Dagger 2 Inject Child Class

Question

I understand that with Dagger 2 I cannot inject in a base class and expect it to carry over to the child class. But why can I not call inject inside of a child class unless I explicitly have an inject method for that class?

I tried following the example in this article: Dagger 2: Even Sharper, Less Square . This solution should allow me to call inject in a subclass, but when I test it I get a NullPointerException for all of my @Inject targets.

public abstract class BaseFragment extends Fragment {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        injectComponent(MyApplication.getComponent());
    }

    protected abstract void injectComponent(AppComponent component);
}

public class MyFragment extends BaseFragment {

    @Inject MyDependency mDependency;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
        mDependency.doSomething(); // NullPointerException
        ...
    }

    ...

    @Override
    public void injectComponent(AppComponent component) {
        component.inject(this);
    }
}

Is there another way to inject into each subclass without creating an inject method for each an every class? Or is that the only way Dagger 2 will work? If that is the case, I will end up with an absurdly long Component class. With an inject method for every Activity , Fragment or various other helper classes:

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

    void inject(MyClass clazz);

    void inject(MyClass2 clazz);

    ...

    void inject(MyClassN clazz);
}

I would much rather have to call component.inject() in every class, than have to do that along with another inject method.

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Although similar, I do not believe my question is a duplicate Dagger 2 Activity Injection Not Working . That question asks why the injection was not working, to which the answer would be: because Dagger 2 has a strong type-association and you must declare an inject method for each and every class. This question focuses more on a way around the boilerplate, while hopefully maintaining strong type-association.

Answer 1

I found a nifty little trick, used in the AsymmetricFingerprintDialog sample application, though I do not know if I fully understand how it works. The sample uses the first iteration of Dagger, but a quick test showed that it works with Dagger 2 as well.

For each Fragment I added an empty constructor with an @Inject annotation, then used an @Inject annotation to get an instance of the Fragment in my Activity .

public class MainActivity extends AppCompatActivity {

    @Inject MyFragment mFragment;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        MyApplication.getComponent().inject(this);
        getSupportFragmentManager().beginTransaction
                .replace(R.id.fragment_container, mFragment).commit();
        ...
    }
}

---

public class MyFragment extends Fragment {

    @Inject MyDependency mDependency;

    @Inject
    public MyFragment() {}

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
        mDependency.doSomething();
        ...
    }
}

This allows me to call MyApplication.getComponent().inject(this) only in each Activity , and the dependencies will be injected to the Fragment classes as well. In this way, I only need an inject method for each Activity , and not each Fragment .

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

    void inject(MainActivity activity);
}

I am curious as to how this works though. From what I can see it has nothing to do with Dagger, instead it seems to use the @Inject annotation for its intended purpose? Though I am new to Dagger as well as JSR-330, so I am unsure.

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Edit: This method only works with setRetainInstanceState(true) , or if you save and restore the instance yourself.