Provide more ways to bail out inside Hooks

There’s a few separate issues but I wanted to file an issue to track them in general:

• useState doesn’t offer a way to bail out of rendering once an update is being processed. This gets a bit weird because we actually process updates during the rendering phase. So we’re already rendering. But we could offer a way to bail on children. Edit: we now do bail out on rendering children if the next state is identical.
• useContext doesn’t let you subscribe to a part of the context value (or some memoized selector) without fully re-rendering. Edit: see https://github.com/facebook/react/issues/15156#issuecomment-474590693 for solutions to this.

[brunolemos]

I would enjoy something like this to avoid unnecessary re-renders:

const { data } = useContext(MyContext, result => [result.data])

where the second parameter would work like the second parameter of useEffect, except it’s () => [] instead of [], with result being the response of useContext(MyContext).

Note: This is supposing the existing second param could change or react internally could check the typeof to see if it’s a function or the observedBits

[sebmarkbage]

My personal summary is that new context is ready to be used for low frequency unlikely updates (like locale/theme). It’s also good to use it in the same way as old context was used. I.e. for static values and then propagate updates through subscriptions. It’s not ready to be used as a replacement for all Flux-like state propagation.