super.noSuchMethod(Invocation.method(#call, [param]),
            returnValue: Future.value(false)) as Future<bool>;
      }
    }

Here we will be able to return Future<bool> values in Mockito.