RFR: 8345348: CSS media feature queries [v13]

[Michael Strauß]

On Mon, 14 Apr 2025 12:44:42 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

>> modules/javafx.graphics/src/main/java/com/sun/javafx/application/preferences/PreferenceProperties.java line 246:
>> 
>>> 244:         }
>>> 245: 
>>> 246:         public synchronized void fireValueChangedIfNecessary() {
>> 
>> I noticed that you made many methods `synchronized` in this class, but I have trouble seeing why this is.  A short explanation may be in order.
>> 
>> So, if I had to guess: it's possible to change the value override and/or the updating of all properties from a different thread, which is not the FX thread.  However, what guarantees are now given to listeners (if any)?  The value changed events can now also be triggered from any thread, unless wrapped in a `Platform.runLater`.
>> 
>> What I mean here is, let's say I listen on `accentColorProperty` of `Platform.Preferences`, on what thread can the change event happen?  The `Platform.Preferences` class does not mention anything special, so I think it is fair to assume it should be on the FX thread, allowing me to make modifications to an active scene graph in my change handler; however that will fail if the change notification was not triggered from the FX thread.
>
> Platform preference change events always happen on the FX thread. The reason for `synchronized` is as follows:
> 
> A `Scene` can be created on any thread as per spec. When the scene is created, the `ScenePreferences` class and its associated properties are instantiated, which in turn subscribe internally to the `Platform.Preferences` properties. The synchronization is to protect the addition and removal of listeners.
> 
> This is not ideal, though. As soon as we're subscribed to `Platform.Preferences`, the `Scene.Preferences` can receive change notifications on the FX thread, which interferes with the mandated capability to be created on any thread.
> 
> I think we have a few options here:
> 1. Don't subscribe scene preferences to platform preferences until the scene is shown. The downside of this is that we don't know the actual values of the preferences up until that point.
> 2. With limited synchronization, fetch the current preference values from the platform when the scene is created, but don't subscribe to change notifications until the scene is shown.
> 3. Specify that scene preference changes can happen on the FX thread, even when the scene is created in a background thread.

I've decided to go with option 2.

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PR Review Comment: https://git.openjdk.org/jfx/pull/1655#discussion_r2047234821