Adding a listener to a variable in Java/JavaFX which gets called on variable change

If you are using JavaFX 2 then it provides an out-of-box solutions for both JavaBeans component architecture and Observer design pattern. Moreover it gives a great flexibility of associating the state of variables by the property bindings. The code below illustrates the property changed events and the binding of property variables. Of course you can wrap the property accessors to hide details by like getFlag() and setFlag() below, and use them in the rest of application.

public class Demo extends Application {

    private BooleanProperty booleanProperty = new SimpleBooleanProperty(true);

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

    @Override
    public void start(Stage primaryStage) {

        // Add change listener
        booleanProperty.addListener(new ChangeListener<Boolean>() {

            @Override
            public void changed(ObservableValue<? extends Boolean> observable, Boolean oldValue, Boolean newValue) {
                System.out.println("changed " + oldValue + "->" + newValue);
                myFunc();
            }
        });

        Button btn = new Button();
        btn.setText("Toggle boolean flag");
        btn.setOnAction(new EventHandler<ActionEvent>() {

            @Override
            public void handle(ActionEvent event) {
                booleanProperty.set(!booleanProperty.get()); //toggle
                System.out.println("toggled to " + booleanProperty.get());
            }
        });

        // Bind to another property variable
        btn.underlineProperty().bind(booleanProperty);

        StackPane root = new StackPane();
        root.getChildren().add(btn);
        primaryStage.setScene(new Scene(root, 300, 250));
        primaryStage.show();
    }

    public boolean getFlag() {
        return booleanProperty.get();
    }

    public void setFlag(boolean val) {
        booleanProperty.set(val);
    }
}

As simple as this:

public void changeBooleanFlag(boolean bEnabled)
{
    if(booleanFlag == bEnabled) return;
    booleanFlag = bEnabled;
    myFunc();
}

and whenever you want to change the boolean flag, you should do it via this method.

I would do it using PropertyChangeListener. Here is a nice tutorial: http://docs.oracle.com/javase/tutorial/uiswing/events/propertychangelistener.html

Example of code:

import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;
import java.util.ArrayList;
import java.util.List;

public class MyClass {
    private final List<PropertyChangeListener> listeners = new ArrayList<>();
    private boolean b1, b2;

    public MyClass() {
    }

    public boolean isB1() {
        return b1;
    }

    public void setB1(boolean b1) {
        boolean oldValue = this.b1;
        this.b1 = b1;
        firePropertyChange("b1", oldValue, b1);
    }

    public boolean isB2() {
        return b2;
    }

    public void setB2(boolean b2) {
        boolean oldValue = this.b2;
        this.b2 = b2;
        firePropertyChange("b2", oldValue, b2);
    }

    public void addPropertyChangeListener(PropertyChangeListener listener) {
        listeners.add(listener);
    }

    private void firePropertyChange(String property, Object oldValue, Object newValue) {
        for (PropertyChangeListener l : listeners) {
            l.propertyChange(new PropertyChangeEvent(this, property, oldValue, newValue));
        }
    }

    /**
     * Main method for tests.
     * @param args
     */
    public static void main(String[] args) {
        MyClass m = new MyClass();

        m.addPropertyChangeListener(new PropertyChangeListener() {
            @Override
            public void propertyChange(PropertyChangeEvent e) {
                String changedProperty = e.getPropertyName();
                System.out.println("Changed property: " + changedProperty);
                System.out.println("New value: " + e.getNewValue());
                System.out.println("Old value: " + e.getOldValue());
                System.out.println();
            }
        });

        m.setB1(true);
        m.setB2(false);
    }
}