How to unit test the presenter in a mvp-design in flutter when it is dependent on the view?

Question

I want to test the functions of a presenter in an mvp-design in flutter, but I fail to instantiate it for testing because it depends on the view and the view is the state of a stateful widget.

When I give null as view, the function to test fails, bc it tries to call a fuction of the view.

Contract:

abstract class View extends BaseView {

  setSelectedWord(String word);

}

abstract class Presenter {
   toTest(word);
}

Presenter:

class PresenterImpl implements Presenter {
  ///
  View _view;

  @override
  toTest(String word) {
    // do sth.
    _view.setSelectedWord(word);
  }
}

View:

class Screen extends BaseScreen {
  Screen({Key key}) : super(key: key);

  @override
  _State createState() => _State();
}

class _State extends BaseState implementsView {
  ///
  Presenter _presenter;

  setSelectedWord(word){
     //do sth.
   }
}

What is the right way to set up a presenter for testing in this setup?

Answer 1

It is unclear from your sample code exactly how the View instance is being injected into the PresenterImpl , but assuming it is through the constructor you would simply provide a different instance of the View for your unit test, one that does not rely on Flutter.

For example, you could make a MockView class that also extends the View abstract class, this MockView should have no dependencies on Flutter and require no context into the rest of the application.

A simple example could look like:

class MockView extends View {
    String _selectedWord;
    // getter

    setSelectedWord(String word) => _selectedWord = word;
}

presenterTest() {
    String word = "test";
    MockView mockView = MockView();

    PresenterImpl underTest = PresenterImpl(mockView);
    underTest.toTest(word);

    assert(word == mockView.selectedWord);
}

As a side note, testing in this manner is one of the main reasons View and Presenter are abstract, so that we can provide different implementations for testing.