Watch

watch runs actions whenever a context (or computed) field changes — in any state, for the machine’s whole lifetime. It’s the declarative form of “subscribe to a field and react.”

import { machine } from '@dunky-dev/state-machine'

const m = machine({
  initial: 'idle',
  context: { query: '' },
  watch: {
    query: ['runSearch'], // whenever `query` changes → run runSearch
  },
  states: { idle: {} },
  implementations: {
    actions: {
      runSearch: ({ context, send }) => send({ type: 'search', q: context.query }),
    },
  },
})

Any time query changes — from any transition, in any state — runSearch fires. No need to add the action to every transition that touches query.

Run-to-completion

Watch actions obey the same run-to-completion rule as events: they’re queued and run after the transition that changed the field fully settles. A watcher writing context can’t re-enter a transition in flight.

Watch vs. computed

	watch	computed
Use for	side-effects (send, log, sync)	derived values (filter, count, sum)
Runs when	field changes	read, if input changed
Returns	nothing	a value

Use watch when you want to do something in response to a field changing. Use computed when you want to derive a value from fields.

Multiple fields

watch: {
  query:       ['runSearch'],
  activeIndex: ['scrollIntoView', 'announceSelection'],
}