A convenience class for creating implementations of javafx.beans.value.ObservableValue. It contains all of the infrastructure support for value invalidation- and change event notification.

This implementation can handle adding and removing listeners while the observers are being notified, but it is not thread-safe.

This class keeps track of the latest value it has seen to determine if change listeners should be called when next fireValueChangedEvent() is called. So while this value is usually the current value of the involved observable, it becomes the "old" value as soon as the observable is changed, until such time it is updated again (by calling fireValueChangedEvent()).

During this brief period, listeners may be added or removed causing the ExpressionHelper to perhaps switch to a different variant of itself. These different variants must be made aware of the currently stored latest value, as obtaining this value from the ObservableValue would (during that brief period) be a different value. Using the incorrect latest value would result in change listeners not being fired as they perform an equality check.