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

[John Hendrikx]

On Thu, 16 Apr 2020 08:24:16 GMT, dannygonzalez <github.com+6702882+dannygonzalez at openjdk.org> wrote:

>> The listeners added by `Node` are apparently internally required for internal properties `TreeShowing` and `TreeVisible`, and are used to take various decisions like whether to play/pause animations.  There is also a couple of listeners registering on these properties in turn (in `PopupWindow`, `SwingNode`, `WebView` and `MediaView`).
>> 
>> A lot of the checks for visibility / showing could easily be done by using the `Scene` property and checking visibility / showing status from there.  No need for so many listeners.  The other classes mentioned might register their own listener, instead of having `Node` do it for them (and thus impacting *every* node).
>> 
>> Alternatively, `Node` may lazily register the listeners for Scene.Window and Window.Showing only when needed (which from what I can see is for pretty specific classes only, not classes that you'd see a lot in a TableView...)
>
> If it is of any help, I have attached a VisualVM snapshot (v1.4.4) where the ExpressionHelper.removeListener is using 61% of the JavaFX thread whilst running our application.
> 
> [snapshot-1587024308245.nps.zip](https://github.com/openjdk/jfx/files/4485728/snapshot-1587024308245.nps.zip)
> 
> If you show only the JavaFX Application thread, press the "HotSpot" and "Reverse Calls" button you can take a look to see which classes are calling the removeListener function.
> 
> ![Screenshot 2020-04-16 at 09 16 11](https://user-images.githubusercontent.com/6702882/79432046-2f788800-7fc3-11ea-930a-98fed0190628.png)

@dannygonzalez  Could you perhaps debug your application and take a look at how large the following array is: a random node -> `scene` -> `value` -> `window` -> `readOnlyProperty` -> `helper` -> `changeListeners`.  I just tested this with a custom control displaying 200 cells on screen at once (each cell consisting of about 30 nodes itself), and I saw about 20000 change listeners registered on this single Scene Window property.

However, this custom control is not creating/destroying cells beyond the initial allocation, so there wouldn't be any registering and unregistering going on, scrolling was still smooth >30 fps.

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