(docs) - custom exchanges

[JoviDeCroock]

In this first commit I've made an exchange that looks at the token and refreshes when needed, we build iteratively up until this point.

This covers the concepts:

• HOC exchange
• Basic exchange
• operations + forward
• link to wonka docs
• usage of mergeMap to decide a direction

Sandboxes:

Server: https://codesandbox.io/s/urql-issue-template-server-0ufyz
Client: https://codesandbox.io/s/recursing-shadow-t8b6g

[andyrichardson]

This looks good! I think we've been needing something like this for a while and the use case is a valid one (although I'm not sure if blocking operations from being forwarded while waiting for async actions is something we want to encourage?).

There's some work here that might be salvageable - https://github.com/FormidableLabs/urql/pull/186/files

Would love to see the docs that you've done get merged 🙌

[JoviDeCroock]

Thanks for the feedback @andyrichardson I do think it's needed since ideally when using a
refreshToken mechanism you don't want to request unless your token is refreshed. When it's still valid it will just complete with the fromValue anyway. I understand your concern though, I'll add a comment why this isn't good.

I'll add one of your examples in there as well thanks for the link!

[kitten]

@andyrichardson @JoviDeCroock I don't think we need to add a comment on this. In a lot of cases there'll be an existing access token mechanism that people will have to work with and refreshing on the fly isn't always an option but trivial to add to this example.

So the main thing I'd like to highlight is that these guides aren't about best practices but just examples so people can write any exchange they need and not just the ones we're writing about :)

It's just not realistic to add a super complex auth example on a guide here

[JoviDeCroock]

I made some small adjustments to the language, removed a link since it was redundant and formatted the long lines.

[andyrichardson]

I think we could use a different example (such as re-creating one of our existing exchanges) but this is huge progress regardless of the example use case 👍

[JoviDeCroock]

Moved the sandbox with the completed refreshToken exchange to the urql codesandbox team