How to write a KeyListener for JavaFX

Question

I want to write a little game where I can move a ball on a JavaFX Panel using the W, A, S, D keys.
I have a getPosX() and setPosX() but I don't know how to write a KeyListener which will e.g. calculate setPosX(getPosX()+1) if I press D.

What do I have to do?

Answer 1

From a JavaRanch Forum post.

Key press and release handlers are added on the scene and update movement state variables recorded in the application. An animation timer hooks into the JavaFX pulse mechanism (which by default will be capped to fire an event 60 times a second) - so that is a kind of game "loop". In the timer the movement state variables are checked and their delta actions applied to the character position - which in effect moves the character around the screen in response to key presses.

import javafx.animation.AnimationTimer;
import javafx.application.Application;
import javafx.event.EventHandler;
import javafx.scene.*;
import javafx.scene.image.*;
import javafx.scene.input.KeyEvent;
import javafx.scene.paint.Color;
import javafx.stage.Stage;

/**
 * Hold down an arrow key to have your hero move around the screen.
 * Hold down the shift key to have the hero run.
 */
public class Runner extends Application {

    private static final double W = 600, H = 400;

    private static final String HERO_IMAGE_LOC =
            "http://icons.iconarchive.com/icons/raindropmemory/legendora/64/Hero-icon.png";

    private Image heroImage;
    private Node  hero;

    boolean running, goNorth, goSouth, goEast, goWest;

    @Override
    public void start(Stage stage) throws Exception {
        heroImage = new Image(HERO_IMAGE_LOC);
        hero = new ImageView(heroImage);

        Group dungeon = new Group(hero);

        moveHeroTo(W / 2, H / 2);

        Scene scene = new Scene(dungeon, W, H, Color.FORESTGREEN);

        scene.setOnKeyPressed(new EventHandler<KeyEvent>() {
            @Override
            public void handle(KeyEvent event) {
                switch (event.getCode()) {
                    case UP:    goNorth = true; break;
                    case DOWN:  goSouth = true; break;
                    case LEFT:  goWest  = true; break;
                    case RIGHT: goEast  = true; break;
                    case SHIFT: running = true; break;
                }
            }
        });

        scene.setOnKeyReleased(new EventHandler<KeyEvent>() {
            @Override
            public void handle(KeyEvent event) {
                switch (event.getCode()) {
                    case UP:    goNorth = false; break;
                    case DOWN:  goSouth = false; break;
                    case LEFT:  goWest  = false; break;
                    case RIGHT: goEast  = false; break;
                    case SHIFT: running = false; break;
                }
            }
        });

        stage.setScene(scene);
        stage.show();

        AnimationTimer timer = new AnimationTimer() {
            @Override
            public void handle(long now) {
                int dx = 0, dy = 0;

                if (goNorth) dy -= 1;
                if (goSouth) dy += 1;
                if (goEast)  dx += 1;
                if (goWest)  dx -= 1;
                if (running) { dx *= 3; dy *= 3; }

                moveHeroBy(dx, dy);
            }
        };
        timer.start();
    }

    private void moveHeroBy(int dx, int dy) {
        if (dx == 0 && dy == 0) return;

        final double cx = hero.getBoundsInLocal().getWidth()  / 2;
        final double cy = hero.getBoundsInLocal().getHeight() / 2;

        double x = cx + hero.getLayoutX() + dx;
        double y = cy + hero.getLayoutY() + dy;

        moveHeroTo(x, y);
    }

    private void moveHeroTo(double x, double y) {
        final double cx = hero.getBoundsInLocal().getWidth()  / 2;
        final double cy = hero.getBoundsInLocal().getHeight() / 2;

        if (x - cx >= 0 &&
            x + cx <= W &&
            y - cy >= 0 &&
            y + cy <= H) {
            hero.relocate(x - cx, y - cy);
        }
    }

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