RFR: 8290040: Provide simplified deterministic way to manage listeners [v6]

[Nir Lisker]

On Thu, 29 Sep 2022 15:02:13 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.
>
> John Hendrikx has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains ten commits:
> 
>  - Merge remote-tracking branch 'origin/master' into
>    feature/conditional-bindings
>    
>    # Conflicts:
>    #	modules/javafx.base/src/test/java/test/javafx/beans/value/LazyObjectBindingTest.java
>  - Fix review comments
>  - Add missing new line
>  - Small wording fixes
>  - Update javadoc of "when" with better phrasing
>  - Rename showing property to shown as it is already used in subclasses
>  - Add conditional bindings to ObservableValue
>  - Change explanatory comment to block comment
>  - Fix bug where ObjectBinding returns null when revalidated immediately
>    
>    Introduced with the fluent bindings PR

I personally think that it's fine to include the changes to `Node` here. It adds a good amount of value to the new method and can help avoid many memory leaks with this combination.

modules/javafx.graphics/src/main/java/javafx/scene/Node.java line 1415:

> 1413:      *
> 1414:      * <p>Note that the {@code Node} does not need to be visible for this property
> 1415:      * to be {@code true}.

I think that the definition of "shown" is important enough to make it the main focus of the description with something like:


Indicates whether or not this {@code Node} is shown. A node is considered shown if it's
part of a {@code Window} whose {@link shown #Window::showingProperty} is {@code true}.
The {@link visibility #visibleProperty} of the node or its scene do not affect this property.


I think that there's no need to repeat the showing conditions for a window.
Also we need to be careful not to confuse `shown` with `showing`, which are properties on `ComboBox`, `ChoiceBox` and others.

modules/javafx.graphics/src/main/java/javafx/scene/Node.java line 1417:

> 1415:      * to be {@code true}.
> 1416:      *
> 1417:      * @defaultValue false

I don't think a default value is relevant. It can't be set and the node is not responsible for its value. It's true, though, that by default, since the node does not belong to a window, it's `false`, but I don't know how helpful it is.

-------------

Changes requested by nlisker (Reviewer).

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