Question

I have an application where I need to change font sizes frequently. A question posted a year ago on this forum (Change just the font size in SWT) gave me some of the information I needed, but I’ve still got some unknowns I haven’t figured out yet.

In particular, someone signing as hudsonb offered a helpful code fragment which I’d like to reproduce below:

FontData[] fontData = label.getFont().getFontData();
for(int i = 0; i < fontData.length; ++i)
    fontData[i].setHeight(14);

final Font newFont = new Font(display, fontData);
label.setFont(newFont);

// Since you created the font, you must dispose it
label.addDisposeListener(new DisposeListener() {
    public void widgetDisposed(DisposeEvent e) {
        newFont.dispose(image);
    }
});

Suppose I used code like this to change font sizes frequently. Aren’t I creating a whole sequence of DisposeListeners, and adding them to the label’s listener queue? Don’t I need to remove the previous listener each time before adding a new listener? Or is there some mechanism I don’t understand that makes this unnecessary?

Answer 1

An alternative would be to add a DisposeListener that maintains a list of Fonts it needs to dispose of when finished with, e.g.

class FontDisposer implements DisposeListener {
    private List<Font> toDispose;

    public FontDisposer() {
        toDispose = new ArrayList<Font>();
    }

    @Override
    public void widgetDisposed(DisposeEvent e) {
            // Dispose all fonts in toDispose
    }

    public void registerFont(Font f) {
        toDispose.add(f);
    }
}

class UIWotsit extends SomeUIClass {
    private FontDisposer disposer;

    public UIWotsit() {
        disposer = new FontDisposer();
        addDisposeListener(disposer);
    }

    public void changeFont(Font f) {
        disposer.registerFont(f);
        // Do all the font changing stuff
    }
}

This may not suit your particular application but having something that maintains a list of Fonts is probably a reasonable start. You could also use an anonymous DisposeListener that calls a method in whatever code manages the Font creation/changing/disposal:

class OtherFontDisposer implements DisposeListener {
    @Override
    public void widgetDisposed(DisposeEvent e) {
        myCleverFontManager.disposeOfAll();
    }
}

It all depends on whether you are creating new labels or not, and whether you want to dispose of Fonts immediately or only when the UI is disposed of.