// Option A: Return "token" like 
// Same pattern as RxJava
// Less API methods on top-level classes
class Realm {
  fun addChangelistener(listener: RealmChangeListener): Cancellable
  fun cancelAllChangeListeners() // Or maybe introduce something like CompositeCancellable like RxJava
}

val token = realm.addChangelistener { } 
token.cancel(); // onDestroy
realm.addChangeListener { }
realm.cancelAllChangeListeners()

// Option B: Java like API
// 1 more method in the API
// Must define listener in a class variable to be able to cancel listener
// Mirrors current Java API
class Realm {
  fun addChangelistener(listener: RealmChangeListener)
  fun removeChangelistener(listener: RealmChangeListener)
  fun removeAllChangelisteners()
}

// Option C: Something else?
// ...

class RealmResults {

    // Option A: Two arguments returned
    // Depends on autocomplete to work "out-of-the-box"
    // Top-level access to the collection
    fun addChangeListener1(listener: (listener: (collection: RealmResults<E>, change: OrderedRealmCollection<E>) -> Unit) { TODO() }

    // Option B: One argument returned
    // `it` is available 
    // collection access is one layer away.
    // Flows only allow one argument. So the API for change listeners and Flows would be consistent
    fun addChangeListener1(listener: (OrderedCollectionChange<E, RealmResults<E>>) -> Unit) { TODO() }
}