RFR: 8252936: Optimize removal of listeners from ExpressionHelper.Generic

[yosbits]

On Mon, 2 Mar 2020 11:47:03 GMT, Nir Lisker <nlisker at openjdk.org> wrote:

>>> 
>>> 
>>> I think that a starting point would be to decide on a spec for the listener notification order: is it specified by their registration order, or that is it unspecified, in which case we can take advantage of this for better performance. Leaving is unspecified and restricting ourselves to having it ordered is losing on both sides.
>> 
>> technically true - but the implementation was linear with a fixed sequence since-the-beginning-of-java-desktop-time (and sometimes, for good ol' beans properties, even exposed as api to access the array of listeners). So technically, we could go the path of explicitely spec'ing that the order is unspecified. Pretty sure that doing so and implementing it will break tons of application code that's subtly relying on fifo notification (f.i. register before or after skin has its own is a simple wide-spread trick) .. which all did it wrong and might deserve it ;-) But then if even internal code does it ..
>
>> technically true - but the implementation was linear with a fixed sequence since-the-beginning-of-java-desktop-time (and sometimes, for good ol' beans properties, even exposed as api to access the array of listeners). So technically, we could go the path of explicitely spec'ing that the order is unspecified. Pretty sure that doing so and implementing it will break tons of application code that's subtly relying on fifo notification (f.i. register before or after skin has its own is a simple wide-spread trick) .. which all did it wrong and might deserve it ;-) But then if even internal code does it ..
> 
> So we can choose to specify it as ordered.
> 
> These are the 2 options I mentioned:
> 1. Not specify the behavior and change the implementation in favor of performance. This could break applications as you've mentioned.
> 2. Specify that the order is preserved and keep the ordered implementation behavior. This will allow applications to rely on the behavior safely.
> 
> Right now we're losing on both sides. We keep a promise and we don't tell anyone about it. The only reason for this is if we intend to change the behavior in the future, in which case we should add a doc comment saying that the order is unspecified and is planned to change in the future, so there will be at least some warning.
> 
> Once we choose what to do here it will make sense to continue to review the code with that decision in mind.

In a minimal test I wrote (not a microbenchmark that removes listeners), I tried this PR code, but did not reproduce the performance improvement. I have attached a test program in my PR(#125)

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PR: https://git.openjdk.java.net/jfx/pull/108