MobX takes normal data (usually class properties) and makes it observable, via an annotation. That very much appears to be what the @State annotation in the Swift code is doing, though I don't know for sure.
setState is a totally special container for data; while it technically lives in a class member (this.state), it can only be modified via setState. It isn't observable, so much as setState just internally triggers a render directly.
useState is different because your state lives out somewhere in React's core framework, associated with your function only through a value and a callback. It doesn't live in your function's local scope itself, because it would get lost if it did. It's very weird and funky because the whole point of non-class components is for them to be pure functions which have no state. useState is this shortsighted workaround for that self-imposed limitation.
Redux, finally, is different because its new states are determined as a pure function of the current state and some change. It's the polar opposite of MobX's mutable-observables pattern. Neither is strictly better, but they're as different as can be.
@observable/useState/React.Component.state are all conceptually identical, just with varying implementations of the magic. All of them summon an observable value from the void and provide some method of updating said observable.
That said, @State seems most identical to useState -- it provides an observed value which you use $/.binding to update.
> It's very weird and funky because the whole point of non-class components is for them to be pure functions which have no state.
That's a false assumption, that's not the "point" of functional components. Functional components were always pure simply because there was no way to make them stateful. There are distinct benefits for using stateful functional components over classes -- mainly less boilerplate and abstractable/re-usable state functions.
setState is a totally special container for data; while it technically lives in a class member (this.state), it can only be modified via setState. It isn't observable, so much as setState just internally triggers a render directly.
useState is different because your state lives out somewhere in React's core framework, associated with your function only through a value and a callback. It doesn't live in your function's local scope itself, because it would get lost if it did. It's very weird and funky because the whole point of non-class components is for them to be pure functions which have no state. useState is this shortsighted workaround for that self-imposed limitation.
Redux, finally, is different because its new states are determined as a pure function of the current state and some change. It's the polar opposite of MobX's mutable-observables pattern. Neither is strictly better, but they're as different as can be.