Presenter injection with Dagger 2

I just started using Dagger 2 and I found online thousands guides each one with a different implementation and I'm a bit confused now. So basically this is what I wrote at the moment:

AppModule.java:

@Module
public class AppModule {

 Application mApplication;

 public AppModule(Application application) {
    mApplication = application;
 }

 @Provides
 @Singleton
 Application providesApplication() {
    return mApplication;
 }
}

DataModule.java:

@Module
public class DataModule {

private static final String BASE_URL = "http://beta.fridgewizard.com:9001/api/";

@Provides
@Singleton
NetworkService provideNetworkService() {
    return new NetworkService(BASE_URL);
}

@Provides
@Singleton
SharedPreferences provideSharedPreferences(Application app) {
    return PreferenceManager.getDefaultSharedPreferences(app);
}
}

PrefsModel.java:

@Module(includes = DataModule.class)
public class PrefsModel {

@Provides
@Singleton
QueryPreferences provideQuery(SharedPreferences prefs) {
    return new QueryPreferences(prefs);
}
}

AppComponent.java (I'm exposing QueryPreferences object since I need it in a presenter, hopefully is correct in this way):

@Singleton
@Component(modules = {AppModule.class, DataModule.class, PrefsModel.class})
public interface AppComponent {

    void inject(HomeFragment homeFragment);

    QueryPreferences preferences();
    NetworkService networkService();
}

Then I have the FwApplication.java:

public class FwApplication extends Application {

private static final String TAG = "FwApplication";

private NetworkService mNetworkService;

private AppComponent mDataComponent;

    @Override
    public void onCreate() {
       super.onCreate();

       buildComponentAndInject();
    }

    public static AppComponent component(Context context) {
      return ((FwApplication)   context.getApplicationContext()).mDataComponent;
    }

    public void buildComponentAndInject() {
       mDataComponent = DaggerComponentInitializer.init(this);
    }

    public static final class DaggerComponentInitializer {
      public static AppComponent init(FwApplication app) {
        return DaggerAppComponent.builder()
                .appModule(new AppModule(app))
                .dataModule(new DataModule())
                .build();
    }
   }
}

Finally I added another module for the presenters:

@Module
public class PresenterModule {

   @Provides
   Presenter<FwView> provideHomePresenter(NetworkService networkService) {
      return new HomePresenterImpl(networkService);
   }

   @Provides
   Presenter<FwView> provideSearchPresenter(NetworkService networkService) {
      return new SearchPresenterImpl(networkService);
   }

}

And the following component (which returns error because I cannot add a scoped dependencies here):

@Component(dependencies = AppComponent.class, modules = PresenterModule.class)
public interface PresenterComponent {

    void inject(HomePresenterImpl presenter);
}

So, I have few questions that are not clear for me reading the documentation online:

How can I fix the error in the presenter component since it depends on NetworkService which is a singleton defined in the AppComponent?
I have an HomeFragment which should implement the HomePresenter with "new HomePresenter(networkService)" but now I don't know how to use the DI defined

EDIT - FIX:

HomeFragment.java:

public class HomeFragment extends Fragment {

private static final String TAG = "FW.HomeFragment";


@Inject
HomePresenterImpl mHomePresenter;

public static HomeFragment newInstance() {
    return new HomeFragment();
}

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    FwApplication.component(getActivity()).inject(this);
}

Then I modified the presenter constructor in this way:

@Inject
public HomePresenterImpl(NetworkService networkService) {
    mNetworkService = networkService;
    mInteractor = new InteractorImpl(mNetworkService);
}

Then NetworkService is injected automatically.

I was wondering if it is correct in this way since I have to call for every fragment I have that needs a presenter constructed in the same way as the one above the following code:

FwApplication.component(getActivity()).inject(this);

回答1:

You are mixing thing up. To provide your presenter, you should switch to something like the following:

Use constructor injection if possible. It will make things much easier

public class HomePresenterImpl {

    @Inject
    public HomePresenterImpl(NetworkService networkService) {
        // ...
    }

}

To provide the interface use this constructor injection and depend on the implementation:

Presenter<FwView> provideHomePresenter(HomePresenterImpl homePresenter) {
    return homePresenter;
}

This way you don't have to call any constructors yourself. And to actually inject the presenter...

public class MyFragment extends Fragment {

    @Inject
    Presenter<FwView> mHomePresenter;

    public void onCreate(Bundle xxx) {
        // simplified. Add your modules / Singleton component
        PresenterComponent component = DaggerPresenterComponent.create().inject(this);
    }
}

This way you will inject the things. Please read this carefully and try to understand it. This will fix your major problems, you still can not provide 2 presenters of the same type from the same module (in the same scope)

// DON'T
@Provides
Presenter<FwView> provideHomePresenter(NetworkService networkService) { /**/ }

@Provides
Presenter<FwView> provideSearchPresenter(NetworkService networkService) { /**/ }

This will not work. You can not provide 2 objects of the same kind. They are indistinguishable. Have a look at @Qualifiers like @Named if you are sure this is the way you want to go.

回答2:

You do not have to provide Presenter if @Inject annotation is used in the constructor. @Inject annotation used in the constructor of the class makes that class a part of dependencies graph. So, it also can be injected when needed.

On the other hand, if you add @Inject annotation to fields, but not to constructors, you have to provide that class.