Better Type-Safety When Using the inject Function

[PCOffline]

This is a proposal for an opt-in behaviour that introduces better type-safety when injecting stores.
The type-safety is improved in two ways:

1. When injecting stores, add types of known stores to ensure that all strings passed as arguments are valid stores
2. The function returned by inject takes a component as an argument. This component could have inferred prop types based on the stores injected. (e.g. if Store1 was injected, then we know that the props have Store1)

I will emphasise this behaviour is opt-in, so it's not a breaking-change.
Here's a POC

[kubk]

Thanks for the idea. Please check this: #3741 (comment)

Note: usually there is no need anymore to use Provider / inject in new code bases; most of its features are now covered by React.createContext.

[PCOffline]

Yes I'm aware, but many people are still stuck on the injection API and it's too costly to migrate, so why not add some fancy types :)

[mweststrate]

@PCOffline how would this work generically, e.g. in your PoC MobxStores is defined in userland code, as is inject. So in principle you'd still have to pass MobXStores as generic to inject anyway, which is not to different from typing the props? Would you say inject<MobXStores>('inject')(....) is still an significant improvement?

[PCOffline]

Hmm, I agree it seems clumsy. What I had in mind is that Mobx internally has a collection of all the stores and can produce a type from them. If that's not possible, I don't have a fancy solution for this problem.
Maybe the user can declare MobxStores and override inject to implicitly be inject<MobxStores>, but that's also cumbersome.
Alternatively, you can declare a type internally called MobxStores which, by default, is a string, and the user can override with module overrides. Again, very unintuitive, but I can't think of anything better.