Retention via ViewModel delegation

RetentionDelegate.kt

import android.arch.lifecycle.ViewModel
import android.arch.lifecycle.ViewModelProvider
import android.arch.lifecycle.ViewModelStoreOwner
import android.support.v4.app.Fragment
import android.support.v7.app.AppCompatActivity
import kotlin.reflect.KProperty

/**
 * This class handles retention of any object using the ViewModel framework. An
 * example of this method being used can be found in the Loader Manager framework
 * in the Android support library.
 */
class RetentionDelegate<T>(
        private val viewModelStoreOwner: ViewModelStoreOwner,
        private val initializer: (() -> T)? = null
) {

    operator fun getValue(thisRef: Any?, property: KProperty<*>): T {
        val (uniqueId, viewModel) = storeInformationFor(property)

        // If the value is in the ViewModel, return it
        @Suppress("UNCHECKED_CAST")
        if (viewModel.values.containsKey(uniqueId)) return viewModel.values[uniqueId] as T

        // If the viewModel doesn't hold the value and the initialiser is null, we're in an illegal state
        initializer ?: throw IllegalStateException("Attempting to invoke getValue when no value is present")

        val value = initializer.invoke() 

        // Add the value to the ViewModel
        viewModel.values[uniqueId] = value as Any

        return value
    }

    /**
     * Convenience getter function for viewModel
     */
    private fun storeInformationFor(property: KProperty<*>): Pair<String, RetentionViewModel> {
        // Get ViewModel
        val viewModel = ViewModelProvider(viewModelStoreOwner)[RetentionViewModel::class.java]

        // Use the property as a unique ID for the viewModel value map
        val uniqueId = property.name

        return uniqueId to viewModel
    }
}

/**
 * ViewModel class for holding the values to be retained in the viewModelStore
 */
class RetentionViewModel: ViewModel() {
    val values: MutableMap<String, Any> = hashMapOf()       
}

/**
 * Extension properties for ease of use
 */
fun <T> Fragment.retained(valInitializer: (() -> T)? = null) = RetentionDelegate(this, valInitializer)

fun <T> AppCompatActivity.retained(valInitializer: (() -> T)? = null) = RetentionDelegate(this, valInitializer)

RetentionTestActivity.kt

class RetentionTestActivity: AppCompatActivity() {
    val component by retained { instantiateDaggerComponent() } 
}