问题
I am using only dagger2 (not dagger-android) in my project. It's working fine to inject the ViewModel using multibinding. But there's one problem with that previously without dagger2 I was using the same instance of viewmodel used in activity in multiple fragments (using fragment-ktx method activityViewModels()), but now since dagger2 is injecting the view model it's always gives the new instance (checked with hashCode in each fragment) of the viewmodel for each fragment, that's just breaks the communication between fragment using viewmodel.
The fragment & viewmodel code is as below:
class MyFragment: Fragment() {
@Inject lateinit var chartViewModel: ChartViewModel
override fun onAttach(context: Context) {
super.onAttach(context)
(activity?.application as MyApp).appComponent.inject(this)
}
}
//-----ChartViewModel class-----
class ChartViewModel @Inject constructor(private val repository: ChartRepository) : BaseViewModel() {
//live data code...
}
Here's the code for viewmodel dependency injection:
//-----ViewModelKey class-----
@MapKey
@Retention(AnnotationRetention.RUNTIME)
@Target(AnnotationTarget.FUNCTION, AnnotationTarget.PROPERTY_GETTER, AnnotationTarget.PROPERTY_SETTER)
internal annotation class ViewModelKey(val value: KClass<out ViewModel>)
//-----ViewModelFactory class------
@Singleton
@Suppress("UNCHECKED_CAST")
class ViewModelFactory
@Inject constructor(
private val viewModelMap: Map<Class<out ViewModel>, @JvmSuppressWildcards Provider<ViewModel>>
) : ViewModelProvider.Factory {
override fun <T : ViewModel> create(modelClass: Class<T>): T {
val creator = viewModelMap[modelClass] ?: viewModelMap.asIterable()
.firstOrNull { modelClass.isAssignableFrom(it.key) }?.value
?: throw IllegalArgumentException("Unknown ViewModel class $modelClass")
return try {
creator.get() as T
} catch (e: Exception) {
throw RuntimeException(e)
}
}
}
//-----ViewModelModule class-----
@Module
abstract class ViewModelModule {
@Binds
internal abstract fun bindViewModelFactory(factory: ViewModelFactory): ViewModelProvider.Factory
@Binds
@IntoMap
@ViewModelKey(ChartViewModel::class)
abstract fun bindChartViewModel(chartViewModel: ChartViewModel): ViewModel
}
Is there any way to achieve the same instance of viewmodel for multiple fragment and also at the same time inject the view model in fragments. Also is there any need for the bindViewModelFactory method as it seems to have no effect on app even without this method.
One workaround could be to make a BaseFragment for fragments which shares the common viewmodel, but that will again include the boilerplate code and also I am not a great fan of BaseFragment/BaseActivity.
This is generated code for ChartViewModel which always create the newInstance of viewModel:
@SuppressWarnings({
"unchecked",
"rawtypes"
})
public final class ChartViewModel_Factory implements Factory<ChartViewModel> {
private final Provider<ChartRepository> repositoryProvider;
public ChartViewModel_Factory(Provider<ChartRepository> repositoryProvider) {
this.repositoryProvider = repositoryProvider;
}
@Override
public ChartViewModel get() {
return newInstance(repositoryProvider.get());
}
public static ChartViewModel_Factory create(Provider<ChartRepository> repositoryProvider) {
return new ChartViewModel_Factory(repositoryProvider);
}
public static ChartViewModel newInstance(ChartRepository repository) {
return new ChartViewModel(repository);
}
}
回答1:
The problem is that when you inject the viewmodel like this
class MyFragment: Fragment() {
@Inject lateinit var chartViewModel: ChartViewModel
dagger simply creates a new viewmodel instance. There is no viewmodel-fragment-lifecycle magic going on because this viewmodel is not in the viewmodelstore of the activity/fragment and is not being provided by the viewmodelfactory you created. Here, you can think of the viewmodel as any normal class really. As an example:
class MyFragment: Fragment() {
@Inject lateinit var anything: AnyClass
}
class AnyClass @Inject constructor(private val repository: ChartRepository) {
//live data code...
}
Your viewmodel is equivalent to this AnyClass because the viewmodel is not in the viewmodelstore and not scoped to the lifecycle of the fragment/activity.
Is there any way to achieve the same instance of viewmodel for multiple fragment and also at the same time inject the view model in fragments
No. Because of the reasons listed above.
Also is there any need for the bindViewModelFactory method as it seems to have no effect on app even without this method.
