RFR: JDK-8303897 ObservableValue's when binding should only invalidate when strictly needed [v2]

[Nir Lisker]

On Fri, 7 Apr 2023 06:36:50 GMT, John Hendrikx <jhendrikx at openjdk.org> wrote:

>> Description copied from issue:
>> 
>> There are up to two additional invalidations performed that really should be avoided, causing downstream fluent bindings to be recomputed with the same values.  This is very confusing as these should only be called when there is an actual change, and not called for the same value multiple times in a row.
>> 
>> These two extra invalidations have two different causes, each causing an additional invalidation to be triggered:
>> 
>> 1) ObjectBinding's `isObserved` is delayed slightly.  When you add a listener, the listener is added internally and the binding is made valid; this triggers some downstream activity which checks the `isObserved` status to decide whether to start observation of properties -- unfortunately this still returns `false` at that time.  A work-around for this existed by calling `getValue` again in `LazyObjectBinding` with a huge comment explaining why this is needed. Although this works, it still means that a downstream function like `map` is called an additional time while it should really only be called once.
>> 
>> The solution is to ensure `isObserved` returns `true` before the `ExpressionHelper` is called.  Already verified this solves the problem.  This also means the work-around in `LazyObjectBinding` is no longer needed, which seems like a big win.
>> 
>> 2) The second additional call originates from a different issue. When `ConditionalBinding` (which implements the `when` construct) sees its condition property changing, it always invalidates itself. This is however only necessary if the current cached value (if it was valid) differs from the current source value. To prevent an unnecessary invalidation, and the resulting revalidation calls that this will trigger, a simple check to see if the value actually changed before invalidating solves this problem.
>
> John Hendrikx has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix review comments

I reviewed the first bug and the fix looks good.

The fix adds a `boolean` field to all `ObjectBinding`s, which could be a slight performance hit. I have a local version of a refactored `ExpressionHelper` class that uses a singleton instance for an empty helper instead of `null`. It makes the code cleaner (no special `null` handling) and will allow to get rid of the new `observed` field because that info can be encoded in `ExpressionHelper` instead of in `ObjectBinding`. After all, the info of being observed really belongs in the class that manages the listeners IMO. Not sure if it's worth pursuing at the moment.

Also, I really wish we could make `add/removeListener` throw on `null` :)

I still need to finish reviewing the second bug. The logic flow is a bit complicated when taking into account the many states the binding can be at (valid, observed, active...). One question I have is about the tests: why split a When test class from an already existing one in the fluent bindings test?

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PR Review: https://git.openjdk.org/jfx/pull/1056#pullrequestreview-1376817231