Flow: Missing React.forwardRef definition

[billyjanitsch]

React 16.3 introduced a new forwardRef() API, for which Flow does not currently have a definition.

[motiz88]

I might try to work on this over the weekend.

Looks like for maximum correctness we need a new opaque type React$RefForwardingComponent<Props> (name?) and to add it as an option to the unions React$ComponentType<Props> and React$ElementType. However, and I'm extrapolating from limited experimentation here, it looks like Flow will not typecheck these props (when used in JSX etc) unless we add special handling for RefForwardingComponent (in react_kit.ml probably?)

And then there's the option of fudging it and pretending forwardRef<Props>() returns React$ComponentType<Props>, but that's obviously unsound 🤷‍♂️

[motiz88]

OK, having done some digging, I'm going to make a PR with the less-correct solution, typing forwardRef as returning ComponentType<Props>. I'm basing this on the existing libdef for createContext: It models Provider and Consumer as ComponentType<Props> even though internally they're plain objects distinguished by their $$typeof property, and therefore are neither callable nor are they classes extending React.Component.

Ideally we would model these React builtins using a first-class "opaque component" type so we could catch certain errors that the current model will happily allow (_and_ consider to have full type coverage).

[tiarebalbi]

Any idea of which release this will be part of? Thanks

[jbrown215]

@tiarebalbi 0.89

[jbrown215]

I will follow up with a blog post on how to use the new AbstractComponent type. It will coincide with the 0.89 release.

[shahzaibkhalid]

@jbrown215 Can we get any idea of timelines for release?

[jbrown215]

At the latest, December 19th, but hopefully December 12th. Call these dates tentative, since unexpected blockers might pop up.