Proposed draft for the thumbnail D-Bus specification

Philip Van Hoof spam at
Wed Sep 10 03:42:07 PDT 2008

On Wed, 2008-09-10 at 11:49 +0200, Philip Van Hoof wrote:

> have been trying to explain us that a class must
> be good at doing one thing ... but only one thing. 

I mis worded this, it's rather

"being good at being one thing" not "being good at doing one thing".
It's small difference in words, but if of course has a different

Of course can a class do more than one thing. Just like how a horse can
both run and memorize.

> A class must not try to solve the world.

Just like a horse wont solve the world, yet it's good at being a horse

My point is:

A thumbnailer must be good at being a thumbnailer.

A thumbnailer must not try to be good at being both a generic resizer
and a thumbnailer.

A thumbnailer could consume a generic resizer, to avoid said code
duplication. Of course. But that's still two different classes of


public interface Resizable {
	public string mime_type;

public interface Resizer {
	pixmap resize (Resizable r, int width, int height)

public class ImageResizer : Resizer {

	public pixmap resize (Resizable r) {

		Requires {
			typeof (r) extends (Image)


public interface Thumbnailer {

	HashTable <string, Resizer> performers;

	public void Queue (Resizable r) {

		pixmap = performers[r.mime_type].resize (r, 256, 256)
		save (pixmap, large_path)

		pixmap = performers[r.mime_type].resize (r, 128, 128)
		save (pixmap, normal_path)


And not:

public interface Thumbnailer {
  public void Queue (Resizable r, int width, int height, out pixmap p) {

	// Required magic if-then-else, making decisions 
	// behind your back

 	if (width < 256) {
		do thumbnailing
		p = get_from_image (thumbnail_path)
	} else {
		do resizing
		p = get_from_memory ()


Philip Van Hoof, freelance software developer
home: me at pvanhoof dot be 
gnome: pvanhoof at gnome dot org

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