Homerun sources and Plasma runners

[Aurélien Gâteau]

Spent yesterday evening trying to think about this topic from all edges.

First, I am convinced using Qt models is the right API design for Homerun. I 
have a very hard time giving up on them for the sake of not breaking source 
compatibility, to the point where I think it is better to do a clean break in 
libplasma2.

Furthermore, apart from the browsing issues, there is another fundamental 
difference between runners and sources which I don't think we can bridge: a 
runner is expected to run for a limited time. As stated in 
AbstractRunner::match() doc: "Returning from this method means to end 
execution of the runner.". Homerun sources, on the other hand, should be able 
to update themselves whenever they detect changes. This is especially 
important when Homerun is used as a containment because it needs to update 
itself without any user intervention.

At one point I thought we could at least make use of the single-runner mode of 
AbstractRunner to implement some of the simplest sources. While that can work 
for, say, the Power or Session sources, it cannot work for other simple 
sources like "Recent Documents", again because such a source needs to be able 
to update itself.

A runner is like a one-time search, whereas a source is more like a live-
search folder.
To me, the runner concept is a subset of the source concept: it should be 
possible to build a runner on top of an existing source. Going the other way 
around means giving up some sources abilities.

Considering prior art, Kickoff and Folder View are not built on top of runners 
either (true, Kickoff uses runners to implement searching, but not for 
browsing), so I am not sure why runners would be appropriate for Homerun but 
not for them? In a sense, ioslaves would make more sense as content providers 
for Homerun, I am actually thinking about it, but they lack features like 
custom order and the API is complicated, not something I would expect lots of 
people to build on.

To sum up, I don't think we should try to shoehorn the source concept into the 
existing libplasma1 runner API. I am all for creating a more complete 
implementation of sources in libplasma2, on top of which runners could be 
based. I would like to see libhomerun as a brainstorm of this work, adding 
there what is missing from existing runner API.

Aurélien