[External] : Re: Prioritized event handlers

Tue Oct 31 20:03:43 UTC 2023

Dear Martin:

I understand John’s point: you want all the user installed filters and handlers across all levels to be processed before switching to system level processing. I also understand this prioritization proposal is designed to push our existing set of system handlers to a separate phase. But is that all we’re talking about here? I need some clarification.

If I understand this correctly, this proposal advocates making drastic changes to the event handling mechanism.  Not only it introduces the prioritization scheme (which I support), but it also alters the way events are bubbled up, by introducing waves/phases.  I am afraid I don’t see the use case for doing that.  In my opinion, the prioritization scheme should work on handlers added to the same EventTarget.  There should be no multiple waves - if an event gets consumed by an event handler, the dispatching should stop (that change I also support), if not - it bubbles up the hierarchy.

Within a given control the order of event processing gets involved. If a Control is subclassed the subclass should get first shot at the event. The same is true for Behaviors and Skins. Beyond that I’m still not clear if the behavior or skin should get the event first or if the skin should get it via the behavior or the other way around. In any case, you’ve got the control, the behavior, the skin, and all of their subclasses trying to sort out the execution order.

Hmmm.  I am not sure I understand exactly what you are saying.

The way I understand this prioritization proposal is that we introduce a priority associated with the handler (and not the filter).  The proposal specifies 3 priority levels, I think there might be more, or perhaps we even have an int priority (might be too much freedom, let’s discuss).

Specifically, I think there might be 5 levels, from high to low (the names are just for the purposes of discussion):

{ AppHigh, SkinHigh, AppMedium, SkinLow, AppLow }

This way there is absolutely no ambiguity in deciding which handler gets invoked first.  I must note that SkinHigh, SkinLow should not be available to the application code.  At the same time, AppHigh,AppMedium,AppLow levels should not be available to skins/behaviors.

Side question: should filters also have priority?

Based on this discussion (and I might be mistaken on this) it sounds like you're trying to handle all this using this proposal, namely registering event handlers with a prioritization scheme.

Wait, I though that’s what you are proposing, based on the doc https://gist.github.com/mstr2/4bde9c97dcf608a0501030ade1ae7dc1

Are you proposing something else, or is something missing from the doc?

Wouldn’t it be easier to just grab the event and pass it around using Java method calls? Perhaps the call is handleEvent(). A control implements handleEvent() by passing the event off to the behavior’s handleEvent() which passes it off to the skin’s handleEvent(). The skin sends it up the superclass chain by calling super.handleEvent(), etc. so on.

Wouldn’t this be a drastic departure from the established event handling method?  Why call an empty method if we are not interested in the event?  Or maybe I misunderstood you here, what are you asking?

To summarize, I think the idea of explicit priority is a good idea as it solves the current issue or exact ordering of handlers in the event of skin change, to give one example.  I think we might also benefit from a limited set of priorities (5) that reflect the reality of fx having two sides - the application side and the “system”, or skin/behavior, side.

Personally, I find some other ideas problematic: I don’t see a good use case for multiple waves in dispatching, as this represent a major departure from the current mechanism, unless I am missing something.

I don’t understand the paragraph about subclassing.  Perhaps you mean that if we have a situation where one class extends the other, they should coordinate the event handling.  For example, the base class would declare the handling method, register it as a listener, for the child class to override and get the events?  Or the base class should not add any handlers, instead leaving it up to the child class?  But that’s implementation detail.

What do you think?

-andy

From: Martin Fox <martin at martinfox.com>
Date: Tuesday, October 31, 2023 at 10:13
To: Andy Goryachev <andy.goryachev at oracle.com>
Cc: Michael Strauß <michaelstrau2 at gmail.com>, openjfx-dev <openjfx-dev at openjdk.org>
Subject: [External] : Re: Prioritized event handlers
I understand John’s point: you want all the user installed filters and handlers across all levels to be processed before switching to system level processing. I also understand this prioritization proposal is designed to push our existing set of system handlers to a separate phase. But is that all we’re talking about here? I need some clarification.

