sagas-watcher.js

// This watcher is listening for a START_CREATE_USER dispatch. 
// It expects that action to have a payload attribute and it passes that attribute along.

function* watchCreateUser() {
  // while (true) tells the watcher to watch forever.
  while (true) {
    // "yield take" =  Wait here until a 'START_CREATE_USER' action is dispatched
    const { payload } = yield take(START_CREATE_USER)
    // "yield fork" = Asyncly call a generator called startCreateUseFlow, pass it the payload.
    yield fork(startCreateUserFlow, payload)
    // Instead of looping back and waiting for another START_CREATE_USER (which you probably don't 
    // want to accept at this moment) you can call take with an array of actions that will occur 
    // before you'd be ok with calling the flow again.
    // when you provide an array to TAKE it will match the first action it receives and then move on.
    yield take([USER_CREATE_FAIL, USER_LOGOUT])
  } 
  // This is a never ending loop so we return to waiting for another START_CREATE_USER dispatch
}