feature: provide config.isPaused indicating idle state and stop revalidation

[huozhi]

Description

give user ability to decide whether they want revalidation in some edge cases:

• page transition on slow connection that revalidations can cause more errors
• page is in background sometimes they don't want any revalidation

Changes

• add config.isPaused()
• stop doing revalidation on config.isPaused() is true

related discussion 841

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[shuding]

This is a good start! We can have built-in beforeunload event handling and other features based on this. 2 questions:
Why using a function (isIdle: () => boolean) instead of a value (isIdle: boolean)? Value should be more consistent with our existing APIs. Another concern is that I think pause might be a better name, idle makes me feel that it means SWR is currently not active (not fetching anything).

[huozhi]

@shuding Thanks for the nice feedback! I was thinking the paused state might be related to sth (like DOM state) out of react, i.e. swr can be paused when document.visible is false like isOnline. So I feel we can rename to isPaused but keep it as a functional property?

will update PR by comments 🙏

[shuding]

But the behavior of this option is to pause requests right (everything will be skipped)?

[huozhi]

yup, I've changed to config.isPuased() to let user add some condition like

isPuased() { return document.visibility === 'hidden' }

and add one more case in revalidation catch branch, which is ignoring the errors when promises are been rejected under paused state

[shuding]

Thanks, this is a good fix!