Support epic-like behavior by passing event stream to invoked observables

[jzarzeckis]

I posted an example as a comment in this thread: #1997 (comment) .

But the general idea is to support similar behavior to epics in Redux-observable where epic has 2 input streams (action$ and state$ from which the output is built).

Here's how I imagine this could look in xState:

const machine = {
  /* ... */
  states: {
    initial: {
      invoke: {
        src: (context, event, event$) =>
          event$ /* OR: from(event$) */
            .pipe(
              filter(({ type }) => type === "FETCH"),
              switchMap(({ url }) => fetch(url)),
              map(({ data }) => ({ type: "FETCH_RESPONSE", data }))
            ),
      },
    },
  },
};