feat: pass a function to the revalidate option in mutate

[koba04]

This PR proposes to allow us to pass a function to the revalidate option.

Motivation

My main motivation is to provide a way to revalidate specific pages with useSWRInfinite. See more details in the section below.
refs. #1590 (comment), #616 (comment)

This also allows us a conditional revalidation based on the result of the mutation.

I can implement this feature as a separate option, but I think having revalidate and revalidatePage (?) is confusing. This proposal only adds a way to pass a function to the revalidate option, but this gives us more capabilities.

Interface

- revalidate?: boolean
+ revalidate?: boolean | (data: Data, key: Arguments) => boolean

data is the latest data, which means this is the mutated data if mutate has the data argument.
key is the key value bounded to the mutation. If fetcher accepts a function as its key argument, this is the return value of the function.

with useSWRInfinite

If you use mutate returned from useSWRInfinite, the revalidate function is called with each page and useSWRInfinite only revalidates the pages that return true, which means you can revalidate specific pages. This drastically improves performance in the case we know what pages we should revalidate.

const { data, mutate, size } = useSWRInfinite((index) => [`/api/?page=${index + 1}`, index + 1], fetcher);

mutate(data, {
  // only revalidate the last page
  revalidate: (pageData, [_, page]) => page === size
});

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 2d9f849:

Sandbox	Source
SWR-Basic	Configuration
SWR-States	Configuration
SWR-Infinite	Configuration
SWR-SSR	Configuration

[shuding]

The code and the feature itself sound good to me 👍