RFC: Context Selectors

[gnoff]

See actual RFC here: reactjs#119

------------------------- DRAFT RFC below ----------------------------

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This RFC describes a new API, currently envisioned as a new hook, for allowing users to make a selection from a context value instead of the value itself. If a context value changes but the selection does not the host component would not update.

This is a new API and would likely remove the need for observedBits, an as-of-yet unreleased bailout mechanism for existing readContext APIs

For performance and consistency reasons this API would rely on changes to context propagation to make it lazier. See RFC for lazy context propagation

Motivation

In summary, adding this useContextSelector API would solve external state subscription incompatibilities with Concurrent React, eliminate a lot of complexity and code size in userland libraries, make almost any Context-using app the same speed or faster, and provide users with a more ergonomic alternative to the observedBits bailout optimization.

Addendum

Example: https://codesandbox.io/s/react-context-selectors-xzj5v
Implementation: https://github.com/gnoff/react/pull/3/files

[gnoff]

opened PR against main RFC repo.

reactjs#119

closing this one