[AngularJS NG-redux] Handle Asynchronous Operations with Middleware

Invariably the question that comes up when talking about Redux is how does one handle asynchronous operations in redux. For instance, how do we hand off an operation to redux that requires a remote call to the server? Where exactly does the async part get handled?

Redux has this concept of middleware that allows us to insert custom logic in the space between dispatching an action, and the moment it reaches the reducer. In this lesson, we are going to set up a few async operations and then use redux thunk middleware to make sure that everything makes it into the reducer properly.

The main key to thunk middleware is that it allows us to return a function instead of an action. This function can encapsulate the async operation and dispatches the appropriate action when the operation is completed.

To handle async opreations, we can install:

npm install --save redux-thunk

Import:

import thunk from 'redux-thunk';

Use it as middle ware:

const config = $ngReduxProvider => {
  'ngInject';
  $ngReduxProvider.createStoreWith(rootReducers, [thunk]);
};

Refactor action creator to use thunk middleware:

export const CategoriesActions = ($http, $q) => {
  'ngInject';
  const FETCH = {
    categories: 'data/categories.json'
  };

  /*const getCategoreis = (categories) => {
   return {type: GET_CATEGORIES, payload: categories}
   };*/

  const getCategoreis = () => {
    return (dispatch, getState) => {
      const { categories } = getState();
      if (categories.length > 0) {
        return $q.when(categories);
      } else {
        return $http.get(FETCH.categories)
          .then(res => res.data)
          .then((payload) => {
            return dispatch({
              type: GET_CATEGORIES,
              payload
            })
          });
      }
    };
  };

....
}

Here we use cache, so we need to set initial value to empty array:

export const categories = (state = [], { type, payload }) => {
  switch (type) {
    case GET_CATEGORIES:
      return payload || state;
    default:
      return state;
  }
};