RFR: 8343956: Focus delegation API

[John Hendrikx]

On Sat, 9 Nov 2024 01:28:53 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

> Implementation of [focus delegation](https://gist.github.com/mstr2/44d94f0bd5b5c030e26a47103063aa29).

This looks really good.  I'm wondering if this could be simplified further.  Specifically, I think the `hoistFocus` flag and manual management of the focus delegate may not be needed.

It seems to me that a Control could share some similarities with a Scene, in that Control has properties that track a focus owner (similar to focus delegate). In effect, a Control is a focus root similar to scene.  When a Scene receives focus, it determines the best Node to "delegate" focus to; similarly, when a Control receives focus, it determines which skin control should be focused.  The normal focus rules should do the right thing here and for example select the TextField of a Spinner as the delegate automatically (some children may need to be marked as not focusable to guide the auto selection, but this is an already existing standard mechanism).

When determining where to send events, if the target is a focus root, it queries its focus owner (or focus delegate) and extends the event to that target.  If that target is also a focus root, the process repeats.

The request focus function should operate differently as well.  It should look for the closest focus root (a Control or Scene) and call the appropriate request focus function on the root it finds.  If that root is Scene, everything works as usual.  If it is another focus root like a Control, Control can determine the best way to focus one of its child nodes (likely you can just apply a normal search for an eligible focusable control for this).

Perhaps the focus root functionality can be captured in an interface that both Scene and Control implement.  I think it would need to specify a `requestFocus` method and `focusOwnerProperty`.  This interface would then replace the `focusScope` flag.

> > It works quite well to say that a `Scene` delegates focus to a `Node`. It doesn't seem to work quite as well (linguistically) to say that a `TextField` is the focus owner of a `ComboBox` (the `ComboBox` is also focused, why would the `TextField` be its focus owner?).
> 
> For what it's worth I am not a fan of using the term focus owner to refer to a focus delegate. The focusOwner of a Scene is the target of key events. The focus delegate of a ComboBox is the _second_ target of key events. Key events are sent to the ComboBox first and the delegate only receives the events that it doesn't consume. To me that's an important distinction.

I don't think the distinction quite holds.  Scene does not delegate all key events.  Menu shortcuts for example are consumed and never dispatched, and I think the same goes for mnemonics.  Navigation keys are dispatched, and only acted upon by Scene when bubbled back up.

However, I now think that "focus delegate" is indeed the better name for this.
 
> Presumably we would need a way of telling the focus scope node which sub-node hoisted the focus so it could select the correct delegate. But it's difficult to imagine a control that's trying to pass itself off as a monolithic entity having two internal TextFields. Wouldn't that require enabling keyboard traversal inside a monolithic control? What does that mean? I don't think this is a scenario we need to solve.

See my example in my reply to Michael, I think it may be worth having a few thoughts about.

Keep open IMHO.

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PR Comment: https://git.openjdk.org/jfx/pull/1632#issuecomment-2569018506
PR Comment: https://git.openjdk.org/jfx/pull/1632#issuecomment-2646636802
PR Comment: https://git.openjdk.org/jfx/pull/1632#issuecomment-3027816893