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I have a GridPane with an image on it. I would like the image to change its position (go to a different grid child node) every 2 seconds. However when I ran the program using another thread I get: Not on FX application thread error. Any help is appreciated!

I have tried using Platform.runLater as well but it still doesn't work. Also by reading through different posts I learned that GridPane is not thread safe. Is there a way to bypass that?

public class BoardController implements Initializable 
 @FXML
 private GridPane board;
 @FXML
 private Text position;

 String pikachuPos = "";
 String pokeballPos = "";
 int i = 0;
 int j = 0;
 boolean selectPikachu = true;
 int count = 0;

 int pikachuRow = 0;
 int pikachuCol = 0;
 int pokeballRow = 0;
 int pokeballCol = 0;

 ImageView pikachu = new ImageView(getClass().getResource("pikachu.png").toExternalForm());
 ImageView pokeball = new ImageView(getClass().getResource("pokeball.png").toExternalForm());

 /**
 * Initializes the controller class.
 */
 @Override
 public void initialize(URL url, ResourceBundle rb) 
 pikachu.setFitWidth(30);
 pikachu.setFitHeight(30);
 pokeball.setFitWidth(30);
 pokeball.setFitHeight(30);

 position.setVisible(true);
 if (count == 2) 
 System.out.println("2");
 
 boolean pikachu = true;

 Button buttons = new Button[10][10];
 Button b;
 for (i = 0; i < 10; i++) 
 for (j = 0; j < 10; j++) 
 b = new Button(j + " " + i);
 buttons[i][j] = b;
 buttons[i][j].setOnAction((event) -> 
 if (this.selectPikachu) 
 Button button = (Button) event.getSource();
 this.pikachuPos = button.getText();
 this.selectPikachu = false;
 this.position.setText("Choose Pokeball position");
 else 
 Button button = (Button) event.getSource();
 this.pokeballPos = button.getText();
 this.position.setText("");
 try 
 this.play();
 catch (InterruptedException ex) 
 Logger.getLogger(BoardController.class.getName()).log(Level.SEVERE, null, ex);
 
 

 );

 board.add(buttons[i][j], i, j);
 GridPane.setHalignment(buttons[i][j], HPos.CENTER);
 
 
 

 public void play() throws InterruptedException 

 Node node = board.getChildren().get(0);
 board.getChildren().clear();
 board.getChildren().add(0, node);

 String pikachuDetails = pikachuPos.split(" ");
 String pokeballDetails = pokeballPos.split(" ");

 pikachuRow = Integer.parseInt(pikachuDetails[1]);
 pikachuCol = Integer.parseInt(pikachuDetails[0]);

 pokeballRow = Integer.parseInt(pokeballDetails[1]);
 pokeballCol = Integer.parseInt(pokeballDetails[0]);

 ImageView pokeball = new ImageView(getClass().getResource("pokeball.png").toExternalForm());
 pokeball.setFitWidth(30);
 pokeball.setFitHeight(30);

 ImageView pikachu = new ImageView(getClass().getResource("pikachu.png").toExternalForm());
 pikachu.setFitWidth(30);
 pikachu.setFitHeight(30);

 GridPane.setHalignment(pokeball, HPos.CENTER);
 board.add(pokeball, pokeballRow, pokeballCol);

 GridPane.setHalignment(pikachu, HPos.CENTER);
 board.add(pikachu, pikachuRow, pikachuCol);

 startTask();

 

 public void startTask() 
 // Create a Runnable
 Runnable task = new Runnable() 
 public void run() 
 runTask();
 
 ;

 // Run the task in a background thread
 Thread backgroundThread = new Thread(task);
 // Terminate the running thread if the application exits
 backgroundThread.setDaemon(true);
 // Start the thread
 backgroundThread.start();
 

 public void runTask() 
 for (int i = 1; i <= 10; i++) 
 try 
 board.add(pikachu, 5, 5);
 Thread.sleep(1000);
 catch (InterruptedException e) 
 e.printStackTrace();
 
 
 

 public void setPikachu(int r, int c) 
 board.add(pikachu, r, c);
 GridPane.setHalignment(pikachu, HPos.CENTER);
 

I learned that GridPane is not thread safe.

There is no such thing as thread-safe when it comes to JavaFX scene nodes. They are expected to be modified only from JavaFX Application Thread.

I have tried using Platform.runLater as well but it still doesn't work.

You have to place it at the correct part too. If you did Platform.runLater(this::startTask); then it would do nothing.

You should use Platform.runLater(() -> board.....) to modify the scene graph from non JavaFX Application Thread.

Lastly, you really should not use board.add(). Adding a control multiple times is going to throw an IllegalArgumentException saying that there is a duplicate children.

So this is what you should do:

Platform.runLater(() -> board.setConstraints(pikachu, 5, 5));

I am not sure why you are always setting it to row 5 and column 5 though.