Inject singleton inside RecyclerView.Adapter<RecyclerView.ViewHolder>

[Sebastian Riethmüller]

Hi guys,

I'm using dagger 2 in project. I provide an Singleton by a Dagger Modul:

@Module
public class MyModule{

    private MySingleton mMySingleton ;

    @Provides
    @Singleton
    MySingleton provideMySingelton() {
        if (mMySingleton == null) {
            mMySingleton = new MySingleton();
        }
        return mMySingleton ;
    }
}

with this app component interface:

@Singleton
@Component(modules={MyModule.class})
public interface AppComponent {
    void inject(MyFragment myFragment);
    void inject(RecyclerView.Adapter recyclerViewAdapter);
}

The dagger components are building in my application class:

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

    // Dagger%COMPONENT_NAME%
    mAppComponent = DaggerAppComponent.builder()
            // list of modules that are part of this component need to be created here too
            .myModule(new MyModule())
            .build();
}

public static MyApp from(@NonNull Context context) {
    return (MyApp) context.getApplicationContext();
}

I can inject my singleton in MyFragment:

@Inject protected MySingleton mMySingleton;

@Override
public void onCreate(Bundle savedInstanceState) {
    MyApp.from(getContext()).getmAppComponent().inject(this);
}


But I can't inject it inside my RecyclerView.Adapter

public class MyAdapter extends RecyclerView.Adapter<RecyclerView.ViewHolder> {

    @Inject protected MySingleton mMySingleton;

    @Override
    public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {

       MyApp.from(context).getmAppComponent().inject(this);
    }
}

In my MyAdapter is mMySingleton after run line  

    MyApp.from(context).getmAppComponent().inject(this);

always null.

Can somebody tell me why?

I understand it.

Much thanks

[Ron Shapiro]

Your inject method must take the concrete subclass that you want injected. In your case, a MembersInjector<RecyclerView.Adapter> has no idea how to inject into MyAdapter. Change the method to be void inject(MyAdapter adapter); and you should be good.

A couple asides:

• Please don't do state management in your module unless you absolutely need to. Moreover, Dagger does this for you in ScopedProvider (which gets used when you use a scope on a provides method), so just return a new MySingleton from your module method and call it a day (or if you want, give it an @Inject constructor and annotate the class @Singleton).
  
• Your module method can then be static, which generates a leaner component
• Your Component builder doesn't need to take instances of components that have no-argument constructors. If all of your modules are like that, you can even call DaggerMyComponent.create() and Dagger will create any modules it needs by itself.

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