A: Dagger.android 2.11 simple example with support for Singleton, PerActivity, PerFragment, and PerChildFragment scopes

Dagger2SimpleExample.java

// This is a super simplified example of how to use the new dagger.android framework
// introduced in Dagger 2.10. For a more complete, in-depth guide to dagger.android
// read https://proandroiddev.com/how-to-android-dagger-2-10-2-11-butterknife-mvp-part-1-eb0f6b970fd

// For a complete codebase using dagger.android 2.11, butterknife 8.7, and MVP,
// see https://github.com/vestrel00/android-dagger-butterknife-mvp

// Note that this example uses Dagger 2.11, where @ContributesAndroidInjector was
// introduced removing the need to define @Subcomponent classes.

// Could also extend DaggerApplication instead of implementing HasActivityInjector
// App.java
public class App extends Application implements HasActivityInjector {
  
    @Inject
    AppDependency appDependency;

    @Inject
    DispatchingAndroidInjector<Activity> activityInjector;

    @Override
    public void onCreate() {
        super.onCreate();
        DaggerAppComponent.create().inject(this);
    }

    @Override
    public AndroidInjector<Activity> activityInjector() {
        return activityInjector;
    }
}

// AppModule.java
@Module(includes = AndroidInjectionModule.class)
abstract class AppModule {
    @PerActivity
    @ContributesAndroidInjector(modules = MainActivityModule.class)
    abstract MainActivity mainActivityInjector();
}

// AppComponent.java
@Singleton
@Component(modules = AppModule.class)
interface AppComponent {
    void inject(App app);
}

// Could also extend DaggerActivity instead of implementing HasFragmentInjector
// MainActivity.java
public final class MainActivity extends Activity implements HasFragmentInjector {

    @Inject
    AppDependency appDependency; // same object from App
  
    @Inject
    ActivityDependency activityDependency;
  
    @Inject
    DispatchingAndroidInjector<Fragment> fragmentInjector;
  
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        AndroidInjection.inject(this);
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main_activity);

        if (savedInstanceState == null) {
            addFragment(R.id.fragment_container, new MainFragment());
        }
    }
  
    @Override
    public final AndroidInjector<Fragment> fragmentInjector() {
        return fragmentInjector;
    }
}

// MainActivityModule.java
@Module
public abstract class MainActivityModule {
    @PerFragment
    @ContributesAndroidInjector(modules = MainFragmentModule.class)
    abstract MainFragment mainFragmentInjector();
}

// Could also extend DaggerFragment instead of implementing HasFragmentInjector
// MainFragment.java
public final class MainFragment extends Fragment implements HasFragmentInjector {

    @Inject
    AppDependency appDependency; // same object from App
  
    @Inject
    ActivityDependency activityDependency; // same object from MainActivity
  
    @Inject
    FragmentDependency fragmentDepency; 

    @Inject
    DispatchingAndroidInjector<Fragment> childFragmentInjector;
  
    @SuppressWarnings("deprecation")
    @Override
    public void onAttach(Activity activity) {
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.M) {
            // Perform injection here before M, L (API 22) and below because onAttach(Context)
            // is not yet available at L.
            AndroidInjection.inject(this);
        }
        super.onAttach(activity);
    }

    @Override
    public void onAttach(Context context) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            // Perform injection here for M (API 23) due to deprecation of onAttach(Activity).
            AndroidInjection.inject(this);
        }
        super.onAttach(context);
    }
  
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container,
                             Bundle savedInstanceState) {
        return inflater.inflate(R.layout.main_fragment, container, false);
    }
  
    @Override
    public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        if (savedInstanceState == null) {
            addChildFragment(R.id.child_fragment_container, new MainChildFragment());
        }
    }
  
    @Override
    public final AndroidInjector<Fragment> fragmentInjector() {
        return childFragmentInjector;
    }
}

// MainFragmentModule.java
@Module
public abstract class MainFragmentModule {
    @PerChildFragment
    @ContributesAndroidInjector(modules = MainChildFragmentModule.class)
    abstract MainChildFragment mainChildFragmentInjector();
}

// MainChildFragment.java
public final class MainChildFragment extends Fragment {
  
    @Inject
    AppDependency appDependency; // same object from App
  
    @Inject
    ActivityDependency activityDependency; // same object from MainActivity
  
    @Inject
    FragmentDependency fragmentDepency; // same object from MainFragment
  
    @Inject
    ChildFragmentDependency childFragmentDepency;
  
    @Override
    public void onAttach(Context context) {
        AndroidInjection.inject(this);
        super.onAttach(context);
    }
  
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container,
                             Bundle savedInstanceState) {
        return inflater.inflate(R.layout.main_child_fragment, container, false);
    }
}

// MainChildFragmentModule.java
@Module
public abstract class MainChildFragmentModule {
}

// PerActivity.java
@Scope
@Retention(RetentionPolicy.RUNTIME)
public @interface PerActivity {
}

// PerFragment.java
@Scope
@Retention(RetentionPolicy.RUNTIME)
public @interface PerFragment {
}

// PerChildFragment.java
@Scope
@Retention(RetentionPolicy.RUNTIME)
public @interface PerChildFragment {
}

// AppDependency.java
@Singleton
public final class AppDependency {
    @Inject
    AppDependency() {
    }
}

// ActivityDependency.java
@PerActivity
public final class ActivityDependency {
    @Inject
    ActivityDependency() {
    }
}

// FragmentDependency.java
@PerFragment
public final class FragmentDependency {
    @Inject
    FragmentDependency() {
    }
} 

// ChildFragmentDependency.java
@PerChildFragment
public final class ChildFragmentDependency {
    @Inject
    ChildFragmentDependency() {
    }
}