Wire in data

So far, we have created isolated stateless components-–great for Storybook, but ultimately not helpful until we give them some data in our app.

This tutorial doesn’t focus on the particulars of building an app, so we won’t dig into those details here. But we will take a moment to look at a common pattern for wiring in data with container components.

Container components

Our TaskList component as currently written is “presentational” in that it doesn’t talk to anything external to its own implementation. To get data into it, we need a “container”.

This example uses Svelte's Stores, Svelte's default data management API, to build a simple data model for our app. However, the pattern used here applies just as well to other data management libraries like Apollo and MobX.

First, we’ll construct a simple Svelte store that responds to actions that change the state of tasks in a file called store.js in the src directory (intentionally kept simple):

// A simple Svelte store implementation with update methods and initial data.
// A true app would be more complex and separated into different files.

import { writable } from 'svelte/store';

const TaskBox = () => {
  // Creates a new writable store populated with some initial data
  const { subscribe, update } = writable([
    { id: '1', title: 'Something', state: 'TASK_INBOX' },
    { id: '2', title: 'Something more', state: 'TASK_INBOX' },
    { id: '3', title: 'Something else', state: 'TASK_INBOX' },
    { id: '4', title: 'Something again', state: 'TASK_INBOX' },
  ]);

  return {
    subscribe,
    // Method to archive a task, think of a action with redux or Pinia
    archiveTask: (id) =>
      update((tasks) =>
        tasks
          .map((task) =>
            task.id === id ? { ...task, state: 'TASK_ARCHIVED' } : task
          )
          .filter((t) => t.state === 'TASK_INBOX' || t.state === 'TASK_PINNED')
      ),
    // Method to archive a task, think of a action with redux or Pinia
    pinTask: (id) =>
      update((tasks) =>
        tasks.map((task) =>
          task.id === id ? { ...task, state: 'TASK_PINNED' } : task
        )
      ),
  };
};
export const taskStore = TaskBox();

Then we'll update our TaskList to read data out of the store. First, let's move our existing presentational version to the file src/components/PureTaskList.svelte and wrap it with a container.

In src/components/PureTaskList.svelte:

<!--This file moved from TaskList.svelte-->
<script>
  import Task from './Task.svelte';
  import LoadingRow from './LoadingRow.svelte';
  export let loading = false;
  export let tasks = [];

  //👇 Reactive declarations (computed props in other frameworks)
  $: noTasks = tasks.length === 0;
  $: emptyTasks = noTasks && !loading;
  $: tasksInOrder = [
    ...tasks.filter((t) => t.state === 'TASK_PINNED'),
    ...tasks.filter((t) => t.state !== 'TASK_PINNED'),
  ];
</script>

{#if loading}
  <div class="list-items">
    <LoadingRow />
    <LoadingRow />
    <LoadingRow />
    <LoadingRow />
    <LoadingRow />
  </div>
{/if}
{#if emptyTasks}
  <div class="list-items">
    <div class="wrapper-message">
      <span class="icon-check" />
      <p class="title-message">You have no tasks</p>
      <p class="subtitle-message">Sit back and relax</p>
    </div>
  </div>
{/if}
{#each tasksInOrder as task}
  <Task {task} on:onPinTask on:onArchiveTask />
{/each}

In src/components/TaskList.svelte:

<script>
  import PureTaskList from './PureTaskList.svelte';
  import { taskStore } from '../store';

  function onPinTask(event) {
    taskStore.pinTask(event.detail.id);
  }
  function onArchiveTask(event) {
    taskStore.archiveTask(event.detail.id);
  }
</script>

<PureTaskList
  tasks={$taskStore}
  on:onPinTask={onPinTask}
  on:onArchiveTask={onArchiveTask}
/>

The reason to keep the presentational version of the TaskList separate is that it is easier to test and isolate. As it doesn't rely on the presence of a store, it is much easier to deal with from a testing perspective. Let's rename src/components/TaskList.stories.js into src/components/PureTaskList.stories.js and ensure our stories use the presentational version:

import PureTaskList from './PureTaskList.svelte';
import MarginDecorator from './MarginDecorator.svelte';

import * as TaskStories from './Task.stories';

export default {
  component: PureTaskList,
  //👇 The auxiliary component will be added as a decorator to help show the UI correctly
  decorators: [() => MarginDecorator],
  title: 'PureTaskList',
  argTypes: {
    onPinTask: { action: 'onPinTask' },
    onArchiveTask: { action: 'onArchiveTask' },
  },
};

const Template = args => ({
  Component: PureTaskList,
  props: args,
  on: {
    ...TaskStories.actionsData,
  },
});
export const Default = Template.bind({});
Default.args = {
  // Shaping the stories through args composition.
  // The data was inherited from the Default story in task.stories.js.
  tasks: [
    { ...TaskStories.Default.args.task, id: '1', title: 'Task 1' },
    { ...TaskStories.Default.args.task, id: '2', title: 'Task 2' },
    { ...TaskStories.Default.args.task, id: '3', title: 'Task 3' },
    { ...TaskStories.Default.args.task, id: '4', title: 'Task 4' },
    { ...TaskStories.Default.args.task, id: '5', title: 'Task 5' },
    { ...TaskStories.Default.args.task, id: '6', title: 'Task 6' },
  ],
};

export const WithPinnedTasks = Template.bind({});
WithPinnedTasks.args = {
  // Shaping the stories through args composition.
  // Inherited data coming from the Default story.
  tasks: [
    ...Default.args.tasks.slice(0, 5),
    { id: '6', title: 'Task 6 (pinned)', state: 'TASK_PINNED' },
  ],
};

export const Loading = Template.bind({});
Loading.args = {
  tasks: [],
  loading: true,
};

export const Empty = Template.bind({});
Empty.args = {
  // Shaping the stories through args composition.
  // Inherited data coming from the Loading story.
  ...Loading.args,
  loading: false,
};

Now that we have some actual data populating our component, obtained from the Svelte store, we could have wired it to src/App.svelte and render the component there. Don't worry about it. We'll take care of it in the next chapter.