JavaFX Slider not invoking valueProperty's ChangeListener for min and max values

Question

I have this issue where the JavaFX Slider doesn't invoke the ChangeListener for the valueProperty when the slider-knob is on the min or max extreme. I want to execute some code only after the slider value has been changed (not while the slider is being dragged). I tried to achieve this with the following code:

Slider slider = new Slider();
slider.setMin(0);
slider.setMax(10);
slider.setMajorTickUnit(5);
slider.setMinorTickCount(5);
slider.setBlockIncrement(1);
slider.setSnapToTicks(true);
slider.setShowTickMarks(true);
slider.setShowTickLabels(true);

ChangeListener<? super Number> valueListener = (observable, oldValue, newValue) -> {
    if (!slider.isValueChanging()) {
        System.out.println("Value changed");
    }
};

slider.valueProperty().addListener(valueListener);

You'll see that when you drag and release the slider the string Value changed is printed out every time except for when the slider-knob is released on the 0 or 10 positions. Is this the expected behaviour? Or am I missing something?

score 3 · Answer 1 · answered Jul 20 '18 at 13:29

The problem is a consequence of the design of fx properties: in contrast to the ol' bean spec, they are completely self-contained, each fires on change independently of the other. A consequence is that listeners to one property must not query any other property when reacting to a change.

Applied to the context of Slider's value/isValueAdjusting properties:

when listening to value, the adjusting state is unspecified
when listening to adjusting, the value state is unspecified

At the end of the day, Slider's api is simply too weak to serve the common use-case (of reacting to changes only if they are completed) without considerable effort.

A way out might be a custom slider with enhanced api. The outline below is based on the observation (beware: implementation detail!) that all internals set the final value via slider.adjustValue(value). The enhanced slider

adds a property adjustedValue that's updated when the change is final
updates the adjustedValue at the end of adjustValue

The outline (incomplete and not formally tested):

public static class AdjustedSlider extends Slider {

    private DoubleProperty adjustedValue;

    public AdjustedSlider() {
        super();
    }

    public AdjustedSlider(double min, double max, double value) {
        super(min, max, value);
    }

    public DoubleProperty adjustedValueProperty() {
        if (adjustedValue == null) {
            adjustedValue = new SimpleDoubleProperty(this, "adjustedValue", 0);
        }
        return adjustedValue;
    }

    public double getAdjustedValue() {
        return adjustedValueProperty().get();
    }

    private void setAdjustedValue(double value) {
        adjustedValueProperty().set(value);
    }

    @Override
    public void adjustValue(double newValue) {
        super.adjustValue(newValue);
        setAdjustedValue(getValue());
    }

}

wcmatthysen · Accepted Answer · 2018-06-28T19:50:05.017

Looks like there is already a solution posted here. It basically comes down to adding explicit checks for the extremes so that the ChangeListener looks like this:

ChangeListener<? super Number> valueListener = (observable, oldValue, newValue) -> {
    boolean isOnMin = newValue.doubleValue() == slider.getMin();
    boolean isOnMax = newValue.doubleValue() == slider.getMax();
    if (!slider.isValueChanging() || isOnMin || isOnMax) {
        System.out.println("Value changed");
    }
};

However, the downside to this solution is that it immediately invokes the System.out.println whenever the slider touches either one of the edges, even though the mouse-button has not been released yet.

An alternative solution that I found was to keep the original ChangeListener that only checks the isValueChanging-value, but to add the following ChangeListener to the valueChangingProperty:

 ChangeListener<? super Boolean> valueChangingListener = (observable, oldUpdating, newUpdating) -> {
    double value = slider.getValue();
    boolean notUpdatingAnymore = oldUpdating && !newUpdating;
    boolean isOnExtreme = value == slider.getMin() || value == slider.getMax();
    if (notUpdatingAnymore && isOnExtreme) {
        System.out.println("Value changed");
    }
};

slider.valueChangingProperty().addListener(valueChangingListener);

It's ugly, but it works. I don't know if there is a cleaner way to achieve the desired behaviour.

thought it was fixed - but obviously not. See also: https://stackoverflow.com/q/35129670/203657 as to why that happens — kleopatra, Jul 20 '18 at 09:27

JavaFX Slider not invoking valueProperty's ChangeListener for min and max values

2 Answers2