RFR: 8345348: CSS media feature queries [v21]

[Michael Strauß]

On Tue, 6 May 2025 21:03:14 GMT, Andy Goryachev <angorya at openjdk.org> wrote:

>> 1. That's how it is implemented. No change events before the scene is shown (only initial values are read).
>> 2. Now this would be a direct contradiction of the specification, which would make the user model much harder. Suddenly the "you can create and modify a scene on a background thread" becomes "you can do that, but for this particular API, you can't". Since we're doing all of this mainly for application developers, the user model must be the first consideration.
>
> PlatformPreferences.update() seems to be called only from the fx thread, if I read the code right.  What happens when the Scene is created in a bg thread and a listener added to its preferences, and an event is fired?  Will the statement `#1` be still valid?  (I don't see any code that delays the dispatch until after the Scene is shown).
> 
> `#2` - all this is good, but we seem to hand the gun to user and say: here, shoot yourself in the foot.  And they do.  I think it (creating things in a background thread) was a mistake in the first place, but there seems to be zero desire to change it, so who am I to say otherwise?

`NullCoalescingPropertyBase` (used in `ScenePreferences`) is initially disconnected. It will set its initial value to the value is reads from `PlatformPreferences`, but it will not subscribe to change notifications. The reason for this is to prevent a scene that is created on a background thread from immediately receiving change notifications on the FX thread, which could not only potentially corrupt the property state (because they're not concurrently thread-safe), but also mess up the application state.

When the scene is shown, `NullCoalescingPropertyBase` will subscribe to change notifications from `PlatformPreferences`, because now we're safely on the FX thread. The code for that is in ScenePreferences:L142.

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PR Review Comment: https://git.openjdk.org/jfx/pull/1655#discussion_r2077196617