redux-effects-example3.js

// When SUBMIT action happens:
// - emit VALIDATE action,
// - validate the form,
// - and when validating the form finishes with success:
//   - emit SUBMIT_START action.
const validateOnSubmitEpic = (action$, state$) => {
  return action$.ofType(SUBMIT).switchMap(() => {
    const validate$ = Observable.of({ type: VALIDATE });
    const onValidate$ = Observable.merge(
        action$.ofType({ type: VALIDATE_SUCCESS }),
        action$.ofType({ type: VALIDATE_FAILURE })
      )
      .first()
      .filter(action => action.type === VALIDATE_SUCCESS)
      .mapTo({ type: SUBMIT_START });
    return Observable.merge(validate$, onValidate$);
  });
}