Within a given control the order of event processing gets involved. If a Control is subclassed the subclass should get first shot at the event. The same is true for Behaviors and Skins. Beyond that I’m still not clear if the behavior or skin should get the event first or if the skin should get it via the behavior or the other way around. In any case, you’ve got the control, the behavior, the skin, and all of their subclasses trying to sort out the execution order.

Based on this discussion (and I might be mistaken on this) it sounds like you're trying to handle all this using this proposal, namely registering event handlers with a prioritization scheme. Wouldn’t it be easier to just grab the event and pass it around using Java method calls? Perhaps the call is handleEvent(). A control implements handleEvent() by passing the event off to the behavior’s handleEvent() which passes it off to the skin’s handleEvent(). The skin sends it up the superclass chain by calling super.handleEvent(), etc. so on.

This would make for an easy sell to outside developers. We can tell them that if they subclass a Control and implement handleEvent() they will get events first during the system phase. The same is true if they subclass a behavior or skin. They don’t need to buy into or even see a complicated event prioritization scheme to get exactly what they expect, namely first access to events.

But, again, maybe I’m off base here. Let me know.

Martin

On Oct 30, 2023, at 12:53 PM, Andy Goryachev <andy.goryachev at oracle.com> wrote:

Dear Michael:

Thank you, this is very helpful.

Questions/Comments:

1. Does this proposal changes the way events are dispatched with respect to priority?  In other words, does it first go through the list of all handlers registred on the leaf Node (high priority first, then lower, then lowest), then bubble up?  Or do they propagate upwards looking for high priority handlers first, then the process restarts for lower priorities, as I saw in some previous emails?  (I could be mistaken)

2. Do you propose to abort event dispatching immediately after the event is consumed?  This probably should be mentioned earlier in the Motivation (the problem statement) section.

3. I wonder if three priority levels are sufficient.  Let me explain.  We have two possible actors who can register an event listener: the application code and the FX (or, rather more specifically, the skin and its behavior, whatever that might be).

Application code might want to add handlers at three possible priorities:

  *   App handler must always be called before any fx handler
  *   App hander does not care
  *   App handler must always be called after any fx handlers

For fx/skin handlers we might have fewer levels:

  *   Skin handler does not care
  *   Skin handler must be called after all other skin handlers

This situation maps to 5 priorities and 4 effective levels (or 5).

We should also mention the fact that when any actor adds two or more handlers for the same event with the same priority, they get invoked in the order added.

Would you agree, or am I missing some critical aspect of the proposed solution?

Thank you
-andy

From: openjfx-dev <openjfx-dev-retn at openjdk.org> on behalf of Michael Strauß <michaelstrau2 at gmail.com>
Date: Friday, October 27, 2023 at 19:41
To: openjfx-dev <openjfx-dev at openjdk.org>
Subject: Re: Prioritized event handlers
Here is the proposal:
https://gist.github.com/mstr2/4bde9c97dcf608a0501030ade1ae7dc1<https://urldefense.com/v3/__https:/gist.github.com/mstr2/4bde9c97dcf608a0501030ade1ae7dc1__;!!ACWV5N9M2RV99hQ!NvO4B-fpHrjczoDGCoctorfNPX48w38MvW-LOf6ElCk0dBqFX_xPlETcr56POnEaBcwENrIOsX4OKDM0OGc07A$>

Comments are welcome.

On Fri, Oct 27, 2023 at 8:21 PM Andy Goryachev
<andy.goryachev at oracle.com> wrote:
>
> Would it be possible to create a proposal in the JEP format outlining the proposed public API?
>
>
>
> Thank you
>
> -andy

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/openjfx-dev/attachments/20231031/2fcbfa61/attachment-0001.htm>