epics.ts

/** Launch a new kernel when the app host token changes. **/
const launchKernelWhenTokenChanged = (action$, state$) =>
  // Whenever a new kernel is launched we execute this logic
  action$.pipe(ofType("LAUNCH_KERNEL_SUCCESSFUL"),
  switchMap((action) => 
  // We subscribe to the StateObservable, which is a stream of changes to the Redux state
  state$.pipe(
    // We pull out the property we care about, the token field in the core state
    pluck("app", "host", "token"),
    // We only move on to the next step if it has changed https://www.learnrxjs.io/operators/filtering/distinctuntilchanged.html
    distinctUntilChanged(),
    // if so we launch a kernel by name
    concatMap(() => of(actions.launchKernelByName({ contentRef: action.payload.contentRef, .... })
 )));