animatedBind

gistfile1.java

    public static class AnimatedBindInfo {
        @Nullable public Timeline timeline;
        public NumberBinding bindFrom;
        public Runnable onAnimFinish;
    }

    public static AnimatedBindInfo animatedBind(Node node, WritableDoubleValue bindTo, NumberBinding bindFrom) {
        bindTo.set(bindFrom.doubleValue());   // Initialise.
        bindFrom.addListener((o, prev, cur) -> {
            AnimatedBindInfo info = (AnimatedBindInfo) node.getUserData();
            if (info.timeline != null)
                info.timeline.stop();
            info.timeline = new Timeline(new KeyFrame(UI_ANIMATION_TIME, new KeyValue(bindTo, cur)));
            info.timeline.setOnFinished(ev -> {
                ((AnimatedBindInfo) node.getUserData()).timeline = null;
                if (info.onAnimFinish != null)
                    info.onAnimFinish.run();
            });
            info.timeline.play();
        });
        // We must pin bindFrom into the object graph, otherwise something like:
        //    animatedBind(node, node.opacityProperty(), when(a).then(1).otherwise(2))
        // will mysteriously stop working when the result of when() gets garbage collected and the listener with it.
        AnimatedBindInfo info = new AnimatedBindInfo();
        info.bindFrom = bindFrom;
        node.setUserData(info);
        return info;
    }

Really nice utility!

Regarding the comment in the code, wouldn't it be sufficient to require the user to store a reference to AnimatedBindInfo to keep the animation working? This would eliminate the need to pass in a Node altogether. The listener would have to be a closure over the AnimatedBindInfo instance instead of over the node.

If you find pinning it to a Node a convenient way to keep the reference, there could be a convenient method AnimatedBindInfo.pin(Node node) to do this.