Creating a `MutableStateFlow` from a Jetpack `ViewModel`'s `SavedStateHandle`.

SavingFlow.kt

import androidx.lifecycle.SavedStateHandle
import kotlinx.coroutines.flow.MutableStateFlow

private class SavingFlow<T> private constructor(
    private val save: (T) -> Unit,
    private val msf: MutableStateFlow<T>
) :
    MutableStateFlow<T> by msf {

    constructor(
        savedStateHandle: SavedStateHandle,
        key: String,
        initialValue: T
    ) : this(
        { savedStateHandle.set(key, it) },
        MutableStateFlow(savedStateHandle.get(key) ?: initialValue)
    )

    override var value: T
        get() = msf.value
        set(value) {
            save(value)
            msf.value = value
        }
}

fun <T> SavedStateHandle.getStateFlow(key: String, initialValue: T): MutableStateFlow<T> {
    return SavingFlow(this, key, initialValue)
}