I try to learn how to use Dagger 2. Please help with follow exception:
Exception: UninitializedPropertyAccessException: lateinit property trips has not been initialized
MainActivityViewModel:
class MainActivityViewModel : ViewModel() {
private lateinit var tripsLiveData: MutableLiveData<List<Trip>>
@Inject
lateinit var trips : List<Trip>
fun getTrips() : LiveData<List<Trip>> {
if (!::tripsLiveData.isInitialized){
tripsLiveData = MutableLiveData()
tripsLiveData.value = trips
}
return tripsLiveData
}
}
TripModule:
@Module
class TripModule{
@Provides
fun provideTrips(): List<Trip> {
var list = ArrayList<Trip>()
list.add(Trip(100,10))
list.add(Trip(200,20))
return list
}
}
AppComponent:
@Singleton
@Component(modules = [
AndroidSupportInjectionModule::class,
ActivityBuilder::class,
TripModule::class])
interface AppComponent{
@Component.Builder
interface Builder {
@BindsInstance
fun application(application: Application): Builder
fun build(): AppComponent
}
fun inject(app: MyApplication)
}
MainActivity:
class MainActivity : AppCompatActivity() {
@Inject
lateinit var tripsAdapter: TripsAdapter
override fun onCreate(savedInstanceState: Bundle?) {
// Inject external dependencies
AndroidInjection.inject(this)
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
setupRecyclerView();
setUpViewModel();
}
private fun setupRecyclerView() {
recycler_view.apply {
layoutManager = LinearLayoutManager(context)
adapter = tripsAdapter
}
}
private fun setUpViewModel(){
val model = ViewModelProviders.of(this).get(MainActivityViewModel::class.java)
model.getTrips().observe(this, Observer { tripsAdapter.trips = it!! })
}
}
If you want your viewmodel's to be part of the dagger graph, you need to do several things - using dagger's multibindings (just once, for newer viewmodels it will be easier). You'd create new viewmodel factory which will take care of instantiating viewmodels. This factory will be part of dagger graph and therefore will have references to anything provided via dagger. You can then have either constructor injection via @Inject constructor(anyParameterFromDagger: Param)
or @Inject lateinit var someParam: Param
inside the body of viewmodel.
1) Create qualifier for view model classes
@MustBeDocumented
@Target(AnnotationTarget.FUNCTION)
@Retention(AnnotationRetention.RUNTIME)
@MapKey
annotation class ViewModelKey(val value: KClass<out ViewModel>)
2) create viewmodel factory which takes values from dagger's multibindings
@Singleton
class DaggerViewModelFactory @Inject constructor(
private val creators: Map<Class<out ViewModel>, @JvmSuppressWildcards Provider<ViewModel>>
) : ViewModelProvider.Factory {
@Suppress("UNCHECKED_CAST")
override fun <T : ViewModel> create(modelClass: Class<T>): T {
var creator: Provider<out ViewModel>? = creators[modelClass]
if (creator == null) {
for ((key, value) in creators) {
if (modelClass.isAssignableFrom(key)) {
creator = value
break
}
}
}
if (creator == null) {
throw IllegalArgumentException("unknown model class $modelClass")
}
try {
return creator.get() as T
} catch (e: Exception) {
throw RuntimeException(e)
}
}
}
3) have dagger module which will provide the factory (from point 2) and then your viewmodels
abstract class YourDaggerModuleWhichThenNeedToBePartOfYourGraphAsIncluded {
@Binds
abstract fun bindViewModelFactory(factory: DaggerViewModelFactory): ViewModelProvider.Factory // this needs to be only one for whole app (therefore marked as `@Singleton`)
@Binds
@IntoMap
@ViewModelKey(MainActivityViewModel::class)
abstract fun bindMainActivityViewModel(vm: MainActivityViewModel): ViewModel // for every viewmodel you have in your app, you need to bind them to dagger
}
4) in your activity, when you get your viewmodel, you need to use the factory from dagger: (places changed marked as // TODO
in the code below)
class MainActivity : AppCompatActivity() {
@Inject
lateinit var tripsAdapter: TripsAdapter
@Inject
lateinit var viewModelFactory: ViewModelProvider.Factory // TODO this was added to the activity
override fun onCreate(savedInstanceState: Bundle?) {
// Inject external dependencies
AndroidInjection.inject(this)
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
setupRecyclerView();
setUpViewModel();
}
private fun setupRecyclerView() {
recycler_view.apply {
layoutManager = LinearLayoutManager(context)
adapter = tripsAdapter
}
}
private fun setUpViewModel(){
val model = ViewModelProviders.of(this, viewModelFactory)[MainActivityViewModel::class.java] // TODO this was changed
model.getTrips().observe(this, Observer { tripsAdapter.trips = it!! })
}
}
I didn't provide the code for including module to dagger component as I hope this is something you already did.
You can read more about this eg in this medium article (I'm not author of the article):
@Provides fun provideTrips(): List<Trip> {
I'm a bit wary of this in the sense that I doubt that it's really Dagger's job to provide this for you directly, but I'll just take that for granted and ignore that for now.
Your code should be:
class MainActivityViewModel @Inject constructor(
trips: List<Trip>
): ViewModel() {
private val tripsLiveData: MutableLiveData<List<Trip>> = MutableLiveData()
init {
tripsLiveData.setValue(trips)
}
fun getTrips() : LiveData<List<Trip>> = tripsLiveData
}
@Module
class TripModule{
@Provides
// @Singleton // <-- possibly should be here?
fun provideTrips(): List<Trip> = listOf(
Trip(100,10),
Trip(200,20)
)
}
@Singleton
@Component(modules = [
AndroidSupportInjectionModule::class,
ActivityBuilder::class,
TripModule::class
])
interface AppComponent{
@Component.Builder
interface Builder {
@BindsInstance
fun application(application: Application): Builder
fun build(): AppComponent
}
fun inject(app: MyApplication)
}
class MainActivity : AppCompatActivity() {
@Inject
lateinit var viewModelProvider: Provider<MainActivityViewModel>
// this is specifically not marked with `@Inject`
lateinit var viewModel: MainActivityViewModel
private val tripsAdapter = TripsAdapter()
override fun onCreate(savedInstanceState: Bundle?) {
// Inject external dependencies
AndroidInjection.inject(this)
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
setupRecyclerView();
setUpViewModel();
}
private fun setupRecyclerView() {
recycler_view.apply {
layoutManager = LinearLayoutManager(context)
adapter = tripsAdapter
}
}
private fun setUpViewModel(){
viewModel = ViewModelProviders.of(this, object: ViewModelProvider.Factory {
override fun <T : ViewModel?> create(modelClass: Class<T>): T {
if(modelClass == MainActivityViewModel::class.java) {
@Suppress("UNCHECKED_CAST")
return viewModelProvider.get() as T
}
throw IllegalArgumentException("Unexpected argument: $modelClass")
}
}).get(MainActivityViewModel::class.java)
viewModel.getTrips().observe(this, Observer {
val trips = it ?: return@observe
tripsAdapter.trips = trips
})
}
}
Aka you should use constructor injection, use the Provider<T>
to only get the ViewModel from Dagger when you actually need it, and otherwise initialize it via a ViewModelProviders.Factory
so that you actually get the ViewModel from Dagger.
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