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As you know an ObservableValue in java can generate two types of events: Invalidation and Change. I don't know what's the difference between them?

When we should use Invalidation or Change Listener?

Mohammad Sadegh
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  • What just reading the JavaDocs, the `InvalidationListener` support comes from the parent class `Observable`, it purpose is a little obscure, but I think that's done on purpose, as the meaning is contextual to the implementation. – MadProgrammer Jun 20 '17 at 01:01
  • However, the [JavaDocs for `ObservableValue`](https://docs.oracle.com/javase/8/javafx/api/javafx/beans/value/ObservableValue.html) state – MadProgrammer Jun 20 '17 at 01:02
  • *"An ObservableValue generates two types of events: change events and invalidation events. A change event indicates that the value has changed. An **invalidation event is generated, if the current value is not valid anymore**. This distinction becomes important, if the ObservableValue supports lazy evaluation, because for a lazily evaluated value one does not know if an invalid value really has changed until it is recomputed. For this reason, generating change events requires eager evaluation while invalidation events can be generated for eager and lazy implementations."* – MadProgrammer Jun 20 '17 at 01:02
  • In layman terms, "Hey, the value has been changed" and "Hey, the value is found to be invalid" are two different events. – KC Wong Jun 20 '17 at 01:10
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    See https://stackoverflow.com/questions/33834215/when-to-use-javafx-properties-setter-and-getter-instead-of-using-the-property-d – James_D Jun 20 '17 at 01:52
  • In a invalidation event the value may or may not have changed. In a changed event the value has changed and you receive the old and new values as parameters. Furthermore changelisteners are used for properties but there is a larger list of types that supports invalidationlisteners (e.g. `ObservableList`...) – fabian Jun 20 '17 at 05:25

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As per documentation:

An ObservableValue generates two types of events: change events and invalidation events. A change event indicates that the value has changed. An invalidation event is generated, if the current value is not valid anymore.This distinction becomes important, if the ObservableValue supports lazy evaluation, because for a lazily evaluated value one does not know if an invalid value really has changed until it is recomputed. For this reason, generating change events requires eager evaluation while invalidation events can be generated for eager and lazy implementations.

ChangeListener enforces eager computation even when the observable value supports lazy evaluation.

If you want to find the change that happened in observed property, you use a change listener. Where as the InvalidationListener only helps us know some change has happened, if you want to know the difference between old and new values, you have to compute that yourself or simply use change listener.

Buddha
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