Add ValueDelegate macro

[ranfdev]

closes #983

It works pretty well I'd say.

Should I add support for generics?

[ranfdev]

Added support to make the newtype nullable, implementing ToValueOptional.
I won't add support for generics, unless requested.
Done

I should probably add another example on the properties macro taking advantage of this.

[pbor]

The code itself looks good to me, though given that this is a new API etc, before merging I would prefer to get a thumbs-up from someone who had approval permission for more than a couple of days :)

In the mean time let me bring up some bikeshedding: which alternatives have we considered to the ValueDelegate name? I do not find it super clear... how about ValueRepresentation or ValueRepr ?

[pbor]

Also, if nullable is specified, should we accept/use TryFrom and set None in case of error?

[ranfdev]

naming

ValueRepresentation is a bit too long. ValueRepr would be ok, but in any case I think ValueDelegate is clearer. The macro doesn't provide a full representation. The representation is delegated to the chosen type. GLib can't distinguish MyInt(i32) and an i32. So MyInt(i32) doesn't have a full representation. That would require registering another glib type.

should we accept/use TryFrom and set None in case of error?

No, because that would silently ignore the error. We would need a specific Checker inside the ToValue trait to handle the conversion and provide a useful error. I agree this can be useful.

[pbor]

As mentioned, this looks good to me, but let's wait for other reviews as well before adding new API

[ranfdev]

@sdroege we would like a thumbs up before merging a new API :)

[sdroege]

Seems good to me, please merge after solving the conflicts :)

[sdroege]

So let's get this in now? @pbor ?

[ranfdev]

I wanted to merge it, I just needed a review to do so

[sdroege]

Ah, because mine was discarded because of the rebasing. I see