It does not have any effect because (I'm assuming that) you are not using the ViewModelFactory anywhere. Since it's not referenced anywhere, this dagger code for the viewmodelfactory is useless.
@Binds
internal abstract fun bindViewModelFactory(factory: ViewModelFactory): ViewModelProvider.Factory
Here's what @binds is doing: 1 2
That's why removing it has no effect on the app.
So what is the solution? You need to inject the factory into the fragment/activity and get the instance of the viewmodel using the factory
class MyFragment: Fragment() {
@Inject lateinit var viewModelFactory: ViewModelFactory
private val vm: ChartViewModel by lazy {
ViewModelProvider(X, YourViewModelFactory).get(ChartViewModel::class.java)
}
What is X here? X is ViewModelStoreOwner. A ViewModelStoreOwner is something that has viewmodels under them. ViewModelStoreOwner is implemented by activity and fragment. So you have a few ways of creating a viewmodel:
- viewmodel in activity
ViewModelProvider(this, YourViewModelFactory)
- viewmodel in fragment
ViewModelProvider(this, YourViewModelFactory)
- viewmodel in fragment (B) scoped to a parent fragment (A) and shared across child fragments under A
ViewModelProvider(requireParentFragment(), YourViewModelFactory)
- viewmodel in fragment scoped to parent activity and shared across fragments under the activity
ViewModelProvider(requireActivity(), YourViewModelFactory)
One workaround could be to make a BaseFragment for fragments which shares the common viewmodel, but that will again include the boilerplate code and also I am not a great fan of BaseFragment/BaseActivity
Yes, this is indeed a bad idea. The solution is to use requireParentFragment() and requireActivity() to get the viewmodel instance. But you'll be writing the same in every fragment/activity that has a viewmodel. To avoid that you can abstract away this ViewModelProvider(x, factory) part in a base fragment/activity class and also inject the factory in the base classes, which will simplify your child fragment/activity code like this:
class MyFragment: Fragment() {
private val vm: ChartViewModel by bindViewModel() // or bindParentFragmentViewModel() or bindActivityViewModel()
回答2:
You can share ViewModel between fragments when instantiating if the fragments has the same parent activity
FragmentOne
class FragmentOne: Fragment() {
private lateinit var viewmodel: SharedViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
viewmodel= activity?.run {
ViewModelProviders.of(this).get(SharedViewModel::class.java)
} : throw Exception("Invalid Activity")
}
}
FragmentTwo
class FragmentTwo: Fragment() {
private lateinit var viewmodel: SharedViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
viewmodel= activity?.run {
ViewModelProviders.of(this).get(SharedViewModel::class.java)
} ?: throw Exception("Invalid Activity")
}
}
回答3:
Add your ViewModel as PostListViewModel inside ViewModelModule:
@Singleton
class ViewModelFactory @Inject constructor(private val viewModels: MutableMap<Class<out ViewModel>, Provider<ViewModel>>) : ViewModelProvider.Factory {
override fun <T : ViewModel> create(modelClass: Class<T>): T = viewModels[modelClass]?.get() as T
}
@Target(AnnotationTarget.FUNCTION, AnnotationTarget.PROPERTY_GETTER, AnnotationTarget.PROPERTY_SETTER)
@kotlin.annotation.Retention(AnnotationRetention.RUNTIME)
@MapKey
internal annotation class ViewModelKey(val value: KClass<out ViewModel>)
@Module
abstract class ViewModelModule {
@Binds
internal abstract fun bindViewModelFactory(factory: ViewModelFactory): ViewModelProvider.Factory
@Binds
@IntoMap
@ViewModelKey(PostListViewModel::class)
internal abstract fun postListViewModel(viewModel: PostListViewModel): ViewModel
//Add more ViewModels here
}
To end with, our activity will have ViewModelProvider.Factory injected and it will be passed to theprivate val viewModel: PostListViewModel by viewModels { viewModelFactory }
class PostListActivity : AppCompatActivity() {
@Inject
lateinit var viewModelFactory: ViewModelProvider.Factory
private val viewModel: PostListViewModel by viewModels { viewModelFactory }
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_post_list)
getAppInjector().inject(this)
viewModel.posts.observe(this, Observer(::updatePosts))
}
//...
}
For more check this post:Inject ViewModel with Dagger2 And Check github
来源：https://stackoverflow.com/questions/61972739/use-the-same-instance-of-view-model-in-multiple-fragments-using-dagger2