RFR: 8313424: JavaFX controls in the title bar [v21]

[Michael Strauß]

On Tue, 5 Nov 2024 21:02:28 GMT, Andy Goryachev <angorya at openjdk.org> wrote:

>> The goal of the JEP is to allow fully customizable header bars (excluding window buttons). `HeaderBarBase` is the basic building block that is required for the headerbar-specific features like dragging, double clicking, and so on. This class is special because its functionality can't be replicated by user code.
>> 
>> For maximum customizability, `HeaderBarBase` is just a `Region`. Subclasses are free to implement any kind of control on top of that. This is what `HeaderBar` is: an implementation on top of the basic functionality, offering a pretty useful API and layout behavior. It covers many common use cases with the separation between a leading, center, and trailing area.
>> 
>> But special applications might have special requirements. For example, the Spotify desktop app selectively hides items in its header bar depending on the available space, and sometimes dynamically changes the alignment of items. This would be a case where an application would use a custom implementation of `HeaderBarBase`.
>
> thanks for explanation.  but does it really warrant a base class, couldn't the app either set different components inside the existing HB, or maybe set a different HB?
> 
> is there anything that HeaderBar does that may not be needed in a custom title bar?
> 
> it just feels that HBBase is not needed.

We need to think of this in terms of the user model. `HeaderBar` gives rise to three named areas (leading, center, trailing) and comes with a certain layout behavior for each of those areas.

Sure, it is possible to place another layout container in the `HeaderBar.center` area and set its prefWidth to infinity in order to effectively get rid of all of the custom layout behavior that `HeaderBar` brings to the table.

But that severely limits application and library developers from customizing the user model of the header bar. For example, using `HeaderBarBase`, a library developer can create alternative header bars with a different set of features and a different user model. Those alternative header bars might have different layout areas, different properties, in short a different API. It would be really awkward to shoehorn all of that into the (opinionated) API of `HeaderBar`.

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PR Review Comment: https://git.openjdk.org/jfx/pull/1605#discussion_r1830013578