A controller for Lit that renders asynchronous tasks.

Often a Lit element needs to request, process, and render remote data, for example when querying a REST API for data to be displayed. The Task controller provides a simple pattern for encapsulating this behavior in an easily reusable way. The controller integrates with a host Lit element. The user provides a task function and a dependencies function. Whenever the element updates, the dependencies are checked and if any have changed, the task is initiated. The controller requests an update of the element whenever the task status changes. Task status is provided via the TaskStatus object which has values for INITIAL, PENDING, COMPLETE, and ERROR. The task result is available via its value property, or via the error property when an error occurs. The task render method may also be used to easily render different task states. It accepts an object which optionally can implement methods for initial, pending, complete(value), and error(error). These methods typically return a Lit TemplateResult to render

From inside your project folder, run:

$ npm install @lit-labs/task

Here's an example:

import {Task, TaskStatus} from '@lit-labs/task';
// ...

class MyElement extends LitElement {

  @state()
  private _userId: number;

  private _apiTask = new Task(
      this,
      ([userId]) =>
        fetch(`//example.com/api/userInfo?${userId}`)
          .then(response => response.json())
      () => [this.userId]
    );

  render() {
    return html`
      <div>User Info</div>
      ${this._apiTask.render({
        pending: html`Loading user info...`;
        complete(user): html`${user.name}`;
      })}
      <!-- ... -->
    `;
  }
}

Please see CONTRIBUTING.md.