SavedStateHandle injection

When using the tangleViewModel delegate function, a scoped subcomponent is created with a binding for SavedStateHandle. This SavedStateHandle is provided by the ViewModel's owning Fragment, Activity, or NavBackStackEntry.

This SavedStateHandle may then be included as a dependency in injected constructors, just as it can in Hilt.

class MyViewModel @VMInject constructor(
  val savedState: SavedStateHandle
) : ViewModel()

In addition, Tangle can automatically extract arguments from the SavedStateHandle and inject them into the constructor, through use of the TangleParam annotation.

If the constructor argument's type is not nullable, then Tangle will assert that the argument is in the bundle while creating the ViewModel.

If the argument is marked as nullable, then Tangle will gracefully handle a missing argument and just inject null.

Given this code:

class MyViewModel @VMInject constructor(
  @TangleParam("userId")
  val userId: String, // must be present in the SavedStateHandle
  @TangleParam("address")
  val addressOrNull: String? // can safely be null
) : ViewModel()

Tangle will generate the following:

public class MyViewModel_Factory @Inject constructor(
  internal val savedStateHandleProvider: Provider<SavedStateHandle>
) {
  public fun create(): MyViewModel {
    val userId = savedStateHandleProvider.get().get<String>("userId")
    checkNotNull(userId) {
      "Required parameter with name `userId` " +
      "and type `kotlin.String` is missing from SavedStateHandle."
    }
    val addressOrNull = savedStateHandleProvider.get().get<String?>("address")
    return MyViewModel(userId, addressOrNull)
  }
}