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

[Kevin Rushforth]

The point you raise is one reason why I wouldn't advocate doing this in 
other places, e.g., for scene graph updates, which do need to run 
synchronously.

I note that the Animation docs currently state that these operations are 
asynchronous (and yes, I know you were proposing the change this). So 
while it might be surprising to some apps, it is within the current 
spec, and I'm not sure it is any more surprising than the exception we 
now throw in JavaFX 22.

If we could safely do what you suggest, and just wrap the part that 
touches the AbstractPrimaryTimer, that might be the best option. I don't 
know if that is feasible, though, especially going from a running state 
to a stopped or paused state.

-- Kevin


On 1/24/2024 11:04 AM, Michael Strauß 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.