RFR: 8332895: Support interpolation for backgrounds and borders [v29]

[Michael Strauß]

On Tue, 3 Sep 2024 17:09:51 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

>> This PR completes the CSS Transitions story (see #870) by adding interpolation support for backgrounds and borders, making them targetable by transitions.
>> 
>> `Background` and `Border` objects are deeply immutable, but not interpolatable. Consider the following `Background`, which describes the background of a `Region`:
>> 
>> 
>> Background {
>>     fills = [
>>         BackgroundFill {
>>             fill = Color.RED
>>         }
>>     ]
>> }
>> 
>> 
>> Since backgrounds are deeply immutable, changing the region's background to another color requires the construction of a new `Background`, containing a new `BackgroundFill`, containing the new `Color`.
>> 
>> Animating the background color using a CSS transition therefore requires the entire Background object graph to be interpolatable in order to generate intermediate backgrounds.
>> 
>> More specifically, the following types will now implement `Interpolatable`.
>> 
>> - `Insets`
>> - `Background`
>> - `BackgroundFill`
>> - `BackgroundImage`
>> - `BackgroundPosition`
>> - `BackgroundSize`
>> - `Border`
>> - `BorderImage`
>> - `BorderStroke`
>> - `BorderWidths`
>> - `CornerRadii`
>> - `Stop`
>> - `Paint` and all of its subclasses
>> - `Margins` (internal type)
>> - `BorderImageSlices` (internal type)
>> 
>> ## Interpolation of composite objects
>> 
>> As of now, only `Color`, `Point2D`, and `Point3D` are interpolatable. Each of these classes is an aggregate of `double` values, which are combined using linear interpolation. However, many of the new interpolatable classes comprise of not only `double` values, but a whole range of other types. This requires us to more precisely define what we mean by "interpolation".
>> 
>> Mirroring the CSS specification, the `Interpolatable` interface defines several types of component interpolation:
>> 
>> | Interpolation type | Description |
>> |---|---|
>> | default | Component types that implement `Interpolatable` are interpolated by calling the `interpolate(Object, double)}` method. |
>> | linear | Two components are combined by linear interpolation such that `t = 0` produces the start value, and `t = 1` produces the end value. This interpolation type is usually applicable for numeric components. |
>> | discrete | If two components cannot be meaningfully combined, the intermediate component value is equal to the start value for `t < 0.5` and equal to the end value for `t >= 0.5`. |
>> | pairwise | Two lists are combined by pairwise interpolation. If the start list has fewer elements than the target list, the...
>
> Michael Strauß has updated the pull request incrementally with one additional commit since the last revision:
> 
>   refactoring

I've made some changes as follows:

1. `TransitionTimer.run` returns a `CancellationToken` object instead of a timer reference, which is only used to cancel the timer. This removes all usages of `TransitionTimer` from `TransitionMediator` (except for the single call to the `run` method).
2. The `cancel` method doesn't use a `forceStop` flag any more, which also removes tracking of `updating` flags from implementations. Instead, `TransitionMediator.onUpdate` now directly calls the `super.set` method to avoid cancelling itself.
3. The conditional clearing of the `mediator` field is removed, it is now cleared unconditionally. To make this work, `TransitionTimer` internally detects situations when the field shouldn't be cleared, and then elides the call to `TransitionMediator.onStop`. This avoids having implementations follow a bespoke clearing protocol.
4. The methods that effectively stop a transition timer have been reduced to `stop`, `complete`, and `interrupt`, each having a precise definition.

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PR Comment: https://git.openjdk.org/jfx/pull/1522#issuecomment-2327046946