在 BaseFragment 中使用 dagger2 实例化视图模型

Question

I am sort of new to Dagger and still learning it. According to the tutorials and blogs I read, currently Android does not have a way of injecting dependencies into ViewModels hence we need to use a custom ViewModelProvider.Factory , that said, I managed to get my hands on this one

public class ViewModelProviderFactory implements ViewModelProvider.Factory {
private static final String TAG = ViewModelProviderFactory.class.getSimpleName();
private final Map<Class<? extends ViewModel>, Provider<ViewModel>> creators;

@Inject
public ViewModelProviderFactory(Map<Class<? extends ViewModel>, Provider<ViewModel>> creators) {
    this.creators = creators;
}

@Override
public <T extends ViewModel> T create(Class<T> modelClass) {
    Provider<? extends ViewModel> creator = creators.get(modelClass);
    if (creator == null) {
        for (Map.Entry<Class<? extends ViewModel>, Provider<ViewModel>> entry : creators.entrySet()) {
            if (modelClass.isAssignableFrom(entry.getKey())) {
                creator = entry.getValue();
                break;
            }
        }
    }

    if (creator == null) {
        throw new IllegalArgumentException("unknown model class " + modelClass);
    }

    try {
        return (T) creator.get();
    } catch (Exception e) {
        throw new RuntimeException(e);
    }
 }
}

It works, it has worked for many of my use cases until now. Originally I had to an instance of a ViewModel with something like this

public AFragment extends BaseFragment{

    @Inject
    ViewModelProviderFactory providerFactory;
    private MyViewModel viewModel;

    MyViewModel getViewModel(){
       return ViewModelProviders.of(this, providerFactory).get(MyViewModel.class);
    }


    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      viewModel = getViewModel();
      tokenAuthenticator.setAuthenticatorListener(this);
    }


 }

But as the project grew I realized this was not neat, I had to do this in all my fragments so I opted for a different approach, I wanted to instantiate my ViewModel in my BaseFragment instead and I did this

public abstract class BaseFragment<T extends BaseViewModel, E extends ViewDataBinding> extends DaggerFragment implements TokenAuthenticator.AuthenticatorListener {
    private static final String TAG = BaseFragment.class.getSimpleName();
    public E binding;
    public final CompositeDisposable disposables = new CompositeDisposable();
    public T viewModel;
    @Inject
    ViewModelProviderFactory providerFactory;
    private int layoutId;


    /**
     * @return view model instance
     */


    public T getViewModel() {
        final Type[] types = ((ParameterizedType) this.getClass().getGenericSuperclass()).getActualTypeArguments();
        return ViewModelProviders.of(this, providerFactory).get((Class<T>)types[0]);
    }

}

This gives me compile error

  A binding with matching key exists in component: xxx.xxx.core.base.dagger.builders.FragmentBuilderModule_ContributeDeliveryPlanFragment.DeliveryPlanFragmentSubcomponent
  .
  .
  .
  A binding with matching key exists in component: xxx.xxx.core.base.dagger.builders.FragmentBuilderModule_ContributePayWithMtmMobileMoneyFragment.PayWithMtmMobileMoneyFragmentSubcomponent
      java.util.Map<java.lang.Class<? extends androidx.lifecycle.ViewModel>,javax.inject.Provider<androidx.lifecycle.ViewModel>> is injected at
         xxx.xxx.core.base.ViewModelProviderFactory(creators)
      xxx.xxx.core.base.ViewModelProviderFactory is injected at
          mika.e.mikaexpressstore.core.base.BaseFragment.providerFactory
      xxx.xxx.xxx.xxx.cashondelivery.CashOnDeliveryFragment is injected at
          dagger.android.AndroidInjector.inject(T) [xxx.xxx.core.base.dagger.component.AppComponent → xxx.xxx.core.base.dagger.builders.FragmentBuilderModule_ContributeCashOnDeliveryFragment.CashOnDeliveryFragmentSubcomponent]
2 errors

From the error message I can tell Dagger is complaining the ViewModelProviderFactory is being injected in the base but used in the child , I need help, is there a way to make this work? surely I want to reduce on boilerplate and repetitive code.

Answer 1

I finally fixed it, not how I wanted but better than having to instantiate each viewmodel from the child class. After reading this answer I came to a realization that this was not possible, so instead I removed @Inject annotation from ViewModelProviderFactory in my BaseFragment and it looked like

public abstract class BaseFragment<T extends BaseViewModel, E extends ViewDataBinding> extends DaggerFragment implements TokenAuthenticator.AuthenticatorListener {
    private static final String TAG = BaseFragment.class.getSimpleName();
    public E binding;
    public final CompositeDisposable disposables = new CompositeDisposable();
    public T viewModel;
    @Inject
    private ViewModelProviderFactory providerFactory;
    private int layoutId;


    /**
     * @return view model instance
     */


    public T getViewModel() {
        final Type[] types = ((ParameterizedType) this.getClass().getGenericSuperclass()).getActualTypeArguments();
        return ViewModelProviders.of(this, providerFactory).get((Class<T>)types[0]);
    }


    @MainThread
    protected final void setProviderFactory(ViewModelProviderFactory providerFactory) {
        this.providerFactory = providerFactory;
    }

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
     super.onCreate(savedInstanceState);
     viewModel = getViewModel();
   }



}

And injected the provider from the child fragments instead then called the setter from the BaseFragment

public AFragment extends BaseFragment{

    @Inject
    ViewModelProviderFactory providerFactory;
    private MyViewModel viewModel;


    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        setLayoutId(R.layout.layout_layout);
        setProviderFactory(providerFactory);
        super.onCreate(savedInstanceState);
    }

}

Key here is to call setProviderFactory(provider) before super.onCreate(savedInstanceState) because when super onCreate is called the provider should not be null, it should be set and ready to create the ViewModel