RFR: 8343956: Focus delegation API [v3]

[Michael Strauß]

On Thu, 17 Jul 2025 05:18:28 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

>> Implementation of [focus delegation](https://gist.github.com/mstr2/44d94f0bd5b5c030e26a47103063aa29).
>
> Michael Strauß has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Avoid variable reassignment

> _Mailing list message from [Andy Goryachev](mailto:andy.goryachev at oracle.com) on [openjfx-dev](mailto:openjfx-dev at mail.openjdk.org):_
> 
> I think I narrowed down my objection to this proposal. My main problem with it is that it tries to add properties to the wrong entity (Node).
> 
> In a typical application, the focus traversal requirements usually come in the context of a higher level aggregate such as a form, or a panel, or a window, and not the individual nodes. An example from Swing and AWT is the FocusTraversalPolicy class. This is no accident - the focus traversal might be dynamically dependent on other aspects of the UI state, for example the kind of information already present in a form.
> 
> Trying to implement it at the Node level would be extremely difficult, and one will inevitably resolve to creating a kind of controller that determines the traversal order at the higher level.
> 
> A better approach, in my opinion, would be to revive the FocusTraversalPolicy API [0], [1] (after some modernization). The most important feature that would be added is enabling ability to plug in a custom traversal policy that can address any application-level requirements with relative ease.

I don't think that these two proposals are at odds, in fact they are completely orthogonal. Focus delegation has nothing to do with focus traversal. We're not trying to answer the question what happens when (or how) we move the input focus to another control.

Focus delegation is an implementation of multi-level focus for composite controls. It spans different levels of abstraction, something that JavaFX already attempts to do right now, but fails due to a lack of supporting infrastructure. I don't see how a customizable focus traversal API addresses _any_ of the two major improvements that focus delegation brings to the table:

1. A consistent focus delegation chain: focusing a descendant control keeps the outer control focused, which is what would be expected in a black-box control model.
2. Events are not duplicated, they are logically consistent for all observers.

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PR Comment: https://git.openjdk.org/jfx/pull/1632#issuecomment-3138275955