they should have a short timeout so that the root page is never blocked by a provider not returning fast enough
realized its not that simple, because you have multiple providers you'd need to wait for. if you didn't wait, there's no point in it being a callback because it would just be synchronous.. if you did, well that get's real complex but is possible for each service that does have callbacks only...
Here are my two cents:
Not of course an optimal solution, but IMHO better than nothing.
It should use a Promise (or the callback counter equivalent) to execute the callback after all have finished. For services that don't support callbacks, I'd say executing the callback immediately is fine, or setting a short timeout since that's what the user will probably be doing anyway. Adding a disclaimer to the README about those services should be sufficient.
I was thinking a really basic implementation could not even care about the services themselves and put any callback behind a ~100ms timeout. (The kind of solution that Mixpanel and KISSmetrics use for their link tracking, click tracking and form tracking helpers.)
Safest would certainly be to use their callbacks, but if that's what they're doing in their libraries anyway, maybe it isn't worth the hassle.
I'm not sure it's "safest" to use their callbacks, or at least I don't know what you mean by safest. But you could make a good argument for fastest (depending on their servers). Even with using their callbacks though, I think there should be a maximum timeout to prevent the callback from never firing if the servers are down... (Which I assume these services are implementing internally... but you never know. Seen some crazy stuff :p)
...or from calling it too soon. I wonder which of the providers execute the callbacks only on success, and which handle response errors, and which set a timeout to ensure the callback is always executed. With only a single callback, I'm not sure which would be preferable? I guess the timeout would be easiest because then it's somewhat normalized across the providers, no matter what they offer.
Added these in with a default timeout length.
This feature is great, but you should probably document it on http://segmentio.github.com/analytics.js/
New documentation is in the works! Should be released tomorrow, stay tuned
Guys: thanks a lot for implementing this!