了解Android架构组件示例GithubBrowserSample：ViewModelModule，ViewModel参数

Question

One of the most up to date samples covering Android Architecture Components is GithubBrowserSample provided by Google. I reviewed the code and a few questions arose:

1. I have noticed that ViewModelModule is included in AppModule . It means that all the viewmodels are added to the DI graph. Why that is done in that way instead of having separate Module for each Activity/Fragment that would provide only needed ViewModel for specific Activity/Fragment?

2. In this specific example, where viewmodels are instantiated using GithubViewModelFactory is there any way to pass a parameter to the specific ViewModel ? Or the better solution would be to create a setter in ViewModel and set needed param via setter?

Answer 1

1. [...] It means that all the viewmodels are added to the DI graph. Why that is done in that way instead of having separate Module for each Activity/Fragment [...]?

They are added to the DI graph, but they are not yet created. Instead they end up in a map of providers, as seen in the ViewModelFacory .

@Inject
public GithubViewModelFactory(Map<Class<? extends ViewModel>, Provider<ViewModel>> creators) { }

So we now have a GithubViewModelFactory that has a list of providers and can create any ViewModel that was bound. Fragments and Activities can now just inject the factory and retrieve their ViewModel.

@Inject
ViewModelProvider.Factory viewModelFactory;

// ...later...
repoViewModel = ViewModelProviders.of(this, viewModelFactory).get(RepoViewModel.class);

As to the why...alternatively you could create a ViewModelProvider.Factory for every Activity / Fragment and register the implementation in every Module. This would be a lot of duplicated boilerplate code, though.

2. In this specific example, where viewmodels are instantiated using GithubViewModelFactory is there any way to pass a parameter to the specific ViewModel? Or the better solution would be to create a setter in ViewModel and set needed param via setter?

It seems like all the ViewModels only depend on @Singleton objects—which is necessary, since they all get provided from the AppComponent. This means that there is no way to pass in "parameters" other than other @Singleton dependencies.

So, as you suggested, you'd either have to move the factory down into the Activity / Fragment component so that you can provide lower-scoped dependencies, or use a setter method.