HEADS-UP: Threading restriction for Animation play, pause, stop now enforced USE CASE

[John Hendrikx]

On 24/01/2024 22:06, Nir Lisker wrote:
> This is the ballpark of what I meant with "the downside might be some 
> surprise when these methods behave differently".
That can be documented, and basically already is (because that is what 
the "asynchronous" is alluding to, the fact that after calling "play" 
the state will change asynchronously).
>
> The example you give will only show a potential change if 'play' is 
> not called from the FX thread. In such a case, what's the chance that 
> this is an undeliberate misuse vs. an informed use? This brings me 
> again to: what is the use case for running an animation from a 
> background thread?

The only possible use case I can think of is when using FX properties 
stand-alone (ie. only using javafx.base), without any FX thread 
involvement.  Even in that case though one has to remember that 
properties themselves are not thread safe either. Any "animation" would 
need to be done on the same thread that is manipulating properties.

However, Animation and Timeline simply can't work in such scenario's, as 
they're tied to javafx.graphics, to the FX system being started, and the 
FX thread.  For such a use case you'd need to write your own system, or 
provide the option of specifying an Executor for Animations/Timelines.


> In your simple example, listening on the Status property would work. 
> Also, while runLater makes no guarantees, I've never seen a 
> non-negligible delay in its execution, so how surprising is it really 
> going to be?
>
> If you want to be able to run an animation from a background thread 
> with no race conditions, why not opt for option 3?

Option 3 is basically document it as "be careful" and remove the thread 
restriction recently introduced, am I correct?

IMHO that can simply can't work at all, because this is what 
(theoretically) can happen:

1. Non-FX thread starts animation
      - Fields are manipulated in AbstractPrimaryTimer
      - There is no synchronization, so no guarantee anything is flushed 
(it may all reside in CPU caches)

2. FX Thread becomes involved to actually process the animation
      - Sees partially flushed state fields, and acts on garbage 
information (ie. number of animations > 0, but array contains only null's)

There just is no safe way to do this in without synchronization or 
having everything immutable (and this extends to references to "thread 
safe" structures).

--John

>
> And option 1 is also new and surprising because now code that worked 
> (or pretended to work) throws, which ruins properly written code (with 
> respect to multithreading), or exposes a bug.
>
> On Wed, Jan 24, 2024 at 9:04 PM Michael Strauß 
> <michaelstrau2 at gmail.com> wrote:
>
>     I am not in favor of option 2 if the implementation was simply "wrap
>     the implementation in runLater", as this would be a surprising change.
>     Consider the following code:
>
>         var transition = new FadeTransition();
>         transition.play();
>
>         // Will always print "RUNNING" currently, but might print
>     "STOPPED"
>         // if we go with the proposed change:
>         System.out.println(transition.getStatus());
>
>     Regardless of the race condition in AbstractPrimaryTimer, this code
>     always seemed to be working quite fine (albeit superficially), because
>     the play/stop/etc. methods change that status of the animation as one
>     would expect.
>
>     You are proposing to change that, such that calling these methods will
>     not predictably change the status of the animation. Instead, these
>     methods now act more like "requests" that may be fulfilled at some
>     later point in time, rather than statements of fact.
>     This is not a change that I think we should be doing on an ad-hoc
>     basis, since the same considerations potentially apply for many
>     methods in many places.
>
>     If we were to allow calling play/stop/etc. on a background thread, I
>     would be in favor of keeping the semantics that these methods
>     instantly and predictably affect the status of the animation. Only the
>     internal operation of adding the animation to AbstractPrimaryTimer
>     should be posted to the FX thread. If that is not possible, this
>     suggests to me that we should choose option 1. Introducing new and
>     surprising semantics to an existing API is not the way to go.
>
>
>     On Wed, Jan 24, 2024 at 7:27 PM Nir Lisker <nlisker at gmail.com> wrote:
>     >
>     > These are the options I see as reasonable:
>     >
>     > 1. Document that these methods *must* be run on the FX thread
>     and throw an exception otherwise. This leaves it to the
>     responsibility of the user. However, this will cause the backwards
>     compatibility problems that Jugen brought up. As a side note, we
>     do this in other methods already, but I always questioned why let
>     the developer do something illegal - if there's only one execution
>     path, force it.
>     > 2. Document that these methods *are* run on the FX thread (the
>     user doesn't need to do anything) and change the implementation to
>     call runLater(...) internally unless they are already on the FX
>     thread. This will be backwards compatible for the most part (see
>     option 3). The downside might be some surprise when these methods
>     behave differently.
>     > 3. Document that it's *recommended* that these methods be run on
>     the FX thread and let the user be responsible for the threading.
>     We can explain that manipulating nodes that are attached to an
>     active scenegraph should be avoided.
>     >
>     > I prefer option 2 over 1 regardless of the backwards
>     compatibility issue even, but would like to know if I'm missing
>     something here because in theory this change could be done to any
>     "must run on the FX thread" method and I question why the user had
>     the option to get an exception.
>     > Option 3 is risky and I wager a guess that it will be used
>     wrongly more often than not. It does allow some (what I would
>     call) valid niche uses. I never did it.
>
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