Dagger 2组件依赖不起作用

Question

The dagger modules

@Module
public class NetModule {

    @Provides
    @Singleton
    Gson provideGson() {
        GsonBuilder gsonBuilder = new GsonBuilder();
        return gsonBuilder.create();
    }

    @Provides
    @Singleton
    OkHttpClient provideOkHttpClient() {
        OkHttpClient client = new OkHttpClient();
        return client;
    }

    @Provides
    @Singleton
    Retrofit provideRetrofit(Gson gson, OkHttpClient okHttpClient) {
        Retrofit retrofit = new Retrofit.Builder()
                .addConverterFactory(GsonConverterFactory.create(gson))
                .baseUrl(BuildConfig.SERVER_BASE_URL)
                .client(okHttpClient)
                .build();
        return retrofit;
    }
}

@Module
public class AppModule{

    private Application application;

    public AppModule(Application application){
        this.application = application;
    }

    @Provides
    @Singleton
    Application provideApplication() {
        return application;
    }
}

@Module
public class ImageModule {

    @Provides
    @Singleton
    Picasso providePicasso(Application application, OkHttpClient client) {
        Picasso.Builder builder = new Picasso.Builder(application);
        builder.downloader(new OkHttp3Downloader(client));
        return builder.build();
    }

}

These are the components

@Singleton
@Component(modules={NetModule.class})
public interface NetComponent {

    void inject(MyFragment fragment);
}

@Singleton
@Component(modules={AppModule.class, ImageModule.class}, dependencies = {NetModule.class})
public interface ImageComponent {

    void inject(MyFragment fragment);
}

This is how I am registering the components

public class MyApp extends Application{

    @Override
    public void onCreate() {

        netComponent = DaggerNetComponent.builder()
                .netModule(new NetModule())
                .build();

        imageComponent = DaggerImageComponent.builder()
                .appModule(new appModule(this))
                .imageModule(new ImageModule())
                .build();

    }

}

And in the fragment

public class MyFragment extends Fragment {

   @Inject
   Retrofit retrofit;

   @Inject
   Picasso picasso;

  @Override
  public View onCreateView(LayoutInflater inflater, ViewGroup container,   
     Bundle savedInstanceState) {

   ((MyApp)getActivity().getApplication()).getNetComponent().inject(this);

   ((MyApp)getActivity().getApplication()).getImageComponent().inject(this);
  ....

  }


}

I am getting compilation error as below

Error:(25, 10) error: com.squareup.picasso.Picasso cannot be provided without an @Inject constructor or from an @Provides- or @Produces-annotated method.

What is the best way to achieve this in dagger 2?

Answer 1

When Component A depends on type B, you're saying that every zero-arg getter on B will be available in A's graph . This means that you don't need to call inject separately on NetComponent, but you do need to expose OkHttpClient in NetComponent's definition.

This means NetComponent would look like this:

@Singleton
@Component(modules={NetModule.class})
public interface NetComponent {
    /** Exposes OkHttpClient. Used by components depending on NetComponent. */
    OkHttpClient getOkHttpClient();
}

And your onCreateView could look like this:

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,   
    Bundle savedInstanceState) {

  ((MyApp)getActivity().getApplication()).getImageComponent().inject(this);

}

Internally, Dagger generates the DaggerNetComponent and DaggerImageComponent in two separate steps, but has ImageComponent call the new getOkHttpClient method . This also means that DaggerImageComponent can accept any implementation of NetComponent, not just the one Dagger generates.

Related resources:

• What is the purpose of the non-inject methods in Components in Dagger 2? (see David Rawson's answer)
• Dagger 2 subcomponents vs component dependencies
• Dagger docs on component dependencies and its link to provision methods