Observable REST API with Vue.js

Just recently I discovered a rather new feature in Vue.js (since 2.6.0+): Vue.observable. Vue.observable is used internally in Vue.js to make the object returned by the data() function of a component reactive.

In this article we’ll take a look at how we can use this new feature to build a very simple reactive polling system for a regular REST API.

REST API utility function with polling

Let’s jump right in. In the following example you can see a simple implementation of a withPolling() wrapper function which takes a callback function and an interval as parameters. If an interval time (in ms) is given, it creates a new observable from the data returned by the given callback function. Otherwise it returns the data in a plain object.

// src/utils/api.js
import Vue from 'vue';
// A simple wrapper around the native
// `fetch()` function.
import quickFetch from './quick-fetch';

async function withPolling(callback, interval) {
  const data = await callback();

  // If no polling interval is given we
  // return a regular object with the data.
  if (!interval) return { data };

  // Otherwise, we create a new `Vue.observable`
  // instance and refetch the data according to
  // the specified polling interval.
  const observableData = Vue.observable({ data });
  const poll = () => {
    setTimeout(async () => {
      observableData.data = { ...(await callback()) };
      poll();
    }, interval);
  };
  poll();

  return observableData;
}

export default function api({ endpoint, interval }) {
  return withPolling(() => quickFetch(endpoint), interval);
}

The api() function at the bottom is a convenience function to quickly create API services for various endpoints.

// src/services/todo.js
import api from '../utils/api';

const ENDPOINT = 'https://jsonplaceholder.typicode.com/todos';

export default {
  fetch: id => api({ endpoint: `${ENDPOINT}/${id}` }),
  poll: id => api({ endpoint: `${ENDPOINT}/${id}`, interval: 2000 }),
};

The todo service shown above uses the util function api() to create two methods to either simply fetch a TODO item or fetch a TODO item and also poll for changes to that item.


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Usage

Here you can see how we can use our newly created todo API service both ways: for fetching static data or with polling enabled.

<template>
  <div id="app">
    <strong>TODO (static)</strong>
    <input
      type="checkbox"
      :checked="response.data.completed"
    >
    {{ response.data.title }}
    <strong>TODO (polling)</strong>
    <input
      type="checkbox"
      :checked="pollResponse.data.completed"
    >
    {{ pollResponse.data.title }}
  </div>
</template>

<script>
import todoService from './services/todo';

export default {
  name: 'App',
  data() {
    return {
      response: { data: {} },
      pollResponse: { data: {} },
    };
  },
  async created() {
    this.response = await todoService.fetch();
    this.pollResponse = await todoService.poll();
  },
};
</script>

If you want to see the code in action you can take a look at the following CodeSandbox. Keep in mind, however, that I had to make some modifications to the code in order to demonstrate polling.

Wrapping it up

I think Vue.observable can be a very powerful tool when it is used the right way. But it can also be a possible foot gun. Although, even the official documentation states a simple global store solution as a possible use case, I’d highly recommend you to not use Vue.observable that way.

Changing the contents of an observable object should always be one way. Like in our example the data is only modified by the API response. You absolutely should not change the data directly in the consuming App.vue component. Otherwise, as your application is growing, you’ll soon be in a world of trouble where a lot of different components make changes to the same global state and there is no way of knowing which component triggered what change (that’s why you’re only allowed to change the state in Vuex through mutations).


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