Proposal: Focus Traversal API

[Michael Strauß]

I think it is a good idea to do the non-controversial part of this
feature first.

With regards to the API, I would prefer a single instance method on Node:

    Node moveFocus(FocusTraversalKind);

Where FocusTraversalKind is an enum with values UP, DOWN, LEFT, RIGHT,
NEXT, and PREVIOUS.
Note that it is not called "TraversalDirection", because NEXT and
PREVIOUS are logical types of navigation, not directional ones.
The return value is the node that will receive focus as a result of
this method call.

The name "moveFocus" pairs with "requestFocus" (and we could also
potentially add "predictFocus" to get the node that would receive
focus, without actully changing focus).

Having a single instance method allows for easier composition, as I
can now pass the FocusTraversalKind to methods, which is a bit awkward
to do with method references.

For example, I could write a method:

    /*
     * Moves the focus if allowed.
     * Returns true if the focus changed, false otherwise
     */
    boolean moveFocusIfAllowedByBusinessLogic(Node node,
FocusTraversalKind kind) {
        return switch (kind) {
            case UP, DOWN -> false;
            default -> isAllowed() && node.moveFocus(kind) != node;
        };
    }

If I would instead pass a method reference to
FocusTraversal::traverseNext into my custom method, I'm very limited
in what I can do with it.
For example, I can't switch() on the method reference, I can't change
it with custom logic, etc.


On Fri, Oct 18, 2024 at 10:25 PM Andy Goryachev
<andy.goryachev at oracle.com> wrote:
>
> Dear fellow developers:
>
>
>
> Thank you for providing a lively discussion here in the mailing list, and in the PR.
>
>
>
> A surprisingly large number of objection was voiced, among them in no particular order (please correct me if I missed anything):
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>
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> - no need for the traversal policy API, developers have all the tools they need
>
> - rather than providing Parent.traversalPolicy, the whole thing should be delegated to Scene
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> - no need for custom traversal policies, provide a number of standard policies (e.g. cyclic/logical; or via enum if possible)
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> - introducing a new category of traversal events (later deemed unnecessary)
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> - adding two properties to Parent for directional and logical traversal
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> - desire to be able to configure traversal policy via CSS
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> - focus traversal methods should be instance methods in the Node class
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>
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> Also, the discussion reiterated some of the design problems we have in JavaFX, namely
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>
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> - controls consuming key events that effect no change in the control's state
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> - interference between controls' key handling and Scene's accelerators
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> - undetermined priority of event handlers registered by the skin and the application
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>
>
> I still believe there is a value in providing the traversal policy APIs, but given the general sentiment in the feedback, I think the right approach would be to scale down the proposal to leave only the focus traversal API [0], addressing a single use case of the custom components [1].
>
>
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> I am sad to think that there will be no support for custom traversal policies in JavaFX in the foreseeable future, as it seems highly unlikely that my company will sponsor a major redesign of the focus subsystem.  I hope there is a non-zero chance of the open source community coming together and designing an alternative that satisfies everyone, so maybe not all is lost.
>
>
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> Please let me know what you think of the new proposal.
>
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> Thanks,
>
> -andy
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>
> [0] https://github.com/andy-goryachev-oracle/Test/blob/main/doc/FocusTraversal/FocusTraversal-v2.md
>
> [1] JDK-8091673 Public focus traversal API for use in custom controls
>
> [2] Draft PR https://github.com/openjdk/jfx/pull/1604