Several classes and interfaces offer addListener() and removeListener(), for example:
javafx.beans.property.ReadOnlyStringProperty
javafx.beans.value.ObservableValueBase
javafx.beans.value.ObservableValue

In dynamic languages such as Clojure it is typical to update/change/develop the UI while the application is running. For this it can be useful to check how many listeners are currently attached. A getter that returns an immutable sequence of the listeners would be helpful.