Manual Delegation

This works. However, arguably, it breaks encapsulation. The consumer of ItemViewModel perhaps should not have direct access to the FavoriteStore. Instead, ItemViewModel should have an API for working with favorites that hides the fact that there even is a FavoriteStore.

So, we could set up manual delegation, where ItemViewModel forwards the FavoriteStore API on to favorites:

class ItemViewModel(private val favorites: FavoriteStore<Item>) : FavoriteStore<Item> {
  override fun isFavorite(thingy: Item) = favorites.isFavorite(thingy)
  
  override fun toggleFavorite(thingy: Item, isFavorite: Boolean) = favorites.toggleFavorite(thingy, isFavorite)
}

fun main() {
  val vm = ItemViewModel(InMemoryFavoriteStore())
  val item = Item("this is my id", "this is my name")
  
  println("item isFavorite: ${vm.isFavorite(item)}")
  vm.toggleFavorite(item, true)
  println("item isFavorite: ${vm.isFavorite(item)}")
}

This also works, and for a small interface like FavoriteStore, it is not too tedious to implement. The bigger the API surface, though, the more work is involved in setting up this manual delegation.