A kotlin property delegate to scope any object to an android lifecycle. Blog post at https://ryanharter.com/blog/easy-android-scopes/

Scoped.kt

import androidx.lifecycle.get
import androidx.lifecycle.ViewModel
import androidx.lifecycle.ViewModelProvider
import androidx.lifecycle.ViewModelStore
import androidx.lifecycle.ViewModelStoreOwner

/**
 * Returns a property delegate to access the wrapped value, which will be retained for the
 * duration of the lifecycle of this [ViewModelStoreOwner].
 *
 * ```
 * class MyFragment : Fragment() {
 *   private val presenter by scoped { MyPresenter() }
 *
 *   override fun onCreate(savedInstanceState: Bundle?) {
 *     super.onCreate(savedInstanceState)
 *     presenter.models.collect {
 *       // ...
 *     }
 *   }
 * }
 * ```
 */
inline fun <reified T> ViewModelStoreOwner.scoped(noinline creator: () -> T): Lazy<T> {
  return LazyScopedValue({ viewModelStore }, { ScopeViewModel.Factory(creator) })
}

class ScopeViewModel<V>(
    val value: V
) : ViewModel() {
  class Factory<V>(val valueFactory: () -> V) : ViewModelProvider.Factory {
    @Suppress("UNCHECKED_CAST")
    override fun <T : ViewModel?> create(modelClass: Class<T>): T =
        ScopeViewModel(valueFactory()) as? T
            ?: throw java.lang.IllegalArgumentException("Unknown type")
  }
}

class LazyScopedValue<T>(
    private val storeProducer: () -> ViewModelStore,
    private val factoryProducer: () -> ViewModelProvider.Factory
) : Lazy<T> {
  private var cached: Any = NotSet

  @Suppress("UNCHECKED_CAST")
  override val value: T
    get() {
      val value = cached
      return if (value == NotSet) {
        val factory = factoryProducer()
        val store = storeProducer()
        val viewModel = ViewModelProvider(store, factory).get<ScopeViewModel<T>>()

        viewModel.value.also {
          cached = it as Any
        }
      } else {
        value as T
      }
    }

  override fun isInitialized() = cached != NotSet

  companion object {
    private val NotSet = Any()
  }
}

Oh so it's a more general solution, which works in Kotlin for all platforms? But I think you can make it shorter to use, no?

This isn't as much about the presenter as it is about getting the scoping of view models for things other than view models.

That's long been a problem on Android that was initially a develoepr problem, then the AndroidX team took on and solved for ViewModels, but the fact that it's limited to that class, which is tightly coupled, is really arbitrary. This shows that there are use cases for the functionality in a non-coupled way that can leverage the ViewModel to get the same effect without limiting you to Android artifacts.

As far as how to make it shorter to use, this is a single file utility that allows you to use it anywhere in your app with a single line, private val myPresenter: MyPresenter by scoped { MyPresenter() }. If you have an example of how to make that call-site shorter, I'd love if you could share. (If you haven't read the linked blog post, I'd recommend it, I tried to explain all of this. If I didn't make it clear, or you disagree with my assumptions I appreciate you sharing.)

I see. Thank you.
You don't think it's important if all I do is to develop Android apps, though, right?

If you don't need it, then no. If you're trying to figure out where to keep things like Dagger scoped Components then this could be helpful, but if you're just doing Android dev with ViewModels then you can just use the by viewModels.

OK thank you.

Nice solution! The main drawback of ViewModel is that it's part of Android framework and in TDD approach it matters. :)

Hello Ryan, thank you for sharing this code.

If I'm not mistaken, the current implementation would only work for a single scoped value per Fragment or Activity.
Because of type erasure, the ViewModelStore can not distinguish a ScopeViewModel<X> from a ScopeViewModel<Y>.
So I would recommend that you also pass the field name as key argument when you call the ViewModelProvider:

val viewModel = ViewModelProvider(store, factory).get(fieldName, ScopeViewModel::class.java)

Also, you could allow nullable values by just changing the type of cached to Any?

how do i use dagger in this ?

i'm curious about your statements about ViewModel is tightly coupled with Android framework

but your whole imports is dependent with androidx.lifecycle.* wouldn't this means using this approach also on kotlin multiplatform needs to coupled with android framework?

Unfortunately, I also can't find your samples on kotlin multiplatform to use this gist?

Did you mean, you were replacing the ViewModel in business logic with *Presenter so the Presenter class is shared among platform?

That way, you can still leverages ViewModel solutions in Android without adding much logic/details in the ViewModel itself

@rharter

Yeah, this code that scopes objects to the Android Lifecycle is inherently coupled to the Android platform, since no other platform shares that lifecycle. The benefit of this is that your business logic class, a Presenter or view model (not AndeoidX View model) is no longer tightly coupled to the Android framework.

That's the code you'd want to share, anyway. There's no point sharing this code, which is glue to connect the shared code to the Framework, on other platforms, since they have their own glue code.

Awesome! Thanks for explaining @rharter