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

[Kevin Rushforth]

On Tue, 9 Feb 2021 10:31:57 GMT, Jeanette Winzenburg <fastegal at openjdk.org> wrote:

>> 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

I took a quick look at your latest change, and it seems reasonable to me now that you are calling a helper method rather than triggering a change method on the listener. I'll take a closer look later in the week. In the mean time @kleopatra and @arapte can provide their feedback.

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