RFR: 8342703: CSS transition is not started when initial value was not specified

[Michael Strauß]

On Mon, 25 Nov 2024 12:24:06 GMT, Marius Hanl <mhanl at openjdk.org> wrote:

>> When the initial value of a styleable property is not specified in a stylesheet, no transition is started:
>> 
>>     .button {
>>         transition: -fx-opacity 1s;
>>     }
>>     
>>     .button:hover {
>>         -fx-opacity: 0.5;
>>     }
>> 
>> The expected behavior is that a transition is started in this case, since the default value of `-fx-opacity` is 1.
>> 
>> The reason for this bug is that `StyleableProperty` implementations do not start a CSS transition when the value is applied for the first time. The intention behind this is that a node that is added to the scene graph should not start transitions. CSS transitions should only be started _after_ the node has been shown for the first time.
>> 
>> The logic to detect this situation is currently as follows:
>> 
>>     // If this.origin == null, we're setting the value for the first time.
>>     // No transition should be started in this case.
>>     TransitionDefinition transition = this.origin != null && getBean() instanceof Node node ?
>>         NodeHelper.findTransitionDefinition(node, getCssMetaData()) : null;
>> 
>> 
>> However, this does not work. When no initial style is specified in the stylesheet, `this.origin` will not be set, and thus no transition will be started even after the node has been shown. The new logic works like this:
>> 
>> A `Node.initialCssState` flag is added. Initially, this is `true`. Manually calling `applyCss` or similar methods will not clear this flag, as we consider all manual CSS processing to be part of the "initial CSS state". Only at the end of `Scene.doCSSPass` will this flag be cleared on all nodes that have expressed their interest. This mechanism ensures that a node will be eligible for CSS transitions only after the following conditions have been satisfied:
>> 1. The node was added to a scene graph
>> 2. CSS processing was completed for a scene pulse
>
> modules/javafx.graphics/src/main/java/javafx/scene/Scene.java line 607:
> 
>> 605:      * CSS state (see {@link Node#initialCssState}.
>> 606:      */
>> 607:     private final Set<Node> clearInitialCssStateNodes = Collections.newSetFromMap(new IdentityHashMap<>());
> 
> why not use a normal `HashSet`? What advantage has this approach?

`HashSet` uses `Object.equals`, which can be overridden by user code, and this would break the logic. It's the _instance_ that wants to be notified.

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PR Review Comment: https://git.openjdk.org/jfx/pull/1607#discussion_r1856544027