task

@lit/task

A controller for Lit that renders asynchronous tasks.

Overview

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 an arguments function. Whenever the element updates, the arguments are checked and if any have changed, the task is initiated.

Sometimes it's important to control exactly when a task runs. For example, task arguments may have changed, but it should not run until an interaction event like a button click. For these types of use cases, the autoRun option can be set to false. This setting can be passed in the task configuration and/or be set on the Task itself. It defaults to true, but when autoRun is false, the task does not run automatically when arguments change. Instead, it can be run explicitly by calling run(arg?). By default, run() uses the task's configured arguments function, but a custom array of arguments may be optionally passed.

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.

Installation

From inside your project folder, run:

$ npm install @lit/task

Usage

Here's an example:

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

class MyElement extends LitElement {
  @state()
  private _userId: number = -1;

  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}`,
      })}
      <!-- ... -->
    `;
  }
}

Argument equality

Task accepts an argsEqual to determine if a task should auto-run by testing a new arguments array for equality against the previous run's arguments array.

The default equality function is shallowArrayEquals, which compares each argument in the array against the previous array with notEqual from @lit/reactive-element (which itself uses ===). This works well if your arguments are primitive values like strings.

If your arguments are objects, you will want to use a more sophisticated equality function. Task provides deepArrayEquals in the deep-equals.js module, which compares each argument with a deepEquals function that can handle primitives, objects, Arrays, Maps, Sets, RegExps, or objects that implement toString() or toValue().

Contributing

Please see CONTRIBUTING.md.