Trajectories.java

package org.jamesd.examples.trajectories;

import javafx.animation.AnimationTimer;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.scene.control.Label;
import javafx.scene.control.Spinner;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.HBox;
import javafx.scene.layout.Pane;
import javafx.scene.paint.Color;
import javafx.stage.Stage;

import java.util.*;

public class Trajectories extends Application {

    private GraphicsContext gc ;

    private final double radius = 3 ;

    class Ball {
        private double x ;
        private double y ;
        private final double xVel ;
        private final double yVel ;

        private final Color color ;
        Ball(double x, double y, double xVel, double yVel, Color color) {
            this.x = x ;
            this.y = y ;
            this.xVel = xVel ;
            this.yVel = yVel ;
            this.color = color ;
        }

        public double x() { return x ;}
        public double y() { return y ;}

        public Color color() { return color ;}
        public void update(double time) {
            x = x + xVel * time ;
            y = y + yVel * time ;
            if (x < -radius) x = gc.getCanvas().getWidth() + radius;
            if (x > gc.getCanvas().getWidth() + radius) x = -radius;
            if (y < -radius) y = gc.getCanvas().getHeight() + radius;
            if (y > gc.getCanvas().getHeight() + radius) y = -radius;
        }
    }
    private final List<Ball> balls = new ArrayList<>();
    private final Random rng = new Random();

    @Override
    public void start(Stage stage) {
        Canvas canvas = new Canvas(800, 800);
        gc = canvas.getGraphicsContext2D();
        Spinner<Integer> spinner = new Spinner<>(500, Integer.MAX_VALUE, 2000, 500);
        spinner.valueProperty().addListener((obs, oldValue, newValue) -> createBalls(newValue));
        spinner.setEditable(true);
        createBalls(spinner.getValue());

        Label fpsLabel = new Label();

        AnimationTimer animation = new AnimationTimer() {
            private long lastUpdate = System.nanoTime();
            @Override
            public void handle(long now) {
                double elapsedSeconds = (now - lastUpdate) / 1_000_000_000.0;
                gc.clearRect(0, 0, canvas.getWidth(), canvas.getHeight());
                for (Ball ball : balls) {
                    ball.update(elapsedSeconds);
                    gc.setFill(ball.color());
                    gc.fillOval(ball.x()-radius, ball.y()-radius, radius, radius);
                }
                lastUpdate = now ;
            }
        };

        animation.start();

        AnimationTimer fpsTimer = new AnimationTimer() {
            private long lastUpdate = System.nanoTime();
            private final Deque<Long> frameTimes = new LinkedList<>();

            @Override
            public void handle(long now) {
                frameTimes.add(now);
                if (frameTimes.size() > 101)  frameTimes.removeFirst();
                double elapsedSeconds = (now - lastUpdate) / 1_000_000_000.0;
                if (elapsedSeconds > 0.1) {
                    double time = (now - frameTimes.peekFirst())/1_000_000_000.0;
                    double fps = (frameTimes.size()-1) / time ;
                    fpsLabel.setText(String.format("FPS: %.1f", fps));
                    lastUpdate = now ;
                }
            }
        };
        fpsTimer.start();

        Pane canvasPane = new Pane(canvas) {
            @Override
            protected void layoutChildren() {
                canvas.relocate(0, 0);
                canvas.setWidth(getWidth());
                canvas.setHeight(getHeight());
            }
        };

        BorderPane root = new BorderPane(canvasPane);
        root.setTop(new HBox(10, spinner, fpsLabel));
        Scene scene = new Scene(root);
        stage.setScene(scene);
        stage.show();
    }

    private void createBalls(int n) {
        balls.clear();
        for (int i = 0 ; i < n ; i++) {
            double x = rng.nextDouble() * gc.getCanvas().getWidth();
            double y = rng.nextDouble() * gc.getCanvas().getHeight();
            double xVel = -200 + rng.nextDouble() * 500 ;
            double yVel = -200 + rng.nextDouble() * 500 ;
            balls.add(new Ball(x, y, xVel, yVel, randomColor()));
        }
    }

    private Color randomColor() {
        return Color.color(rng.nextDouble(), rng.nextDouble(), rng.nextDouble());
    }

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