Properties and Bindings - JavaFX 8: A Definitive Guide to Building Desktop, Mobile, and Embedded Java Clients - page 149

■ an invalidation event is fired only once by any of the implementations of the Property interface in the JavaFX
properties and bindings framework if you call the setter with the same value several times in a row.

Note

Another place where invalidation events are fired is from Binding objects. You haven't seen an example of a
Binding object yet, but there are plenty of Binding objects in the second half of this chapter. For now we just note that
a Binding object may become invalid, for example, when its invalidate() method is called or, as we show later in
this chapter, when one of its dependencies fires an invalidation event.

■ an invalidation event is fired only once by any of the implementations of the Binding interface in the JavaFX
properties and bindings framework if it becomes invalid several times in a row.

Note

Understanding the ObservableValue Interface
Next up in the hierarchy is the ObservableValue interface. It's simply an Observable that has a value. Its getValue()
method returns its value. The getValue() method that we called on the SimpleIntegerProperty objects in the
motivating example can be considered to have come from this interface. You can register ChangeListener objects to
an ObservableValue object to receive change events.
You saw change events being fired in the motivating example in the last section. When the change event fires, the
ChangeListener receives two more pieces of information: the old value and the new value of the ObservableValue object.

■ a change event is fired only once by any of the implementations of the ObservableValue interface in the
JavaFX properties and bindings framework if you call the setter with the same value several times in a row.

Note

The distinction between an invalidation event and a change event is made so that the JavaFX properties and
bindings framework may support lazy evaluations . We show an example of this by looking at three lines of code from
the motivating example:

otherProperty.bind(intProperty);
intProperty.set(7168);
System.out.println("otherProperty.get() = " + otherProperty.get());

When intProperty.set(7168) is called, it fires an invalidation event to otherProperty . On receiving this
invalidation event, otherProperty simply makes a note of the fact that its value is no longer valid. It does not
immediately perform a recalculation of its value by querying intProperty for its value. The recalculation is performed
later when otherProperty.get() is called. Imagine if instead of calling intProperty.set() only once as in the
preceding code we call intProperty.set() multiple times; otherProperty still recalculates its value only once.

■ the ObservableValue interface is not the only direct subinterface of Observable . there are four other
direct subinterfaces of Observable that live in the javafx.collections package: ObservableList , ObservableMap ,
ObservableSet , and ObservableArray with corresponding ListChangeListener , MapChangeListener ,
SetChangeListener , and ArrayChangeListener as callback mechanisms. these JavaFX observable collections are
covered in Chapter 7, “Collections and Concurrency.”

Note