Reactjs – How to manage asynchronous Store operations with Flux

In a Flux app there should only be one Dispatcher. All data flows through this central hub. Having a singleton Dispatcher allows it to manage all Stores. This becomes important when you need Store #1 update itself, and then have Store #2 update itself based on both the Action and on the state of Store #1. Flux assumes this situation is an eventuality in a large application. Ideally this situation would not need to happen, and developers should strive to avoid this complexity, if possible. But the singleton Dispatcher is ready to handle it when the time comes.

Stores are singletons as well. They should remain as independent and decoupled as possible -- a self-contained universe that one can query from a Controller-View. The only road into the Store is through the callback it registers with the Dispatcher. The only road out is through getter functions. Stores also publish an event when their state has changed, so Controller-Views can know when to query for the new state, using the getters.

In your example app, there would be a single PostStore. This same store could manage the posts on a "page" (pseudo-page) that is more like FB's Newsfeed, where posts appear from different users. Its logical domain is the list of posts, and it can handle any list of posts. When we move from pseudo-page to pseudo-page, we want to reinitialize the state of the store to reflect the new state. We might also want to cache the previous state in localStorage as an optimization for moving back and forth between pseudo-pages, but my inclination would be to set up a PageStore that waits for all other stores, manages the relationship with localStorage for all the stores on the pseudo-page, and then updates its own state. Note that this PageStore would store nothing about the posts -- that's the domain of the PostStore. It would simply know whether a particular pseudo-page has been cached or not, because pseudo-pages are its domain.

The PostStore would have an initialize() method. This method would always clear the old state, even if this is the first initialization, and then create the state based on the data it received through the Action, via the Dispatcher. Moving from one pseudo-page to another would probably involve a PAGE_UPDATE action, which would trigger the invocation of initialize(). There are details to work out around retrieving data from the local cache, retrieving data from the server, optimistic rendering and XHR error states, but this is the general idea.

If a particular pseudo-page does not need all the Stores in the application, I'm not entirely sure there is any reason to destroy the unused ones, other than memory constraints. But stores don't typically consume a great deal of memory. You just need to make sure to remove the event listeners in the Controller-Views you are destroying. This is done in React's componentWillUnmount() method.

What is wrong with this approach? Why would I want to use Redux Thunk or Redux Promise, as the documentation suggests?

There is nothing wrong with this approach. It’s just inconvenient in a large application because you’ll have different components performing the same actions, you might want to debounce some actions, or keep some local state like auto-incrementing IDs close to action creators, etc. So it is just easier from the maintenance point of view to extract action creators into separate functions.

You can read my answer to “How to dispatch a Redux action with a timeout” for a more detailed walkthrough.

Middleware like Redux Thunk or Redux Promise just gives you “syntax sugar” for dispatching thunks or promises, but you don’t have to use it.

So, without any middleware, your action creator might look like

// action creator
function loadData(dispatch, userId) { // needs to dispatch, so it is first argument
  return fetch(`http://data.com/${userId}`)
    .then(res => res.json())
    .then(
      data => dispatch({ type: 'LOAD_DATA_SUCCESS', data }),
      err => dispatch({ type: 'LOAD_DATA_FAILURE', err })
    );
}

// component
componentWillMount() {
  loadData(this.props.dispatch, this.props.userId); // don't forget to pass dispatch
}

But with Thunk Middleware you can write it like this:

// action creator
function loadData(userId) {
  return dispatch => fetch(`http://data.com/${userId}`) // Redux Thunk handles these
    .then(res => res.json())
    .then(
      data => dispatch({ type: 'LOAD_DATA_SUCCESS', data }),
      err => dispatch({ type: 'LOAD_DATA_FAILURE', err })
    );
}

// component
componentWillMount() {
  this.props.dispatch(loadData(this.props.userId)); // dispatch like you usually do
}

So there is no huge difference. One thing I like about the latter approach is that the component doesn’t care that the action creator is async. It just calls dispatch normally, it can also use mapDispatchToProps to bind such action creator with a short syntax, etc. The components don’t know how action creators are implemented, and you can switch between different async approaches (Redux Thunk, Redux Promise, Redux Saga) without changing the components. On the other hand, with the former, explicit approach, your components know exactly that a specific call is async, and needs dispatch to be passed by some convention (for example, as a sync parameter).

