Review of the thumbnailer spec

[Jannis Pohlmann]

On Tue, 19 May 2009 10:15:07 +0200
Philip Van Hoof <spam at pvanhoof.be> wrote:

> On Tue, 2009-05-19 at 01:30 +0200, Jannis Pohlmann wrote:
> > On Mon, 18 May 2009 16:01:27 +0200
> > Jannis Pohlmann <jannis at xfce.org> wrote:
> > 
> > > On Mon, 18 May 2009 13:48:08 +0200
> 
> [CUT]
> 
>  
> > > As a consequence of the above I'm wondering about a new method
> > > called Failed(). But even that doesn't really solve the problem
> > > of where the failed thumbnails are stored of course.
> > > 
> > > > org.freedesktop.thumbnails.Manager
> > > > 	- Register
> > > > 	- GetSupported
> > > 
> > > I'm thinking about the situation where manager and generic
> > > thumbnailer are implemented as separate daemons. In that case
> > > we'd probably want additional methods like this one:
> > > 
> > >   - GetThumbnailers (returning the D-Bus names of all registered
> > >     specialized thumbnailers).
> > > 
> > > And maybe a signal 
> > > 
> > >   - Registered() (including the D-Bus name of a newly registered
> > >     specialized thumbnailer).
> > 
> > Ideas for an additional method:
> > 
> >   - Unregister (to explicitely unregister the D-Bus name of a
> >     thumbnailer).
> > 
> > At the moment implementations of the manager interface have to
> > listen to the "destroy" signal of proxies created for dynamically
> > registered thumbnailers. This is more of an implicit unregister
> > which I don't think is nice.
> > 
> > That would also lead to another signal:
> > 
> >   - Unregistered (sent when the D-Bus name of a thumbnailer is
> >     unregistered using Manager.Unregister()).
> > 
> > Based on that, clients of the generic thumbnailer/manager services
> > could cache the supported MIME types and listen for Registered and
> > Unregistered signals to update this cache.
> 
> If the only use-case we can find for "Unregistered" is updating the 
> list of supported MIME parts, then we should have a signal for 
> "SupportChanged" or something like that instead.

One more use case for Unregistered would be that a generic thumbnailer
running in a separate process could could remove (or add, in the case
of Registered) the D-Bus name from the list of specialized thumbnailers
it talks to.

> However, do note that GetSupported itself isn't complete:
> 
> A supported MIME part must always fall under a UriScheme too.
> 
> It's for example possible that image/jpeg for file:// is supported by
> a specific specialized thumbnailer, but not for http://, a good
> example would be the EPeg library which can't work with VFS URIs at
> this moment.
> 
> So if you want to take GetSupported more serious than what the spec
> offers now, then you should involve the UriScheme information too.

Definitely. At the moment GetSupported only returns half the "truth"
of what is *really* supported. 

  - Jannis
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