Question

In order to understand how JFace databindings is working, I have a model object with two properties. Changing one property should set the other to the same value:

public class Model {
  private double x;
  private double y;
  private PropertyChangeSupport changeSupport = new PropertyChangeSupport(this);

    public void addPropertyChangeListener(String propertyName,
            PropertyChangeListener listener) {
        propertyChangeSupport.addPropertyChangeListener(propertyName, listener);
    }

    public void removePropertyChangeListener(PropertyChangeListener listener) {
        propertyChangeSupport.removePropertyChangeListener(listener);
    }   

    public void setX(double x) {
        propertyChangeSupport.firePropertyChange("x", this.x, this.x = x);
    }

    public double getX() {
        return x;
    }

    public void setY(double y) {
        propertyChangeSupport.firePropertyChange("y", y, this.y = y);
        setX(y);
    }

    public double y() {
        return y;
    }
}

Now in a separate class I define two Text widgets, xText and yText, which are bound to this object like this:

    DataBindingContext bindingContext = new DataBindingContext();
    bindingContext.bindValue(WidgetProperties.text(SWT.Modify).observe(xText),
            BeanProperties.value(HumidityScanParameters.class,"x").observe(getModel()));
    bindingContext.bindValue(WidgetProperties.text(SWT.Modify).observe(yText),
            BeanProperties.value(HumidityScanParameters.class, "y").observe(getModel()));

I have found that if I change the text in yText, then the setter is automatically called as expected, and this sets both y and x in the model. However, xText is not updated. Why is this? Shouldn’t the firePropertyChange() call arrange for the Text to be updated?

Thanks,
Graham.

Answer 1

The compiler was optimising away the initial value of this.x and this.y, which led the PropertyChangeSupport instance to discard the change notification. It didn’t think anything had changed. If I introduce a temporary variable like this:

public void setX(double x) {
    double oldValue = this.x;
    this.x = x;
    propertyChangeSupport.firePropertyChange("x", oldValue, x);
}

then the notifications occur as I might expect.