如何将 ViewModelProvider.Factory 注入片段

Question

In short: How can I inject a ViewModelProvider.Factory into my Fragment to get access to the appropriate ViewModel?

I have an activity with tabs managed by a BottomNavigationBar. Each tab screen is represented by a Fragment. Switching between tabs is done by replacing Fragments.

Each TabFragment has an associated ViewModel class. These ViewModel classes take arguments in the constructor, and for this reason I have to create a ViewModelProvider.Factory to use ViewModelProvider.

Naturally, my first instinct would be to inject this Factory into the fragment upon construction, and then retrieve a ViewModel instance via the ViewModelProvider:

class TabA(private val viewModelFactory: ViewModelProvider.Factory) : Fragment() {
    ...
    tabAViewModel = ViewModelProvider(this, viewModelFactory)
            .get(TabAViewmodel::class.java)
    ...
}

Fragments, however, require no-arguments constructors. Instead instantiation can be done through a static factory method and simple arguments can be supplied through a Bundle, but it seems that this Bundle cannot contain complex types such as a ViewModelFactory.

How can I then inject my ViewModelFactory into the Fragment? Is dependency injection of custom types even possible for fragments?

Answer 1

If you want to use a Fragment with constructor arguments you can use FragmentFactory - it is no longer a requirement to have no-arg constructors on fragments.

1. Create the injectable factory:

    class MyFragmentFactory @Inject constructor(private val viewModelFactoryTabA: ViewModelProvider.Factory): FragmentFactory() { override fun instantiate(classLoader: ClassLoader, className: String): Fragment = when (loadFragmentClass(classLoader, className)) { TabA::class.java -> TabA(viewModelFactoryTabA) else -> super.instantiate(classLoader, className) }

   }

2. Set the factory in parent Activity:

    class MyActivity: AppCompatActivity() { @Inject lateinit var fragmentFactory: MyFragmentFactory override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) supportFragmentManager.fragmentFactory = fragmentFactory }

   }

More info in the documentation: https://developer.android.com/reference/androidx/fragment/app/FragmentFactory