Dagger 2 - injecting non Android classes

问题:

I'm implimenting Dagger 2 in my Android app. I have it setup in the following way:

AppComponent.java

@Singleton
@Component(modules = {
  AndroidInjectionModule.class,
  AndroidSupportInjectionModule.class,
  ActivityBuilder.class,
  AppModule.class,
  DataBaseDaoModule.class
})

public interface AppComponent {
  @Component.Builder
  interface Builder {
    @BindsInstance
    Builder application(Application aApplication);

    AppComponent build();
  }

  Application application();
  void inject(MyApplication aApplication);
}

AppInjector.java

ublic class AppInjector {

  public static void init(MyApplication aApplication) {

    //Initialize dagger and inject the aApplication
    DaggerAppComponent.builder().application(aApplication).build().inject(aApplication);

    aApplication.registerActivityLifecycleCallbacks(new Application.ActivityLifecycleCallbacks() {
      @Override
      public void onActivityCreated(Activity aActivity, Bundle aBundle) {
        handleActivity(aActivity);
      }

      @Override
      public void onActivityStarted(Activity aActivity) {
      }

      @Override
      public void onActivityResumed(Activity aActivity) {
      }

      @Override
      public void onActivityPaused(Activity aActivity) {
      }

      @Override
      public void onActivityStopped(Activity aActivity) {
      }

      @Override
      public void onActivitySaveInstanceState(Activity aActivity, Bundle aBundle) {
      }

      @Override
      public void onActivityDestroyed(Activity aActivity) {
      }
    });
  }

  private static void handleActivity(Activity aActivity) {
    if (aActivity instanceof HasActivityInjector) {
      AndroidInjection.inject(aActivity);
      Timber.d("injected Activity");
    }
    if (aActivity instanceof FragmentActivity) {
      ((FragmentActivity) aActivity).getSupportFragmentManager()
        .registerFragmentLifecycleCallbacks(
          new FragmentManager.FragmentLifecycleCallbacks() {
            @Override
            public void onFragmentCreated(FragmentManager fm, Fragment f,
                                          Bundle savedInstanceState) {
              if (f instanceof Injectable) {
                Timber.d("injected Fragment");
                AndroidSupportInjection.inject(f);
              }
            }
          }, true);
    }
  }
}

AppModule.java

Module(includes = ViewModelModule.class)
class AppModule {

  @Singleton
  @Provides
  ApiService providesApiService(OkHttpClient aClient, MyInterceptor aInterceptor) {

    //Build a Retrofit object here
  }

  @Singleton
  @Provides
  OkHttpClient providesOkHTTPClient(MyInterceptor aInterceptor) {
   //Setup OKHTTP here
  }
}

And finally in MyApplication.Java in the onCreate method I just call the AppInjector like so: AppInjector.init(this);

All of this works and anything I put in my AppComponent's moduels, I can inject into Activities, Fragments and ViewModels.

However, I have cases where I would need a utility class, that depends on Application, for contex - and I use the utility class in various places. Or I will have a Manager class, that depends on Application, or needs something from AppModule. However, since I use these classes outside of Activities, Fragments and ViewModels I cannot just inject. How would I provide my utility classes with their dependencies and any other type of class - like a manager class?

My first thought was to create a UtilityComponent and a ManagerCompoent of sorts, however I have no idea how I would get them to work with anything in AppModuel or through my AppComponent.

回答1:

Please don't just use component.inject(myObject) for everything. Always prefer constructor injection or provide it from a module where you can do additional setup steps. .inject(myObject) is intended for Framework components where you don't have access to the constructor.

My first thought was to create a UtilityComponent and a ManagerCompoent of sorts, however I have no idea how I would get them to work with anything in AppModuel or through my AppComponent.

You don't need a separate component for that. See below.

However, since I use these classes outside of Activities, Fragments and ViewModels I cannot just inject.

That has nothing to do with injection. You're talking about scopes, and it sound like your utilities are a @Singleton. Your AppComponent is a @Singleton scoped component, hence it can be used to provide your utils, too.

However, I have cases where I would need a utility class, that depends on Application, for context

If they are part of the @Singleton component, which has access to your Application, they can also be provided anywhere else. No need for more components or anything. Just declare your dependencies and don't overthink it.

---

Just declare your util, annotate it with @Singleton and mark the constructor with @Inject for constructor injection. @Singleton ensures that it will be provided by your AppComponent and can access the Application on which it depends.

@Singleton public class MyUtil {

  private Application application;

  @Inject public MyUtil(Application application) {
    this.application = application;
  }

}

And then you can just inject it in your Activities, Fragments, or even into other Utilities....

@Singleton public class MyUtilWrapper {

  private MyUtil myUtil;

  @Inject public MyUtilWrapper(MyUtil myUtil) {
    this.myUtil = myUtil;
  }

}

And you can inject either or both into your activity or fragment...

@Inject MyUtil myUtil;
@Inject MyUtilWrapper myUtilWrapper;

void onCreate(..) {
  AndroidInjection.inject(this);
}

You do not need any modules, provides methods, or components to provide simple classes. Just make sure to add the right scope!