Helper classes for JavaFX to repeatedly register and unregister the same listener, while avoiding duplicate registration of the listener.

ChangeSubscriber.java

import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;

public final class ChangeSubscriber<T> {

    private final ObservableValue<T> observable;
    private final ChangeListener<T> listener;
    private boolean isSubscribed;

    public ChangeSubscriber(ObservableValue<T> observable, ChangeListener<T> listener) {
        this.observable = observable;
        this.listener = listener;
        this.isSubscribed = false;
    }

    public void subscribe() {
        if(!isSubscribed) {
            isSubscribed = true;
            observable.addListener(listener);
        }
    }

    public void unsubscribe() {
        if(isSubscribed) {
            isSubscribed = false;
            observable.removeListener(listener);
        }
    }

    public boolean isSubscribed() {
        return isSubscribed;
    }
}

InvalidationSubscriber.java

import javafx.beans.InvalidationListener;
import javafx.beans.Observable;

public final class InvalidationSubscriber {

    private final Observable observable;
    private final InvalidationListener listener;
    private boolean isSubscribed;

    public InvalidationSubscriber(Observable observable, InvalidationListener listener) {
        this.observable = observable;
        this.listener = listener;
        this.isSubscribed = false;
    }

    public void subscribe() {
        if(!isSubscribed) {
            isSubscribed = true;
            observable.addListener(listener);
        }
    }

    public void unsubscribe() {
        if(isSubscribed) {
            isSubscribed = false;
            observable.removeListener(listener);
        }
    }

    public boolean isSubscribed() {
        return isSubscribed;
    }
}