在JavaFX 8中评估选择绑定时发生异常

Question

Here is an example from Pro JavaFx 8:

    package projavafx.reversi.examples;

    import javafx.application.Application;
import javafx.beans.binding.Bindings;
import javafx.geometry.Pos;
import javafx.scene.Node;
import javafx.scene.Scene;
import javafx.scene.effect.DropShadow;
import javafx.scene.effect.InnerShadow;
import javafx.scene.layout.*;
import javafx.scene.paint.Color;
import javafx.scene.paint.CycleMethod;
import javafx.scene.paint.RadialGradient;
import javafx.scene.paint.Stop;
import javafx.scene.shape.Ellipse;
import javafx.scene.text.Font;
import javafx.scene.text.FontWeight;
import javafx.scene.text.Text;
import javafx.stage.Stage;
import projavafx.reversi.model.Owner;
import projavafx.reversi.model.ReversiModel;
    /**
     * @author Stephen Chin <steveonjava@gmail.com>
     */
    public class BorderLayoutExample extends Application {

        TilePane scoreTiles;
        TilePane titleTiles;

        public static void main(String[] args) {
                                                     launch(args);
                                                                      }

        @Override
        public void start(Stage primaryStage) {
            BorderPane borderPane = new BorderPane();
            borderPane.setTop(createTitle());
            borderPane.setCenter(createBackground());
            borderPane.setBottom(createScoreBoxes());
            Scene scene = new Scene(borderPane, 600, 400);
            primaryStage.setScene(scene);
            primaryStage.show();
           // scoreTiles.prefTileWidthProperty().bind(Bindings.selectDouble(scoreTiles.parentProperty(), "width").divide(2));
           // titleTiles.prefTileWidthProperty().bind(Bindings.selectDouble(titleTiles.parentProperty(), "width").divide(2));

        }

        private Node createTitle() {
            StackPane left = new StackPane();
            left.setStyle("-fx-background-color: black");
            Text text = new Text("JavaFX");
            text.setFont(Font.font(null, FontWeight.BOLD, 18));
            text.setFill(Color.WHITE);
            StackPane.setAlignment(text, Pos.CENTER_RIGHT);
            left.getChildren().add(text);
            Text right = new Text("Reversi");
            right.setFont(Font.font(null, FontWeight.BOLD, 18));
            titleTiles = new TilePane();
            titleTiles.setSnapToPixel(false);
            TilePane.setAlignment(right, Pos.CENTER_LEFT);
            titleTiles.getChildren().addAll(left, right);
            titleTiles.setPrefTileHeight(40);
            titleTiles.prefTileWidthProperty().bind(Bindings.selectDouble(titleTiles.parentProperty(), "width").divide(2));
            return titleTiles;
        }

        private Node createBackground() {
            Region answer = new Region();
            RadialGradient rg = new RadialGradient(225, 0, 0, 0, 1, true, CycleMethod.NO_CYCLE,
                    new Stop(0.0, Color.WHITE),
                    new Stop(1.0, Color.GRAY)
            );
            answer.setBackground(new Background(new BackgroundFill(rg, null, null)));
            //       answer.setStyle("-fx-background-color: radial-gradient(radius 100%, white, gray)");
            return answer;
        }

        private Node createScoreBoxes() {
            scoreTiles = new TilePane(createScore(Owner.BLACK), createScore(Owner.WHITE));
            scoreTiles.setSnapToPixel(false);
            scoreTiles.setPrefColumns(2);
            scoreTiles.prefTileWidthProperty().bind(Bindings.selectDouble(scoreTiles.parentProperty(), "width").divide(2));

            return scoreTiles;
        }

        private Node createScore(Owner owner) {
            Region background;
            Ellipse piece = new Ellipse(32, 20);
            piece.setFill(owner.getColor());
            DropShadow pieceEffect = new DropShadow();
            pieceEffect.setColor(Color.DODGERBLUE);
            pieceEffect.setSpread(.2);
            piece.setEffect(pieceEffect);

            Text score = new Text();
            score.setFont(Font.font(null, FontWeight.BOLD, 100));
            score.setFill(owner.getColor());
            Text remaining = new Text();
            remaining.setFont(Font.font(null, FontWeight.BOLD, 12));
            remaining.setFill(owner.getColor());
            VBox remainingBox = new VBox(10, piece, remaining);
            remainingBox.setAlignment(Pos.CENTER);
            FlowPane flowPane = new FlowPane(20, 10, score, remainingBox);
            flowPane.setAlignment(Pos.CENTER);
            background = new Region();
            background.setStyle("-fx-background-color: " + owner.opposite().getColorStyle());
            ReversiModel model = ReversiModel.getInstance();
            StackPane stack = new StackPane(background, flowPane);
            InnerShadow innerShadow = new InnerShadow();
            innerShadow.setColor(Color.DODGERBLUE);
            innerShadow.setChoke(.5);
            background.effectProperty().bind(Bindings.when(model.turn.isEqualTo(owner))
                    .then(innerShadow)
                    .otherwise((InnerShadow) null));
            DropShadow dropShadow = new DropShadow();
            dropShadow.setColor(Color.DODGERBLUE);
            dropShadow.setSpread(.2);

            piece.effectProperty().bind(Bindings.when(model.turn.isEqualTo(owner))
                    .then(dropShadow)
                    .otherwise((DropShadow) null));
            score.textProperty().bind(model.getScore(owner).asString());
            remaining.textProperty().bind(model.getTurnsRemaining(owner).asString().concat(" turns remaining"));
            return stack;
        }
    }

A warning pops up in the console when running this app:

      sept. 20, 2015 11:07:03 AM com.sun.javafx.binding.SelectBinding$SelectBindingHelper getObservableValue
      WARNING: Exception while evaluating select-binding [width]
      sept. 20, 2015 11:07:03 AM com.sun.javafx.binding.SelectBinding$SelectBindingHelper getObservableValue
      WARNING: Exception while evaluating select-binding [width]

What went wrong here?

Answer 1

The problem is the following bit of code in the method createTitle() :

titleTiles.prefTileWidthProperty().bind(
    Bindings.selectDouble(
        titleTiles.parentProperty(), "width").divide(2));

At this moment, the titleTiles have not yet been added to the borderPane , so the value of the parentProperty is null, hence the width property can not be found on it.

Same in createScoreBoxes().

Next time, though, it would be nice, if you cut down your sample code a bit, especially remove references to classes from your project (import projavafx.reversi.model.ReversiModel;), do that one can paste it into his IDE and run it right away.