Request: useSWR status

[icyJoseph]

Hi!

I've been using this library, and I've collected some feedback from colleagues or people I show this hook to.

One persistent subject is that, often developers want to derive the status of the hook, that is, is it loading? is it validating? is there a difference between them at all? is there any data, etc. Refer to: #563

I've made a PR with some tested changes, that do not break other tests, which can be used to discuss and perhaps refine this feature further. At the moment I don't have so much time to spend on discussions, so hopefully the code can explain itself 😅

#1215

In short:

• When there's no data, and no error, loading
• If there's data and no revalidation happening, stale
• If there's error and no revalidating happening, error
• If there's revalidation, validating

These would be hard-coded strings, which would allow developers to simply match them.

Today developers have to write the same checks every time, const isLoading = !data && !error and eventually end up getting mixed with isValidating. I also noticed that isValidating seems to get stuck on error cases. I am not sure my PR totally fixed that though.

I have also found use cases where people simply mix isValidating and isLoading under one loading flag, with total disregard to #563, because I guess people don't want to bother searching for answers.

What are your thoughts?