RFR: JDK-8304439: Subscription based listeners [v9]

[Kevin Rushforth]

On Tue, 11 Jul 2023 21:04:42 GMT, John Hendrikx <jhendrikx at openjdk.org> wrote:

>> Makes `Subscription` public (removing some of its methods that are unnecessary), and adds methods that can provide `Subscription`s in `ObservableValue`.
>
> John Hendrikx has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Extend tests to test unsubscribe

I note one more javadoc change needed (a missing `@since` tag).

modules/javafx.base/src/main/java/javafx/beans/Subscription.java line 33:

> 31: /**
> 32:  * A subscription encapsulates how to cancel it without having
> 33:  * to keep track of how it was created.

This needs an `@since 21` javadoc tag here.

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PR Review: https://git.openjdk.org/jfx/pull/1069#pullrequestreview-1525234547
PR Review Comment: https://git.openjdk.org/jfx/pull/1069#discussion_r1260342187