Reimplement ObservablePairs and eliminate notify keyword

[timholy]

The old implementation of ObservablePairs seems unnecessarily
complicated. Perhaps more importantly, this change allows us
to eliminate the notify kwarg of setindex!, which should reduce
type-diversity and may help reduce latency.

EDIT: I also suspect that the current implementation has a bug if you make two pairs, one with a and b and the other with b and c and use the same update function for both. This implementation should not suffer from any such problems.

[timholy]

I'd also propose we rename f and g to something more memorable that doesn't require one to check the source code to see which one does which. Maybe first2second and second2first?

[piever]

Thanks! I agree that your implementation of ObservablePair is cleaner, but I'm not 100% sure if the notify keyword argument can be eliminated. Other than the ObservablePair application, I can't think of many other applications, and your done[] trick could probably be applied to those as well.

Pinging @jkrumbiegel because I think some of his Makie code (link) relies on the notify keyword. If that can also be fixed, I guess we can get rid of the keyword.

[timholy]

Thanks @piever!

So folks are aware, you can update the observable with obs.val = x and not notify. My next commit in this series (see #55 (comment); currently I have 8 likely PRs queued after this one 😄, not including the ones already submitted) will split setindex! into

obs.val = x
notify(obs)

to make it easier to control notification manually. (Currently we have notify! which calls setindex! but that seems a bit backwards; setindex! should be built from notify instead.)

[piever]

I know that this is internal, but I guess we could keep something along the lines of

setexcludinghandlers(obs::AbstractObservable, val) = observe(obs).val = val

so that the "silent update" is not done by touching an internal field as in #56 (comment). In particular, the observe(obs) pattern is needed because Interact widgets are AbstractObservables, whose "concrete observable" can be accessed with observe(obs). As mentioned in that comment, setindex! would then just by setexcludinghandlers followed by notify!.

[timholy]

Added.

[piever]

Ok, I've just left a small comment (to basically keep a method for the silent update, because accessing the field directly won't work on AbstractObservables), but other than that this looks good to me.

[timholy]

Thanks @piever!