RFR: 8349091: Charts: exception initializing in a background thread [v6]

[John Hendrikx]

On Mon, 3 Mar 2025 21:10:39 GMT, Andy Goryachev <angorya at openjdk.org> wrote:

>> modules/javafx.controls/src/main/java/javafx/scene/chart/AreaChart.java line 546:
>> 
>>> 544:             symbol.setAccessibleRole(AccessibleRole.TEXT);
>>> 545:             symbol.setAccessibleRoleDescription("Point");
>>> 546:             symbol.setFocusTraversable(isAccessibilityActive());
>> 
>> So, if I understand correctly, we can't directly do this `bind`:
>> 
>>     symbol.focusTraversableProperty().bind(Platform.accessibilityActiveProperty())
>>     
>> Because on a background thread there may not yet be an initialized `Platform`?  Or perhaps, `Platform` would need to initialize and we don't want to do that yet on a background thread?
>
> The reason we `should not` be binding in this PR is because in part it's **bad design(tm)**: we are creating a zillion of listeners (one per plot data point); a better solution would be to create one listener and use (already existing series) to toggle the data points.
> 
> The other reason is https://bugs.openjdk.org/browse/JDK-8351067 - the `Platform::accessibilityActive` property getter is not thread safe, but even if was, registering a listener with it may cause the change to come in **before** the node becomes a part of the scene graph, leading to unsynchronized multi-threaded access.

Was this solution considered:

    ObservableValue<Boolean> partOfSceneGraph = this.sceneProperty()
      .flatMap(Scene::windowProperty)
      .flatMap(Window::showingProperty)
      .orElse(false);
      
    symbol.focusTraversableProperty()
        .bind(Platform.accessibilityActiveProperty().when(partOfSceneGraph));
        
What the above code does is create a binding on the `when` statement (a listener is added on the `when` result).  However, the `when` property will not add a listener on `Platform.accessibilityActiveProperty()` until `partOfSceneGraph` is `true`.  As soon as it does though, a listener is installed on  `Platform.accessibilityActiveProperty()` and its latest value is set to `focusTraversableProperty` via the bind.

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PR Review Comment: https://git.openjdk.org/jfx/pull/1697#discussion_r1978264839