Also think about how this code will change. Say we want to have a second data loading function, and to combine them in a single action creator.

With the first approach we need to be mindful of what kind of action creator we are calling:

// action creators
function loadSomeData(dispatch, userId) {
  return fetch(`http://data.com/${userId}`)
    .then(res => res.json())
    .then(
      data => dispatch({ type: 'LOAD_SOME_DATA_SUCCESS', data }),
      err => dispatch({ type: 'LOAD_SOME_DATA_FAILURE', err })
    );
}
function loadOtherData(dispatch, userId) {
  return fetch(`http://data.com/${userId}`)
    .then(res => res.json())
    .then(
      data => dispatch({ type: 'LOAD_OTHER_DATA_SUCCESS', data }),
      err => dispatch({ type: 'LOAD_OTHER_DATA_FAILURE', err })
    );
}
function loadAllData(dispatch, userId) {
  return Promise.all(
    loadSomeData(dispatch, userId), // pass dispatch first: it's async
    loadOtherData(dispatch, userId) // pass dispatch first: it's async
  );
}


// component
componentWillMount() {
  loadAllData(this.props.dispatch, this.props.userId); // pass dispatch first
}

With Redux Thunk action creators can dispatch the result of other action creators and not even think whether those are synchronous or asynchronous:

// action creators
function loadSomeData(userId) {
  return dispatch => fetch(`http://data.com/${userId}`)
    .then(res => res.json())
    .then(
      data => dispatch({ type: 'LOAD_SOME_DATA_SUCCESS', data }),
      err => dispatch({ type: 'LOAD_SOME_DATA_FAILURE', err })
    );
}
function loadOtherData(userId) {
  return dispatch => fetch(`http://data.com/${userId}`)
    .then(res => res.json())
    .then(
      data => dispatch({ type: 'LOAD_OTHER_DATA_SUCCESS', data }),
      err => dispatch({ type: 'LOAD_OTHER_DATA_FAILURE', err })
    );
}
function loadAllData(userId) {
  return dispatch => Promise.all(
    dispatch(loadSomeData(userId)), // just dispatch normally!
    dispatch(loadOtherData(userId)) // just dispatch normally!
  );
}


// component
componentWillMount() {
  this.props.dispatch(loadAllData(this.props.userId)); // just dispatch normally!
}

With this approach, if you later want your action creators to look into current Redux state, you can just use the second getState argument passed to the thunks without modifying the calling code at all:

function loadSomeData(userId) {
  // Thanks to Redux Thunk I can use getState() here without changing callers
  return (dispatch, getState) => {
    if (getState().data[userId].isLoaded) {
      return Promise.resolve();
    }

    fetch(`http://data.com/${userId}`)
      .then(res => res.json())
      .then(
        data => dispatch({ type: 'LOAD_SOME_DATA_SUCCESS', data }),
        err => dispatch({ type: 'LOAD_SOME_DATA_FAILURE', err })
      );
  }
}

If you need to change it to be synchronous, you can also do this without changing any calling code:

// I can change it to be a regular action creator without touching callers
function loadSomeData(userId) {
  return {
    type: 'LOAD_SOME_DATA_SUCCESS',
    data: localStorage.getItem('my-data')
  }
}

So the benefit of using middleware like Redux Thunk or Redux Promise is that components aren’t aware of how action creators are implemented, and whether they care about Redux state, whether they are synchronous or asynchronous, and whether or not they call other action creators. The downside is a little bit of indirection, but we believe it’s worth it in real applications.

Finally, Redux Thunk and friends is just one possible approach to asynchronous requests in Redux apps. Another interesting approach is Redux Saga which lets you define long-running daemons (“sagas”) that take actions as they come, and transform or perform requests before outputting actions. This moves the logic from action creators into sagas. You might want to check it out, and later pick what suits you the most.

I searched the Redux repo for clues, and found that Action Creators were required to be pure functions in the past.

This is incorrect. The docs said this, but the docs were wrong.
Action creators were never required to be pure functions.
We fixed the docs to reflect that.