Subsidiary note which I forgot in the commit message:

I've decided to give each view a short string name to aid in differentiating which view a hook is being called from. Since hooks are functions and not subclasses, and can get called from different places in the URL hierarchy, it's sometimes difficult to distinguish what data you're actually operating on. I think this will come in handy for other hooks as well.

Do you have an example renderer plugin I can look at?

This is the minimal example (I also included it in the docs):

from datasette import hookimpl

def render_test(args, data, view_name):
    return {
       'body': 'Hello World',
       'content_type': 'text/plain'
    }

@hookimpl
def register_output_renderer():
    return {
        'extension': 'test',
        'callback': render_test
    }

I'm working on the GeoJSON one now and it should be ready soon. (I forgot I was going to run into the same problem as before - that Spatialite's stupid binary format isn't WKB and I have no way of altering the query to change that - but I've just managed to write some code to rearrange the bytes from Spatialite blob-geometry into WKB...)

I also just realised that I should be passing the datasette object into the hook function...as I just found I need it. So hold off merging until I've fixed that.

I updated the hook to pass the datasette object through now.

You can see the working GeoJSON render function here - the hook function is here.

Also I just pushed a change to add registered output renderers to the templates:

Just for the record, this PR is now finished and ready to merge from my perspective.

I plan to merge this in about 7 hours time (after work).

I may tweak the plugin format a little bit before the next Datasette release though (best to merge first, make changes later I think) - for consistency with some other upcoming hooks (the facet hook in particular). I'll discuss that with you while I'm working on it.

Thanks so much for this, it's a really neat addition!