RFR: 8252935: Add treeShowing listener only when needed [v4]

[Jeanette Winzenburg]

On Mon, 8 Feb 2021 13:42:52 GMT, Ambarish Rapte <arapte at openjdk.org> wrote:

>> Isn't it quite error prone to repeat this logic again (especially with all the null cases), not to mention that you would need to test the code for the initial case (with/without Scene, with/without Window), the "in use" case and again for the disposal case?
>> 
>> I personally use helpers for this kind of property chaining as it is far to easy to get wrong:
>> 
>>     public Binding<Boolean> isShowing(Node node) {
>>         Values.of(node.sceneProperty())
>>            .flatMap(s -> Values.of(s.windowProperty()))
>>            .flatMap(w -> Values.of(w.showingProperty()))
>>            .orElse(false)
>>            .toBinding();
>>     }
>> 
>> The implementation here takes care of `null` and automatically tracks property changes and is type safe to boot.  I think JavaFX in general could really benefit from this, as I've seen this chaining logic repeated a lot.
>
> My concern is about having a similar way of doing something. It would keep the code uniform. We have been following the earlier pattern under a cleanup task [JDK-8241364](https://bugs.openjdk.java.net/browse/JDK-8241364). Several bugs under this task are being fixed in earlier way.
> May be we can discuss the new way of handling properties under a separate issue and plan to modify all such instances at once. Does that sound ok ?

hmm ... might appear convenient (in very controlled contexts) but looks like a precondition violation: the sender of the change must not be null (concededly not explicitly spec'ed but logically implied, IMO)

so would tend to _not_ see this as blueprint for a general pattern fx code base

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PR: https://git.openjdk.java.net/jfx/pull/185