CODEWITHSUNDEEP
androiddependency-injectiondagger-2
user1299412
user1299412

Reputation: 399

Dagger 2 - how to inject only to base activity/fragment

I am studying a Dagger 2 from many sources such as this one: http://fernandocejas.com/2015/04/11/tasting-dagger-2-on-android/ but I still haven't found an answer to my question.

I work on quite complex application with tens of fragments and several activities in which I want to use DI (dagger 2). For all of those fragments and activities I have one BaseActivity and one BaseFragment. However, as far as I read and tried, in order to use @Inject in my let's say MainActivity, I have to specify it in Component interface and also invoke getApplicationComponent().inject(this) in onCreate method. When I do this for BaseActivity only, @Inject annotated fields in MainActivity is never injected. And what is even worse, I do not find out about that until that specific part of code is executed and NPE is thrown.

So far it is a deal breaker for me, because this can be source of many crashes. I would need to specify tens of fragments and activities in Component interface and not forget to call inject in each onCreate method.

I would be very glad to hear any solution to this since I would really like to use DI..

code example: 

@Singleton
@Component(modules = ApplicationModule.class)
public interface ApplicationComponent {
    void inject(BaseActivity baseActivity);
    Analytics analytics();
}

public class BaseActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        this.getApplicationComponent().inject(this);
    }
}

public class MainActivity extends BaseActivity {
    @Inject
    Analytics analytics;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        analytics.log("event1"); // THROWS NPE!
    }
}

Upvotes: 4

Views: 4839

Answers (2)

David Medenjak
David Medenjak

Reputation: 34522

You can not inject properties in your subclass by injecting the super (since dagger2 works at compile time and there is no way to dynamically check subclasses for annotated properties.)

You can move analytics up to the super, then it will be injected there. To inject annotated fields in your subclass you will have to call the injection there again.

You can make an abstract method in your baseclass e.g. inject(App app)where you just handle the injection. That way you can't 'miss' it.

As stated in the official documentation:

While a members-injection method for a type will accept instances of its subtypes, only Inject-annotated members of the parameter type and its supertypes will be injected; members of subtypes will not.

Upvotes: 5

Javad Jafari
Javad Jafari

Reputation: 730

move the

@Inject Analytics analytics;

to your BaseActivity class, the Analytics object is initialized in the superclass and is inherited by sub-classes automatically, therefor u wouldn't get null any more. 

 public class MainActivity extends BaseActivity {
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         analytics.log("event1");
     } }

Upvotes: 0

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