RFR: 8290310: ChangeListener events are incorrect or misleading when a nested change occurs [v5]

[Ambarish Rapte]

On Tue, 14 Feb 2023 16:03:52 GMT, John Hendrikx <jhendrikx at openjdk.org> wrote:

>> This contains the following:
>> - Nested changes or invalidations using ExpressionHelper are delayed until the current emission completes
>>   - This fixes odd change events being produced (with incorrect oldValue)
>>   - Also fixes a bug in ExpressionHelper where a nested change would unlock the listener list early, which could cause a `ConcurrentModificationException` if a nested change was combined with a remove/add listener call
>> - A test for ExpressionHelper to verify the new behavior
>> - A test for all *Property and *Binding classes that verifies correct listener behavior at the API level (this tests gets 85% coverage on ExpressionHelper on its own, the only thing it is not testing is the locking behavior, which is not relevant at the API level).
>> - A fix for `WebColorFieldSkin` which triggered a nested change which used a flag to prevent an event loop (I've changed it now to match how `DoubleFieldSkin` and `IntegerFieldSkin` do it
>
> John Hendrikx has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Fix javadoc link
>  - Update doc

modules/javafx.base/src/main/java/javafx/beans/value/ChangeListener.java line 57:

> 55:      * differs from a call to {@link ObservableValue#getValue}. All listeners are
> 56:      * then notified again with an old value equal to the initial new value,
> 57:      * and a new value with the latest value.

This seems like a right place.
It is still a bad practice to change the value in this method, so I would recommend to keep the warning.
I did little rewording, please check how does this look:


* In general, it is considered a bad practice to modify the observed value in
* this method. But if still modified, then the notifications for this change
* are deferred until all the listeners are notified of current change.
* This results in a minor inconsistency of new value with the listeners
* that are still pending to be notified of current change that the listeners
* would receive a new value that differs from a call to {@link ObservableValue#getValue}.

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PR: https://git.openjdk.org/jfx/pull/837