Question

I’m trying to use a Model-View-Presenter pattern in my Android project. I am using the excellent RoboGuice project with AssistedInject to manage my dependencies. I am struggling with the style of creating instances of my presenters.

AssistedInject appears to require me to first inject a factory to create a presenter and then use that factory to create an instance of a presenter. Injection (appears) to only work at the class scope level; I cannot inject local variables. My methods do not need both a factory and a presenter; I only care about the factory long enough to generate a single presenter. I’d like to avoid keeping the mostly useless factory around.

More Detail

In my implementation of the pattern, I choose for each presenter and view to hold a reference to the other. A presenter must usually be passed one or more “Service” objects that are used to interact with the model. An Android Activity is an implementation of a (MVP) View. An Activity must be the composition root of any Android application. Therefore, each activity must instantiate a presenter, and that presenter needs a service as well as a reference to the view.

In general, presenters look like

public class GreatStuffPresenter {
    private final SomeService service;
    private final GreatStuffView view;

    @Inject
    public GreatStuffPresenter(SomeService service, @Assisted GreatStuffView view) {
        this.service = service;
        this.view = view;
        bind();
    }

    public void bind() {
        Record r = service.getSomeRecord();
        view.setField(r.field);
    }
}

and Activites look like

public class GreatStuffActivity extends RoboActivity {
    @Inject private final GreatStuffPresenterFactory presenterFactory;
    private GreatStuffPresenter presenter;

    @Override
    public void onCreate(...) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.create_update_record);

        presenter = presenterFactory.create(this);
    }
}

Now What?

I am dissatisfied that I must scope the presenterFactory at the instance level; I only need it during onCreate(). Am I missing some additional magic that RoboGuice could perform for me? If not, is there a better practice or pattern I should be using to avoid this unnecessary scoping?

Answer 1

In the end, I decided to get the RoboGuice injector, ask it for an instance of my factory, and create the presenter I’m looking for. I am reasonably happy with this approach; I’m not polluting my class with variables I won’t use, and the line seems reasonably simple.

public class GreatStuffActivity extends RoboActivity {
    private GreatStuffPresenter presenter;

    @Override
    public void onCreate(...) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.create_update_record);

        presenter = RoboGuice.getInjector(this).getInstance(GreatStuffPresenterFactory.class).create(this);
    }
}