RFR: 8290310: ChangeListener events are incorrect or misleading when a nested change occurs [v3]

[Nir Lisker]

On Sat, 1 Feb 2025 12:57:34 GMT, John Hendrikx <jhendrikx at openjdk.org> wrote:

>> This provides and uses a new implementation of `ExpressionHelper`, called `ListenerManager` with improved semantics.
>> 
>> # Behavior
>> 
>> |Listener...|ExpressionHelper|ListenerManager|
>> |---|---|---|
>> |Invocation Order|In order they were registered, invalidation listeners always before change listeners|(unchanged)|
>> |Removal during Notification|All listeners present when notification started are notified, but excluded for any nested changes|Listeners are removed immediately regardless of nesting|
>> |Addition during Notification|Only listeners present when notification started are notified, but included for any nested changes|New listeners are never called during the current notification regardless of nesting|
>> 
>> ## Nested notifications:
>> 
>> | |ExpressionHelper|ListenerManager|
>> |---|---|---|
>> |Type|Depth first (call stack increases for each nested level)|(same)|
>> |# of Calls|Listeners * Depth (using incorrect old values)|Collapses nested changes, skipping non-changes|
>> |Vetoing Possible?|No|Yes|
>> |Old Value correctness|Only for listeners called before listeners making nested changes|Always|
>> 
>> # Performance
>> 
>> |Listener|ExpressionHelper|ListenerManager|
>> |---|---|---|
>> |Addition|Array based, append in empty slot, resize as needed|(same)|
>> |Removal|Array based, shift array, resize as needed|(same)|
>> |Addition during notification|Array is copied, removing collected WeakListeners in the process|Appended when notification finishes|
>> |Removal during notification|As above|Entry is `null`ed (to avoid moving elements in array that is being iterated)|
>> |Notification completion with changes|-|Null entries (and collected WeakListeners) are removed|
>> |Notifying Invalidation Listeners|1 ns each|(same)|
>> |Notifying Change Listeners|1 ns each (*)|2-3 ns each|
>> 
>> (*) a simple for loop is close to optimal, but unfortunately does not provide correct old values
>> 
>> # Memory Use 
>> 
>> Does not include alignment, and assumes a 32-bit VM or one that is using compressed oops.
>> 
>> |Listener|ExpressionHelper|ListenerManager|OldValueCaching ListenerManager|
>> |---|---|---|---|
>> |No Listeners|none|none|none|
>> |Single InvalidationListener|16 bytes overhead|none|none|
>> |Single ChangeListener|20 bytes overhead|none|16 bytes overhead|
>> |Multiple listeners|57 + 4 per listener (excluding unused slots)|57 + 4 per listener (excluding unused slots)|61 + 4 per listener (excluding unused slots)|
>> 
>> # About nested changes
>> 
>> Nested changes are simply changes...
>
> John Hendrikx has updated the pull request incrementally with five additional commits since the last revision:
> 
>  - Clean-up and add tests
>  - Pass in listener data directly for fireValueChanged calls
>  - Fix documentation in a few places
>  - Remove unused interface
>  - Update copyrights and add missing

I'd say that the biggest surprise a user would have is about the nested addition of listeners. By the way, your table says that added listeners don't receive any event, but the "The PR" section says that they do. I've observed that they don't.
I think that not receiving events for added listeners is confusing (we discussed this briefly under https://github.com/openjdk/jfx/pull/837). Is there a good reason for this? It prevents SO exceptions in some cases, but I would say that that's the user's fault. I need to think about this more, but was wondering what you concluded.

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PR Comment: https://git.openjdk.org/jfx/pull/1081#issuecomment-2661784907