[External] : Re: Eager Evaluation of Invalidation Listeners

[Nir Lisker]

I would also answer "no" to both points. This is also what the docs say.

So the question is: how likely do we think that changing this behavior will
> break existing applications?
>

That's the main question. I tested the JavaFX code with the new behavior
and some tests break immediately, though a few I've looked at seemed to be
testing the invalidation listener behavior itself (in their own context). I
don't know what would break outside of the tests. If we go this route, we
might want to create PRs to fix the tests before we actually change
the behavior (in contrast to doing it all in the same PR). I think that
tests fail in several modules and it might require several people to fix
these tests depending on their areas of expertise. Then we would need to
test runtime behavior to see what breaks outside of the tests.

In my own codebase nothing breaks, but it's relatively small.

On the related question, I like the idea of nulling out the current value
> when a property is bound.
>

I can pick up from where the older PR stopped. It's a simple change.

On Wed, Oct 6, 2021 at 3:15 AM Kevin Rushforth <kevin.rushforth at oracle.com>
wrote:

> Given that the intention of InvalidationListener was to be a light-weight
> way to listen to properties without causing a binding chain to be
> revalidated, it does seem reasonable to me that the listener is only fired
> on the valid --> invalid transition, which is the specified behavior, even
> if the implementation doesn't currently do that in all cases.
>
> The two related questions then are:
>
> 1. Should adding an invalidation listener to property do an immediate
> get(), which will ensure that the property is then valid? This will force
> an eager evaluation of the property and "arm" the property to fire an
> invalidation even the next time it is invalidated.
>
> 2. Should adding an invalidation listener to a currently invalid property
> cause the listener to be called (either immediately or the next time the
> object is invalidated)?
>
> I think the ideal answer to both questions is "no", although I share
> John's concern that changing this behavior for InvalidationListeners could
> break applications. So the question is: how likely do we think that
> changing this behavior will break existing applications?
>
> I think it's something we can and should consider changing. Unless there
> are serious concerns, I would support changing this behavior as part of
> avoiding eager evaluation when using invalidation listeners. It would need
> to be well tested (of course), and would need a CSR describing the
> compatibility risk, and should probably get a release note.
>
> Any concerns in doing this?
>
> On the related question, I like the idea of nulling out the current value
> when a property is bound.
>
> -- Kevin
>
>
> On 9/11/2021 9:41 AM, Nir Lisker wrote:
>
> I think that we need your input on this to move it forward.
>
> On Fri, Sep 3, 2021 at 7:49 AM Nir Lisker <nlisker at gmail.com> wrote:
>
>> so the value field should perhaps be nulled out when bound.
>>
>>
>> There was a PR for something like that in the old repo:
>> https://github.com/javafxports/openjdk-jfx/pull/110
>> <https://urldefense.com/v3/__https://github.com/javafxports/openjdk-jfx/pull/110__;!!ACWV5N9M2RV99hQ!bIbtLsC0tg02c9a_lgKnM1Xta2USX8QkKRL4imOUSw8xshJsVquOeulplJR7dfEzQUf6$>
>>
>> On Fri, Sep 3, 2021 at 5:35 AM John Hendrikx <hjohn at xs4all.nl> wrote:
>>
>>>
>>>
>>> On 02/09/2021 11:57, Nir Lisker wrote:
>>> >     So in order
>>> >     to make sure that a new interested invalidation listener does not
>>> miss
>>> >     the fact that a property was *already* invalid, the easiest
>>> solution
>>> >     might have been to revalidate it upon registration of a new
>>> listener
>>> >
>>> >
>>> > But why does an invalidation listener need to know the past state of
>>> the
>>> > property? It is only interested in the valid -> invalid transition. If
>>> > the property was invalid then the listener (in theory) shouldn't
>>> receive
>>> > any events anyway on subsequent invalidations. (I understand that you
>>> > don't justify this, I'm posing it as a general question.)
>>>
>>> Strictly speaking, no, if users are using InvalidationListener correctly
>>> then this is definitely correct behavior. I'm not really advocating a
>>> change, and I'd even prefer that it be brought in line with the
>>> documentation.
>>>
>>> I think however that many users are not using it correctly and expect an
>>> invalidation event always the next time the value changes (and their
>>> listener will read that value, validating it again), making it act like
>>> a light-weight ChangeListener. I know that I probably have written code
>>> that made that assumption, and would in the past not even think twice
>>> about replacing a change with an invalidation listener or vice versa if
>>> that happened to be a better fit. Which is sort of what happened as well
>>> in the bidirectional binding PR, and the issue slipped past the author
>>> and two reviewers.
>>>
>>> > I suggest that we split the problem into 2: one is the case where the
>>> > property was valid when the listener was attached, and the other is
>>> when
>>> > it was invalid.
>>> > * A valid starting state. In this case attaching a listener shouldn't
>>> > need to do anything. A subsequent invalidation event will be sent
>>> > regardless. Currently, it is calling get() redundantly.
>>>
>>> True, the call to get is redundant in this case. Ugly too, calling get
>>> and discarding its result, while the intention is to force the property
>>> to become valid.
>>>
>>> > * An invalid starting state. In this case the documentation says that
>>> > nothing needs to happen, but get() is called anyway. Here, the
>>> > difference is that a subsequent invalidation event is sent in one case
>>> > and not in the other. The only way to advance here is to make a design
>>> > decision on what should happen, at least that's how I see it.
>>>
>>> The docs are even more specific I think, they say no more events will be
>>> generated until it becomes valid -- it doesn't leave any option open
>>> that it could generate events if it wanted to.
>>>
>>> > As to the implementation of a possible solution, suppose we add the
>>> > isValid method. Upon attaching an invalidation listener, if the
>>> property
>>> > is valid, we can skip the get() call. That solves the valid starting
>>> > state issue. The question is what to do if the property is not valid.
>>> >
>>> > I also noticed an odd design choice in the implementation of
>>> properties:
>>> > the value field does not update if the property is bound, instead, the
>>> > result of the binding is returned and the value field holds an outdated
>>> > value (until the property is unbound).
>>>
>>> Yeah, that might not be a wise decision as that can lead to memory being
>>> referenced that users might expect to be freed. I didn't see anywhere
>>> defined what will happen to the value of the property when it is unbound
>>> again. The current implementation will keep its last value (during the
>>> unbind it will take the last value and assign it to its own value
>>> field), so the value field should perhaps be nulled out when bound.
>>>
>>> --John
>>>
>>
>