RFR: 8290040: Provide simplified deterministic way to manage listeners

[John Hendrikx]

On Fri, 15 Jul 2022 09:03:35 GMT, John Hendrikx <jhendrikx at openjdk.org> wrote:

> This PR adds a new (lazy*) property on `Node` which provides a boolean which indicates whether or not the `Node` is currently part of a `Scene`, which in turn is part of a currently showing `Window`.
> 
> It also adds a new fluent binding method on `ObservableValue` dubbed `when` (open for discussion, originally I had `conditionOn` here).
> 
> Both of these together means it becomes much easier to break strong references that prevent garbage collection between a long lived property and one that should be shorter lived. A good example is when a `Label` is bound to a long lived property:
> 
>      label.textProperty().bind(longLivedProperty.when(label::isShowingProperty));
> 
> The above basically ties the life cycle of the label to the long lived property **only** when the label is currently showing.  When it is not showing, the label can be eligible for GC as the listener on `longLivedProperty` is removed when the condition provided by `label::isShowingProperty` is `false`.  A big advantage is that these listeners stop observing the long lived property **immediately** when the label is no longer showing, in contrast to weak bindings which may keep observing the long lived property (and updating the label, and triggering its listeners in turn) until the next GC comes along.
> 
> The issue in JBS also describes making the `Subscription` API public, but I think that might best be a separate PR.
> 
> Note that this PR contains a bugfix in `ObjectBinding` for which there is another open PR: https://github.com/openjdk/jfx/pull/829 -- this is because the tests for the newly added method would fail otherwise; once it has been integrated to jfx19 and then to master, I can take the fix out.
> 
> (*) Lazy means here that the property won't be creating any listeners unless observed itself, to avoid problems creating too many listeners on Scene/Window.

modules/javafx.base/src/main/java/javafx/beans/value/ObservableValue.java line 311:

> 309:      * @since 20
> 310:      */
> 311:     default ObservableValue<T> when(ObservableValue<Boolean> condition) {

Discussion: name? `when`, `conditionOn`, other suggestions?  Note that this is not the same as a possible future method `filter`, as a `filter` would still keep observing its source at all times (and would use a `Predicate` as parameter).

modules/javafx.graphics/src/main/java/javafx/scene/Node.java line 1422:

> 1420:     }
> 1421: 
> 1422:     public final ReadOnlyBooleanProperty showingProperty() {

Discussion: Note I had to wrap the `ObservableValue<Boolean>` to become a `ReadOnlyBooleanProperty`, even though `ObservableValue` is already read-only and quite usable without the wrapper.  I would have preferred to just have the showing property signature be:

     ObservableValue<Boolean> showingProperty();

However, I think it would not be detected as a JavaFX property then, and it would also break with the tradition that properties are, well, properties.  Thoughts?

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PR: https://git.openjdk.org/jfx/pull/830