RFR: JDK-8304439: Subscription based listeners [v13]

[John Hendrikx]

On Fri, 14 Jul 2023 14:59:00 GMT, John Hendrikx <jhendrikx at openjdk.org> wrote:

>> I also hadn't considered potential usage of Subscription outside of observables, but I can see some uses for it.
>> 
>> John: I would support moving it to javafx.util (via a separate issue) if you think it is worth doing.
>
> Correct packaging is often overlooked, so I think it is worth doing.  I'll test to see if it doesn't cause any problems, and if not, I'll create an issue for it.

> Good point, I didn't think about subscriptions outside of the listeners contexts, let alone outside of the JavaFX content. It does look now like it belongs in the utils package. Maybe a quick correction can be made if Kevin agrees with this and you think it's worth it.
> 
> Can you think where else in JavaFX subscriptions could be used? I'm thinking about the deprecated `finalize` and try-with-resource, anywhere where we need to release resources. This would give a better idea of what the class revolves around and thus what documentation fits it.

It's very general, basically a specifically purposed `Runnable` where `run` is called `unsubscribe` with some chaining methods. You could use it for purposes we may not be able to think of right now.  I've looked at some `dispose` methods, which often have clean-up code for some inspiration:

1. Use it to clean up references: `() -> { this.skinnable = null; this.scrollBar = null; }`
2. Use it to stop animation timers or timelines : `() -> animationTimer.stop();`
3. You could wrap other add/remove style API that don't offer a `Subscription`: `() -> getSkinnable().removeEventHandler( ... )`

I'm not saying you always should do this, but if you are releasing a `Subscription` anyway, you could combine some of these.  Also, if new API decides to provide a subscription, they're not limited to only remove a listener, they could also stop a thread, a timer or release some resources that was started/allocated when the subscription was created.

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PR Review Comment: https://git.openjdk.org/jfx/pull/1069#discussion_r1263857263