A JavaFX application that can be programmatically launched to display a given Node (for testing)

Preview.java

import io.reactivex.observers.DisposableObserver;
import io.reactivex.subjects.PublishSubject;
import io.reactivex.subjects.Subject;
import io.vavr.control.Option;
import javafx.application.Application;
import javafx.beans.value.ChangeListener;
import javafx.scene.Group;
import javafx.scene.Node;
import javafx.scene.Scene;
import javafx.scene.layout.Region;
import javafx.stage.Stage;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.lang.invoke.MethodHandles;
import java.util.function.Supplier;

import static io.vavr.API.None;
import static io.vavr.API.Some;

public class Preview extends Application {

  // ------------------------------------------------------------
  // Class fields

  private static final Logger logger = LoggerFactory.getLogger(MethodHandles.lookup().lookupClass());
  private static Subject<Preview> previews = PublishSubject.create();

  // ------------------------------------------------------------
  // Instance fields

  private final Group root;
  private Option<Stage> stage = None();

  // ------------------------------------------------------------
  // Constructor

  public Preview() {
    this.root = new Group();
  }

  // ------------------------------------------------------------
  // Application

  @Override
  public void start(Stage stage) {
    this.stage = Some(stage);
    Scene scene = new Scene(root);
    stage.setScene(scene);
    previews.onNext(this);
    stage.show();
    stage.sizeToScene();
  }

  // ------------------------------------------------------------
  // Public methods

  /**
   * Launches a preview window that will display the specified node.
   *
   * @param node A {@code Supplier} of the node to be displayed. Guaranteed
   *             to be called on the JavaFX platform thread.
   */
  public static void preview(Supplier<Node> node) {
    previews.subscribeWith(new SetNodeObserver(node));
    launch(Preview.class);
  }

  // ------------------------------------------------------------
  // Private methods

  /**
   * Sets the specified node as the only child of the root group. If the
   * node is a {@code Region}, also adds a listener that will resize the
   * stage when the region's preferred size changes.
   *
   * @param node The node.
   */
  private void setNode(Node node) {
    root.getChildren().setAll(node);
    if (node instanceof Region) {
      Region region = (Region) node;
      ChangeListener<Number> resizeListener = (o, v0, v1) -> stage.forEach(Stage::sizeToScene);
      region.prefHeightProperty().addListener(resizeListener);
      region.prefWidthProperty().addListener(resizeListener);
    }
  }

  // ------------------------------------------------------------
  // Helper classes

  /**
   * A one-shot observer that will call {@code setNode()} on an observed {@code Preview}
   * and then {@code dispose()} itself.
   */
  private static class SetNodeObserver extends DisposableObserver<Preview> {

    private final Supplier<Node> node;

    SetNodeObserver(Supplier<Node> node) {
      this.node = node;
    }

    @Override
    public void onNext(Preview preview) {
      preview.setNode(node.get());
      dispose();
    }

    @Override
    public void onError(Throwable e) {
      logger.error("Observed error", e);
      dispose();
    }

    @Override
    public void onComplete() {
      // does nothing
    }
  }
}