Extend `loading` indicator

[mikhin]

Could we extend loading from boolean to enum maybe? For now, I miss, for example, some initial state, when the request has not yet been made. In current version for this case we have loading: true, but this is not consistent with reality.

[dkzlv]

So the loading: true issue is real. It will be fixed so that the default state of a store is loading: false, and loading key will always be tied to the actual fetcher function execution state.

After giving it a long thought I decided against adding state/status key to the store. The reason is rather simple: it will either mimic all the combinations of possible states, but as a string, or it will lack some of states.

The example that really gave me an understanding of why I don't want this is the following:

1. store is initialized. Its state is idle
2. we start the loading. Its state is loading
3. fetch ends. Its state is done (or error if function throws)

But then we have a refetch strategy, right? Say, by interval.

4. we start the fetching again, but there is cached data already! Is it loading? If so, why do we need it at all if all it does is just a replication of loading flag? So let's assume it is cachedDataAndLoading or whatever
5. but this fetch ends with an error. Again, it shouldn't be just error, it should be cachedDataAndError

There's a reasonable chance I'll introduce something like loadingSlow for cases where fetching lasts longer than a certain timeout. So all loading flags will double.

So in one case we have a replication of 3 boolean checks, in other—a lot of strings (imo, rather cryptic as well), that, on top of it all, can be created in userland rather easily if one needs it very much.

In the end it seems that it would be better to have a thorough documentation on what each store value means and when it can have certain values. The combinatorics of loading, error and data are not that scary, I would say 😄