Android ViewModel & Navigation

The koin-android Gradle module introduces a new viewModel DSL keyword that comes in complement of single and factory, to help declare a ViewModel component and bind it to an Android Component lifecycle. The viewModelOf keyword is also available, to let you declare a ViewModel with its constructor.

val appModule = module {

    // ViewModel for Detail View
    viewModel { DetailViewModel(get(), get()) }

    // or directly with constructor
    viewModelOf(::DetailViewModel)

}

Your declared component must at least extends the android.arch.lifecycle.ViewModel class. You can specify how you inject the constructor of the class and use the get() function to inject dependencies.

info

The viewModel/viewModelOf keyword helps to declare a factory instance of ViewModel. This instance will be handled by internal ViewModelFactory and reattach ViewModel instance if needed. It also will let inject parameters.

Injecting your ViewModel

To inject a ViewModel in an Activity, Fragment or Service use:

• by viewModel() - lazy delegate property to inject a ViewModel into a property
• getViewModel() - directly get the ViewModel instance

class DetailActivity : AppCompatActivity() {

    // Lazy inject ViewModel
    val detailViewModel: DetailViewModel by viewModel()
}

note

ViewModel key is calculated against Key and/or Qualifier

Activity Shared ViewModel

One ViewModel instance can be shared between Fragments and their host Activity.

To inject a shared ViewModel in a Fragment use:

• by activityViewModel() - lazy delegate property to inject shared ViewModel instance into a property
• getActivityViewModel() - directly get the shared ViewModel instance

note

The sharedViewModel is deprecated in favor of activityViewModel() functions. The naming of this last one is more explicit.

Just declare the ViewModel only once:

val weatherAppModule = module {

    // WeatherViewModel declaration for Weather View components
    viewModel { WeatherViewModel(get(), get()) }
}

Note: a qualifier for a ViewModel will be handled as a ViewModel's Tag

And reuse it in Activity and Fragments:

class WeatherActivity : AppCompatActivity() {

    /*
     * Declare WeatherViewModel with Koin and allow constructor dependency injection
     */
    private val weatherViewModel by viewModel<WeatherViewModel>()
}

class WeatherHeaderFragment : Fragment() {

    /*
     * Declare shared WeatherViewModel with WeatherActivity
     */
    private val weatherViewModel by activityViewModel<WeatherViewModel>()
}

class WeatherListFragment : Fragment() {

    /*
     * Declare shared WeatherViewModel with WeatherActivity
     */
    private val weatherViewModel by activityViewModel<WeatherViewModel>()
}

Passing Parameters to the Constructor

The viewModel keyword API is compatible with injection parameters.

In the module:

val appModule = module {

    // ViewModel for Detail View with id as parameter injection
    viewModel { parameters -> DetailViewModel(id = parameters.get(), get(), get()) }
    // ViewModel for Detail View with id as parameter injection, resolved from graph
    viewModel { DetailViewModel(get(), get(), get()) }
    // or Constructor DSL
    viewModelOf(::DetailViewModel)
}

From the injection call site:

class DetailActivity : AppCompatActivity() {

    val id : String // id of the view

    // Lazy inject ViewModel with id parameter
    val detailViewModel: DetailViewModel by viewModel{ parametersOf(id)}
}

SavedStateHandle Injection (3.3.0)

Add a new property typed SavedStateHandle to your constructor to handle your ViewModel state:

class MyStateVM(val handle: SavedStateHandle, val myService : MyService) : ViewModel()

In Koin module, just resolve it with get() or with parameters:

viewModel { MyStateVM(get(), get()) }

or with Constructor DSL:

viewModelOf(::MyStateVM)

To inject a state ViewModel in a Activity,Fragment use:

• by viewModel() - lazy delegate property to inject state ViewModel instance into a property
• getViewModel() - directly get the state ViewModel instance

class DetailActivity : AppCompatActivity() {

    // MyStateVM viewModel injected with SavedStateHandle
    val myStateVM: MyStateVM by viewModel()
}

info

All stateViewModel functions are deprecated. You can just use the regular viewModel function to inject a SavedStateHandle

Navigation Graph ViewModel

You can scope a ViewModel instance to your Navigation graph. Just retrieve with by koinNavGraphViewModel(). You just need your graph id.

class NavFragment : Fragment() {

    val mainViewModel: NavViewModel by koinNavGraphViewModel(R.id.my_graph)

}

ViewModel Scope API

see all API to be used for ViewModel and Scopes: ViewModel Scope

ViewModel Generic API

Koin provides some "under the hood" API to directly tweak your ViewModel instance. The available functions are viewModelForClass for ComponentActivity and Fragment:

ComponentActivity.viewModelForClass(
    clazz: KClass<T>,
    qualifier: Qualifier? = null,
    owner: ViewModelStoreOwner = this,
    state: BundleDefinition? = null,
    key: String? = null,
    parameters: ParametersDefinition? = null,
): Lazy<T>

note

This function is still using state: BundleDefinition, but will convert it to CreationExtras

Note that you can have access to the top level function, callable from anywhere:

fun <T : ViewModel> getLazyViewModelForClass(
    clazz: KClass<T>,
    owner: ViewModelStoreOwner,
    scope: Scope = GlobalContext.get().scopeRegistry.rootScope,
    qualifier: Qualifier? = null,
    state: BundleDefinition? = null,
    key: String? = null,
    parameters: ParametersDefinition? = null,
): Lazy<T>

ViewModel API - Java Compat

Java compatibility must be added to your dependencies:

// Java Compatibility
implementation "io.insert-koin:koin-android-compat:$koin_version"

You can inject the ViewModel instance to your Java codebase by using viewModel() or getViewModel() static functions from ViewModelCompat:

@JvmOverloads
@JvmStatic
@MainThread
fun <T : ViewModel> getViewModel(
    owner: ViewModelStoreOwner,
    clazz: Class<T>,
    qualifier: Qualifier? = null,
    parameters: ParametersDefinition? = null
)