[FIXED] Trouble setting program flow to correctly run a function

Issue

I have a class tasks, which handles multiple tasks using a menu layout, class tasks check the application flow by setting up the stage, with the menu scene to list all individual task. I want to run some task from the available list using there own classes, something like this:

Tasks.java:

    package tasks;
    
    import javafx.application.Application;
    import javafx.stage.Stage;
    import javafx.scene.Scene;
    import javafx.scene.layout.VBox;
    import javafx.scene.control.Button;
    
    public class Tasks extends Application
    {
        private Stage window;
        private Scene menuScene;
        private Task1 task1;
    
        public Tasks()
        {
            this.window=null;
            this.menuScene=null;
            this.task1=null;
        }
    
        private void setMenu()
        {
            VBox menu=new VBox();
    
            Button newTask1Button=new Button("New Task 1");
            newTask1Button.setOnAction(clickEvent -> this.startNewTask1());
            menu.getChildren().add(newTask1Button);
    
            //More buttons
    
            this.menuScene=new Scene(menu,400,600);
        
            this.window.setScene(this.menuScene);
        }
    
        private void startNewTask1()
        {
            this.task1=new Task1(this.window);
            this.launchTask1();
        }
    
        private void launchTask1()
        {
            if(this.task1!=null)
            {
                int task1State=1;
                //while(task1State==1) //To re-run for pause state
                //{
                    task1State=this.task1.runTask1();
                    System.out.println("Task1 is in state "+task1State); //In no way part of program, just for debugging. Always give state=-1
                    //If 1-Paused, then display pause Menu for task1, by calling this.task1.paused(); and then again based on user input re-run runTask1
                    //If 0-Exit, then change the scene back to menuScene and quit the function
                //}
            }
        }
    
        @Override
        public void start(Stage primaryStage)
        {
            this.window=primaryStage;
            this.window.setTitle("Tasks");
    
            this.setMenu();
    
            this.window.show();
        }
    
        public static void main(String[] args)
        {
            Application.launch(args);
        }
    }

Task1.java:

    package tasks;
    
    import javafx.stage.Stage;
    import javafx.stage.Stage;
    import javafx.scene.Scene;
    import javafx.scene.layout.HBox;
    import javafx.scene.control.Button;
    
    class Task1
    {
        private Stage window;
        private Scene task1Scene;
        private boolean intialised;
    
        private int state;
    
        public Task1()
        {
        }
        public Task1(Stage _window)
        {
            this.window=_window;
            this.task1Scene=null; //Will be set later
            this.intialised=false;
            this.state=-1;
        }
    
        private Scene createScene()
        {
            //Creates some GUI to interact
            
            //Buttons in End, to control exit
            HBox menu=new HBox();
    
            Button pauseButton=new Button("Pause");
            pauseButton.setOnAction(clickEvent -> this.state=1);
            menu.getChildren().add(pauseButton);
            
            Button exitButton=new Button("Exit");
            exitButton.setOnAction(clickEvent -> this.state=0);
            menu.getChildren().add(exitButton);
    
            Scene scene=new Scene(menu,400,600);
    
            return scene;
        }
    
        private void setupControls()
        {
            //To assign event handlers to interact with GUI
        }
    
        public int runTask1()
        {
            if(!this.intialised)
                this.task1Scene=this.createScene();
            this.window.setScene(this.task1Scene);
    
            this.setupControls();

            //while(this.state==-1);
        
            return this.state;
        }
    }

The problem with this I face is, function runTask1() is always instantly returning, even though operation assigned using event handlers for Task1 are still running and no event for exit has been generated.

I tried to solve this by setting an instance variable named state and setting it to -1, and putting a while loop till this state variable is not modified. But that completely stops the GUI.

I realised its reason later by Googling, but couldn't determine which way to solve this.

At places, it was suggested to use Threads (not sure how, I don't want multiple processes running in the program) and at places, it was also suggested to set another Event Handler (but, they were running the different process in the start() function (inherited from Application) itself and it was more of transferring the flow rather than returning backwards).

How should I code to keep running only runTask1() till it is not finished, and return to launchTask1() sequentially?

Solution

The infinite while loop in method runTask1() in class Task1 is freezing the JavaFX application thread. Just remove it.

Basically your Task1 class is another Scene so when you click on button newTask1Button in class Tasks you simply want to set a new Scene.

