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.
-------------
Commit messages:
- Add conditional bindings to ObservableValue
- Change explanatory comment to block comment
- Fix bug where ObjectBinding returns null when revalidated immediately
Changes: https://git.openjdk.org/jfx/pull/830/files
Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=830&range=00
Issue: https://bugs.openjdk.org/browse/JDK-8290040
Stats: 587 lines in 7 files changed: 576 ins; 8 del; 3 mod
Patch: https://git.openjdk.org/jfx/pull/830.diff
Fetch: git fetch https://git.openjdk.org/jfx pull/830/head:pull/830
PR: https://git.openjdk.org/jfx/pull/830
