Avoiding event chains with asynchronous data dependencies

问题

The Facebook Flux dispatcher explicitly prohibits ActionCreators from dispatching other ActionCreators. This restriciton is probably a good idea since it prevents your application from creating event chains.

This however becomes an issue as soon as you have Stores containing data from asynchronous ActionCreators that depend on each other. If CategoryProductsStore depends on CategoryStore there doesn't seem to be a way to avoid event chains when without resorting to deferring the follow-up action.

Scenario 1: A store containing a list of products in a category needs to know from which category ID it should fetch products from.

var CategoryProductActions = {
  get: function(categoryId) {
    Dispatcher.handleViewAction({
      type: ActionTypes.LOAD_CATEGORY_PRODUCTS,
      categoryId: categoryId
    })

    ProductAPIUtils
      .getByCategoryId(categoryId)
      .then(CategoryProductActions.getComplete)
  },

  getComplete: function(products) {
    Dispatcher.handleServerAction({
      type: ActionTypes.LOAD_CATEGORY_PRODUCTS_COMPLETE,
      products: products
    })
  }
}

CategoryStore.dispatchToken = Dispatcher.register(function(payload) {
  var action = payload.action

  switch (action.type) {
    case ActionTypes.LOAD_CATEGORIES_COMPLETE:
      var category = action.categories[0]

      // Attempt to asynchronously fetch products in the given category, this causes an invariant to be thrown.
      CategoryProductActions.get(category.id)

      ...

Scenario 2: Another scenario is when a child component is mounted as the result of a Store change and its componentWillMount/componentWillReceiveProps attempts to fetch data via an asynchronous ActionCreator:

var Categories = React.createClass({
  componentWillMount() {
    CategoryStore.addChangeListener(this.onStoreChange)
  },

  onStoreChange: function() {
    this.setState({
      category: CategoryStore.getCurrent()
    })
  },

  render: function() {
    var category = this.state.category

    if (category) {
      var products = <CategoryProducts categoryId={category.id} />
    }

    return (
      <div>
        {products}
      </div>
    )
  }
})

var CategoryProducts = React.createClass({
  componentWillMount: function() {
    if (!CategoryProductStore.contains(this.props.categoryId)) {
      // Attempt to asynchronously fetch products in the given category, this causes an invariant to be thrown.
      CategoryProductActions.get(this.props.categoryId)
    }
  }
})

Are there ways to avoid this without resorting to defer?

回答1:

Whenever you are retrieving the state of the application, you want to be retrieving that state directly from the Stores, with getter methods. Actions are objects that inform Stores. You could think of them as being like a request for a change in state. They should not return any data. They are not a mechanism by which you should be retrieving the application state, but rather merely changing it.

So in scenario 1, getCurrent(category.id) is something that should be defined on a Store.

In scenario 2, it sounds like you are running into an issue with the initialization of the Store's data. I usually handle this by (ideally) getting the data into the stores before rendering the root component. I do this in a bootstrapping module. Alternatively, if this absolutely needs to be async, you can create everything to work with a blank slate, and then re-render after the Stores respond to an INITIAL_LOAD action.

回答2:

For scenario 1:

I would dispatch new the action from the view itself, so a new action -> dispatcher -> store -> view cycle will trigger.

I can imagine that your view needs to retrieve the category list and also it has to show, by default, the list of products of the first category.

So that view will react to changes con CategoryStore first. Once the category list is loaded, trigger the new Action to get the products of the first category.

Now, this is the tricky part. If you do that in the change listener of the view, you will get an invariant exception, so here you have to wait for the payload of the first action to be completely processed.

One way to solve this is to use timeout on the change listener of the view. Something similar to what is explained here: https://groups.google.com/forum/#!topic/reactjs/1xR9esXX1X4 but instead of dispatching the action from the store, you would do it from the view.

function getCategoryProducts(id) {
setTimeout(() => {
    if (!AppDispatcher.isDispatching()) {
        CategoryProductActions.get(id);
    } else {
        getCategoryProducts(id);
    }
}, 3);
}

I know, it is horrible, but at least you won't have stores chaining actions or domain logic leaking to action creators. With this approach, the actions are "requested" from the views that actually need them.

The other option, which I haven't tried honestly, is to listen for the DOM event once the component with the list of categories is populated. In that moment, you dispatch the new action which will trigger a new "Flux" chain. I actually think this one is neater, but as said, I haven't tried yet.

来源：https://stackoverflow.com/questions/25860642/avoiding-event-chains-with-asynchronous-data-dependencies

标签

reactjs

reactjs-flux