Here is class Task1 with the required change.

package tasks;

import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.layout.HBox;
import javafx.scene.control.Button;

public class Task1 {
    private Stage window;
    private Scene task1Scene;
    private boolean intialised;

    private int state;

    public Task1() {
    }

    public Task1(Stage _window) {
        this.window = _window;
        this.task1Scene = null; // Will be set later
        this.intialised = false;
        this.state = -1;
    }

    private Scene createScene() {
        // Creates some GUI to interact

        // Buttons in End, to control exit
        HBox menu = new HBox();

        Button pauseButton = new Button("Pause");
        pauseButton.setOnAction(clickEvent -> this.state = 1);
        menu.getChildren().add(pauseButton);

        Button exitButton = new Button("Exit");
        exitButton.setOnAction(clickEvent -> this.state = 0);
        menu.getChildren().add(exitButton);

        Scene scene = new Scene(menu, 400, 600);

        return scene;
    }

    private void setupControls() {
        // To assign event handlers to interact with GUI
    }

    public int runTask1() {
        if (!this.intialised)
            this.task1Scene = this.createScene();
        this.window.setScene(this.task1Scene);

        this.setupControls();

//        while (this.state == -1)
//            ;

        return this.state;
    }
}

As you can see, I simply commented out the while loop. The JavaFX application thread contains a loop that waits for user actions to occur such as moving the mouse or typing a key on the keyboard. You don't have to handle that in your code.

EDIT

As a result of the typo in the code in your question, that you mentioned in your comment to my answer and that you corrected in your question in a subsequent edit, I am editing my answer.

The way a JavaFX application works is that it reacts to user actions. You want class Tasks to be notified when the "state", in class Task1, is changed and the "state" is changed when the user clicks on either pauseButton or exitButton in class Task1. According to the code you posted, you could callback to class Tasks from the event handler of pauseButton.

Class Task1.
(Note comment CHANGE HERE and extra parameter in constructor.)

package tasks;

import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.layout.HBox;
import javafx.scene.control.Button;

public class Task1 {
    private Tasks tasks;
    private Stage window;
    private Scene task1Scene;
    private boolean intialised;

    private int state;

    public Task1(Stage _window, Tasks tasks) {
        this.tasks = tasks;
        this.window = _window;
        this.task1Scene = null; // Will be set later
        this.intialised = false;
        this.state = -1;
    }

    private Scene createScene() {
        HBox menu = new HBox();
        Button pauseButton = new Button("Pause");
        pauseButton.setOnAction(clickEvent -> tasks.setState(this.state = 1)); // CHANGE HERE
        menu.getChildren().add(pauseButton);
        Button exitButton = new Button("Exit");
        exitButton.setOnAction(clickEvent -> this.state = 0);
        menu.getChildren().add(exitButton);
        Scene scene = new Scene(menu, 400, 600);
        return scene;
    }

    private void setupControls() {
        // To assign event handlers to interact with GUI
    }

    public int runTask1() {
        if (!this.intialised) {
            this.task1Scene = this.createScene();
        }
        this.window.setScene(this.task1Scene);
        this.setupControls();
        return this.state;
    }
}

---

Class Tasks
(Added method setState(int).)

package tasks;

import javafx.application.Application;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.layout.VBox;
import javafx.scene.control.Button;

public class Tasks extends Application {
    private Stage window;
    private Scene menuScene;
    private Task1 task1;

    public Tasks() {
        this.window = null;
        this.menuScene = null;
        this.task1 = null;
    }

    private void setMenu() {
        VBox menu = new VBox();
        Button newTask1Button = new Button("New Task 1");
        newTask1Button.setOnAction(clickEvent -> this.startNewTask1());
        menu.getChildren().add(newTask1Button);
        this.menuScene = new Scene(menu, 400, 600);
        this.window.setScene(this.menuScene);
    }

    private void startNewTask1() {
        this.task1 = new Task1(this.window, this);
        this.launchTask1();
    }

    private void launchTask1() {
        if (this.task1 != null) {
            this.task1.runTask1();
        }
    }

    @Override
    public void start(Stage primaryStage) {
        this.window = primaryStage;
        this.window.setTitle("Tasks");
        this.setMenu();
        this.window.show();
    }

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

    public void setState(int task1State) {
        System.out.println("Task1 is in state " + task1State); // In no way part of program,
                                                               // just for debugging.
    }
}

Answered By - Abra