JavaFX MultiThreading Flashing Lights - javafx

Question

I have two circles redCircle and greenCircle : -
Circle greenCircle = new Circle(250,150 ,100, Color.TRANSPARENT);
Circle redCircle = new Circle(250,450,100,Color.TRANSPARENT);
greenCircle.setStroke(Color.GREEN);
greenCircle.setStrokeWidth(4);
group.getChildren().add(greenCircle);
redCircle.setStroke(Color.RED);
redCircle.setStrokeWidth(4);
group.getChildren().add(redCircle);
Basically I want Circle to turn on and off twice in 2 seconds. So I am able to turn on Light then wait for 0.5 second , turn off and again wait for 0.5 second and turn on Light. I am not about to turn off after 0.5 second.
public class LightOn {
public Task<Void> runLightOn() throws InterruptedException {
return new Task<Void>(){
#Override
protected Void call() throws Exception {
greenCircle.setFill(Color.GREEN);
return null;
}
};
}
}
public class LightOff {
public void perform() throws InterruptedException {
LightOn onL = new LightOn();
Task<Void> runLinghtOnTask = onL.runLightOn();
runLinghtOnTask.setOnSucceeded(new EventHandler<WorkerStateEvent>() {
#Override
public void handle(WorkerStateEvent event) {
try {
Thread.sleep(500);
} catch (InterruptedException ex) {
Logger.getLogger(MovementEventsDemo.class.getName()).log(Level.SEVERE, null, ex);
}
greenCircle.setFill(Color.TRANSPARENT);
nextFunction();
}
});
new Thread(runLinghtOnTask).start();
}
}
public void nextFunction(){
Task<Void> sleeper2 = new Task<Void>() {
#Override
protected Void call() throws Exception {
try {
Thread.sleep(500);
} catch (InterruptedException e) {
}
return null;
}
};
sleeper2.setOnSucceeded(new EventHandler<WorkerStateEvent>() {
#Override
public void handle(WorkerStateEvent event) {
if(greenCircle.getFill()==Color.GREEN) {
greenCircle.setFill(Color.TRANSPARENT);
}else {
greenCircle.setFill(Color.GREEN);
}
}
});
new Thread(sleeper2).start();
}
I use this to execute : -
LightOff lf = new LightOff();
lf.perform();

Answer 1

The reason your code is not working is that an exception is being thrown in the call() method of the Task returned by runLightOn(), because you are changing the UI from a background thread. If you catch the exception or register an onFailed handler with the Tasks you will be able to log the exception.
For functionality like this, where you have specific timepoints (KeyFrames) at which you want values to change, use a Timeline instead of messing with multithreading:
import javafx.animation.KeyFrame;
import javafx.animation.Timeline;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.layout.Pane;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;
import javafx.stage.Stage;
import javafx.util.Duration;
public class FlashingLight extends Application {
#Override
public void start(Stage primaryStage) {
Circle circle = new Circle(250, 150, 100, Color.TRANSPARENT);
circle.setStroke(Color.GREEN);
circle.setStrokeWidth(4);
Timeline timeline = new Timeline(
new KeyFrame(Duration.seconds(0.5), e -> circle.setFill(Color.GREEN)),
new KeyFrame(Duration.seconds(1.0), e -> circle.setFill(Color.TRANSPARENT))
);
timeline.setCycleCount(2);
Pane pane = new Pane(circle);
Scene scene = new Scene(pane, 600, 600);
primaryStage.setScene(scene);
primaryStage.show();
timeline.play();
}
public static void main(String[] args) {
launch(args);
}
}
As a slight alternative, you can replace
Timeline timeline = new Timeline(
new KeyFrame(Duration.seconds(0.5), e -> circle.setFill(Color.GREEN)),
new KeyFrame(Duration.seconds(1.0), e -> circle.setFill(Color.TRANSPARENT))
);
with
BooleanProperty on = new SimpleBooleanProperty();
Timeline timeline = new Timeline(
new KeyFrame(Duration.seconds(0.5), new KeyValue(on, true)),
new KeyFrame(Duration.seconds(1.0), new KeyValue(on, false))
);
circle.fillProperty().bind(
Bindings.when(on)
.then(Color.GREEN)
.otherwise(Color.TRANSPARENT));
The added benefits here are that you create a boolean value representing whether or not the light is on, which may be useful elsewhere in your logic, and that you separate the logic (on/off, and the timing) from the display (color, etc).