Dagger 2场注入在Android中不起作用

Question

The issue is that I try to use field injection with Dagger 2, but at runtime field, that should be injected, always is null . Also I try to use MVVM pattern. Here is my code:

ProfileActivity.java:

@Override
protected void onStart() {
    super.onStart();
    Log.d(TAG, "ProfileActivity: onStart: ");

    final ProfileViewModel profileViewModel 
         = ViewModelProviders.of(this).get(ProfileViewModel.class);
    profileViewModel.init();
    profileViewModel.getUser().observe(this, new Observer<User>() {
        @Override
        public void onChanged(@Nullable User user) {
            if (user != null) {
                Log.d(TAG, "ProfileActivity: onStart: " + user.toString());
            } else {
                Log.d(TAG, "ProfileActivity: onStart: user == null");
            }
        }
    });
}

ProfileViewModel.java:

public class ProfileViewModel extends ViewModel {
    private LiveData<User> user;

    @Inject
    UserRepository userRepository;

    public ProfileViewModel() {
        Log.d(TAG, "ProfileViewModel: Constructor: ");
    }

    public void init() {
        Log.d(TAG, "ProfileViewModel: init: ");
        user = userRepository.getUser();
    }

    public LiveData<User> getUser() {
        Log.d(TAG, "ProfileViewModel: getUser: ");
        return user;
    }
}

UserRepository.java:

@Singleton
public class UserRepository {
    private LiveData<User> user;

    @Inject
    public UserRepository() {
        Log.d(TAG, "UserRepository: Constructor: ");
    }

    public LiveData<User> getUser() {
        Log.d(TAG, "UserRepository: getUser: ");
        if (user != null) {
            return user;
        } else {
            // There should be userDao.load() call,
            // but it had been omitted for brevity.
            MutableLiveData<User> user = new MutableLiveData<>();
            user.setValue(DB.getUser());
            return user;
        }
    }
}

MyApplication.java:

public class MyApplication extends MultiDexApplication implements HasActivityInjector {
    @Inject
    DispatchingAndroidInjector<Activity> dispatchingAndroidInjector;

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

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

MyApplicationModule.java:

@Module
public abstract class MyApplicationModule {
    @ContributesAndroidInjector
    abstract ProfileActivity contributeActivityInjector();
}

MyApplicationComponent.java:

@Component(modules = { AndroidInjectionModule.class, MyApplicationModule.class})
@Singleton
public interface MyApplicationComponent extends AndroidInjector<MyApplication> {
    void inject(ProfileActivity profileActivity);
}

At runtime I can see the next logs:

ProfileActivity: onStart:
ProfileViewModel: Constructor:
ProfileViewModel: init:

And the app crashes on user = userRepository.getUser(); inside ProfileViewModel 's init() method.

It means that UserRepository had not been injected. Also it is indicated by missing UserRepository: Constructor: log.

Where is my mistake? Thank you.

Answer 1

Basically what you need to do is to use ViewModel Factory to pass injected UserRepository into your ViewModels constructor, initialize it and then you will be able to use it. You cannot use field or parameter injections in ViewModels .

I would suggest you to follow this article: Add the new ViewModel to your MVVM

It provides enough sufficient information to begin using Dagger 2 with Architecture Components.

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Hope it helps.