useStateTransition.ts37 lines · main

1	import { useEffect, useRef } from 'react'
2
3	export function useStateTransition<
4	State extends { type: string },
5	PrevType extends State['type'],
6	NewType extends State['type'],
7	>(
8	state: State,
9	// Documentary only — the entry-detection logic below intentionally does not
10	// require `savedPrevState.type === prevTest`. React 18+ auto-batches
11	// dispatches across awaits (e.g. dispatch SUBMIT → await → mutation
12	// onError dispatch ERROR), which collapses `prevTest → newTest` into a
13	// single render where the intermediate `prevTest` state is never observed,
14	// so the previous state may be any variant other than `newTest`. `cb`'s
15	// first parameter is typed accordingly.
16	_prevTest: PrevType,
17	newTest: NewType,
18	cb: (
19	prevState: Exclude<State, { type: NewType }>,
20	currState: Extract<State, { type: NewType }>
21	) => void
22	): void {
23	const prevState = useRef(state)
24
25	useEffect(() => {
26	const savedPrevState = prevState.current
27
28	if (savedPrevState.type !== newTest && state.type === newTest) {
29	cb(
30	savedPrevState as Exclude<State, { type: NewType }>,
31	state as Extract<State, { type: NewType }>
32	)
33	}
34
35	prevState.current = state
36	}, [cb, newTest, state])
37	}