Using Renderless Components in Vue.js

Using Renderless Components in Vue.js

Most Vue apps need asynchronous HTTP requests and there are many ways to realize them: in the mounted() lifecycle hook, in a method triggered by a button, within the store (when using vuex) or in the asyncData() and fetch() methods (with Nuxt).

While a simple request is very easy with axios, we usually want to cover at least two additional states:

  1. Show something to the user while the request is pending
  2. Handle errors gracefully

Handling these states adds additional code and can quickly lead to code-duplication when having to implement many different requests.

To cut right to the meat, jump to The Async Renderless Component.

Note: Axios is used to make HTTP requests in this example, but it works just as well with any other library for AJAX requests. Also, this example uses this wonderful free Dog API: https://dog.ceo/dog-api/ 🐶.

Origin of the Idea

The idea is not my own, but borrowed from Vue.js creator Evan You @youyuxi who voiced it secondarily while talking about Advanced Vue Components with Adam Whatan on the Full Stack Radio Podcast during Episode 81.

HTTP request in Vue Components: A typical example

Let's start with a minimal example to request a random dog image. The mounted() hook contains the axios call which populates the image variable.

Vue.component("example", {
  el: "#example",
  data() {
    return {
      image: null
    };
  },
  mounted() {
    axios
      .get("https://dog.ceo/api/breeds/image/random")
      .then(function(response) {
        this.image = response.data;
      });
  }
});

Simple enough. However, we want to show a loading animation and handle request errors. So in addition to the image variable pending: false and error: null are added. The mounted() hook then looks as follows:

Vue.component("example", {
  [...]
  mounted() {
    this.pending = true;
    axios
      .get("https://dog.ceo/api/breeds/image/random")
      .then(function(response) { this.image = response.data })
      .catch(function(error) { this.error = error })
      .finally(function () { this.pending = false });
  }
});

Now a loading indicator can be shown for pending === true and a basic error message can be displayed if error !== null. It's really simple, but it can get tedious to implement this pending/success/error behavior repeatedly. Besides, if the request contains parameters that can be changed by the user, e.g. filters or sorting options, then the request has to move to a method which has to be called, whenever the parameters changes, to reload the data.

One easy and effective way to abstract away this simple behavior and make it reusable is ...

The Async Renderless Component

This component makes use of the incredibly versatile Scoped Slot feature. A slot is any piece of HTML that can be passed to a component, telling the component: "Here, render this somewhere". With scoped slots the component which receives the HTML snipped answers: "Awesome, I will put your HTML right there. And here is some data you can use with your snipped if you like".

The Async Renderless component is just such a component that receives a snippet of HTML, a URL and parameters and answers: "Hey look, I am requesting this data for you, here is data, pending and error for you to use."

The Async Renderless Component in full:

Vue.component("async", {
  props: {
    url: { type: String, default: "", required: true },
    params: { type: Object, default: () => ({}) }
  },
  data() {
    return {
      pending: true,
      error: false,
      data: null
    };
  },
  watch: {
    url() {
      this.requestData();
    },
    params: {
      handler() {
        this.requestData();
      },
      deep: true
    }
  },
  mounted() {
    this.requestData();
  },
  methods: {
    async requestData() {
      this.pending = true;
      try {
        const { data } = await axios.get(this.url, { params: this.params });
        this.data = data;
        this.error = false;
      } catch (e) {
        this.data = null;
        this.error = e;
      }
      this.pending = false;
    }
  },
  render() {
    return this.$scopedSlots.default({
      pending: this.pending,
      error: this.error,
      data: this.data
    });
  }
});

The "renderless" happens in the render() tag. Instead of an HTML tag, these components only renders the HTML snippet it receives in its slot as scoped slot, passing three data points to it: pending, error and data.

The watch functions make sure that the data is reloaded whenever either url or params change.

We use the async component inside our template like this:

<async url="https://dog.ceo/api/breed/husky/images">
  <template v-slot:default="{ pending, error, data }">
    <div v-if="pending">Loading ...</div>
    <div v-else-if="error">{{ error }}</div>
    <div v-else>{{ data }}</div>
  </template>
</async>

Why a renderless component and not a mixin or directive?

Components are not the only way to reuse code in Vue, another way is to use a Mixin or a Custom Directive. Both are fine ways to solve this problem. Renderless components utilizing scoped slots are operating the way Vue wants to work, it can be imported when needed just like you are used to with any other component. Thus it's a very explicit way to reuse code as opposed to mixins or directives which don't have to be included separately. In the end, it comes down to preference.

An applied example

I constantly find myself implementing lists when working with APIs which usually feature things like pagination, filters, sorting and search. So I decided to put together a "real-life" example which renders a simple list of dog images with a very simple filter option for some different breeds (and a wrong API call to see the error state):

https://codepen.io/lhermann/pen/jgGxyQ

Whenever one of the filter buttons is clicked the URL, which is passed to the async component, is updated with the appropriate breed. The async component takes care of the HTTP request. No more HTTP request logic is needed in the parent component, separation of concerns is obeyed, our minds are freed and the universe is in harmony 😄.

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