无法从非 - @nullable @provides方法错误返回null

Question

i有一个名为viewModelModule的模块。就是这样：

@Module(includes = RepositoryModule.class)
public class ViewModelModule {
    StepListFragment stepListFragment;
    StepViewFragment stepViewFragment;
    @Provides
    public StepListFragment getStepListFragment() {
        return stepListFragment;
    }
    @Provides
    public StepViewFragment getStepViewFragment() {
        return stepViewFragment;
    }
    public ViewModelModule(StepListFragment stepListFragment) {
        this.stepListFragment = stepListFragment;
    }
    public ViewModelModule(StepViewFragment stepViewFragment) {
        this.stepViewFragment = stepViewFragment;
    }
    @Provides
    IngredientsViewModel ingredientsViewModel(StepListFragment stepListFragment, RecipesRepository repository) {
        return ViewModelProviders.of(stepListFragment, new ViewModelProvider.Factory() {
            @Override
            public <T extends ViewModel> T create(Class<T> modelClass) {
                return (T) new IngredientsViewModel(repository);
            }
        }).get(IngredientsViewModel.class);
    }
    @Provides
    StepsViewModel stepsViewModel(StepViewFragment stepViewFragment, RecipesRepository repository) {
        return ViewModelProviders.of(stepViewFragment, new ViewModelProvider.Factory() {
            @Override
            public <T extends ViewModel> T create(Class<T> modelClass) {
                return (T) new StepsViewModel(repository);
            }
        }).get(StepsViewModel.class);
    }
}

将提供我所有的ViewModules组件。但不是同时。

我有一个针对每个片段的组件：

@Component(modules = {RepositoryModule.class, ContextModule.class, ViewModelModule.class})
public interface StepListComponent {
    void inject (StepListFragment stepListFragment);
}

@Component(modules = {RepositoryModule.class, ContextModule.class, ViewModelModule.class})
public interface StepViewComponent {
    void inject (StepViewFragment stepViewFragment);
}

在第一个时刻显示了阶段段，我将组件实例化如下：

DaggerStepListComponent.builder()
        .applicationModule(new ApplicationModule(this.getActivity().getApplication()))
        .contextModule(new ContextModule(this.getActivity()))
        .viewModelModule(new ViewModelModule(this)).build().inject(this);

正如您在条款的末尾看到的那样，我注入了片段。

之后，当我启动另一个片段时，我会做同样的事情。但是当它调用上面的代码时，我收到此错误：

Caused by: java.lang.NullPointerException: Cannot return null from a non-@Nullable @Provides method

当然，错误原因是我尚未实例化StepViewFragment stepViewFragment；但是我现在不想使用它，所以不会引起问题。

我尝试添加@Nullable子句如下：

@Nullable
@Provides
public StepListFragment getStepListFragment() {
    return stepListFragment;
}
@Nullable
@Provides
public StepViewFragment getStepViewFragment() {
    return stepViewFragment;
}

，但是我得到了一个编译时间错误：

    Error:(15, 10) error: StepListFragment is not nullable, but is being provided by @android.support.annotation.Nullable @Provides 
com.github.alexpfx.udacity.nanodegree.android.baking_app.step.master.steps.StepListFragment com.github.alexpfx.udacity.nanodegree.android.baking_app.step.master.di.ViewModelModule.getStepListFragment()
    at:     com.github.alexpfx.udacity.nanodegree.android.baking_app.step.master.steps.StepListFragment is injected at
    com.github.alexpfx.udacity.nanodegree.android.baking_app.step.master.di.ViewModelModule.ingredientsViewModel(stepListFragment, …)
    com.github.alexpfx.udacity.nanodegree.android.baking_app.step.master.ingredients.IngredientsViewModel is injected at
    com.github.alexpfx.udacity.nanodegree.android.baking_app.step.master.steps.StepListFragment.ingredientsViewModel
    com.github.alexpfx.udacity.nanodegree.android.baking_app.step.master.steps.StepListFragment is injected at
    com.github.alexpfx.udacity.nanodegree.android.baking_app.step.master.di.StepListComponent.inject(stepListFragment)

因此，问题是：是否有一种方法可以通过使用相同的模块保持配置来解决此问题，还是我应该将每个@provides分离到其各自的模块中？这是一个好习惯吗？

Answer 1

片段不应是 ViewModel的依赖项 - ViewModel应该比Fragment的范围更大。

请参阅此GitHub回购，其中包含一个示例项目，该项目将Android architecture components与Dagger2一起使用。