requiresReaction v.s. keepAlive: what's the difference?

[ygnoh]

Hello. These days I'm closely looking into the MobX doc. I have many questions.

Anyway...

AFAIK, keepAlive keeps computed values being cached even if they are not being observed, which means computed is always cached anywhere. But requiresReaction ... what is it?

The doc says:

If you try to read its value outside of the reactive context, in which case it might not be cached, it will cause the computed to throw instead of doing an expensive re-evalution.

That means if I use requiresReaction then even if the computed is not being observed(outside of the reactive context), it will return computed. right?

If yes, what is the difference between requiresReaction and keepAlive?

Thank you.

[urugator]

keepAlive - computed can be accessed anywhere, the cache is permanent, it does not depend on the presence of reaction/autorun/observer.
requiresReaction - computed can be accessed only inside reaction/autorun/observer, the cache behavior does not change - result is cached only as long as there is at least one reaction/autorun/observer observing. Observers can still come and go overtime and so does the cached result. It may reveal accidentally missing observer. There is also a global version of this flag.