RemoteData, slaying a UI Antipattern with Angular

Kris Jenkins How Elm slays a UI antipattern blog post about fixing a common UI state bug when fetching remote data has been around for a few years now, and its popularity has transcended the Elm ecosystem. A few blog posts explain how to achieve the same results with other languages/frameworks, but none of them is based on Angular2+.
In this article, I'll try to explain how to achieve something similar with Angular and TypeScript.

What we are trying to solve

You are making an API call, and you want to display different things based on the status of the request.

⠀
Taken from "How to fix a bad user interface", by Scott Hurff

The traditional approach

export interface CakeOfTheDay {
  isInProgress: boolean;
  error: string;
  data: {
    name: string;
    image: string;
    recipe: string;
  };
}

Let's see what each property means:

• isInProgress: It's true while the remote data is being fetched.
• error: It's either null (no errors) or any string (there are errors).
• data: It's either null (no data) or an object (there is data).

There are a few problems with this approach but the main one is that it is possible to create invalid states such:

{
  isInProgress: true,
  error: 'Fatal error',
  data: {
    name: "Bacon cake",
    image: "cake.png",
    recipe: "Bacon, sugar, mix, stir, bake, enjoy!"
  }
}

How do we interpret this payload? Our Html template will have to use complex *ngIf statements to make sure that we are displaying what we should.

The RemoteData approach

The proposed solution uses an adaptation of Kris Jenkins' RemoteData library where instead of using a complex object, we use a single data type to express all possible request states:

export type RemoteData<T, E = string> =
  | NotAsked
  | InProgress<T>
  | Failure<E>
  | Success<T>;

This approach makes it impossible to create invalid states.
This pattern has served our team well for a long time, so, I decided to create the ngx-remotedata library. It works exceptionally well with ngrx!
With this library creating a robust UI stack will be as easy as throwing a few ngx-remotedata pipes and let the data do the rest:

<h4 *ngIf="remoteData | isNotAsked">No requests yet!</h4>
<h4 *ngIf="remoteData | isInProgress">In progress...</h4>
<h4 *ngIf="remoteData | isSuccess" style="color: green">
  {{ remoteData | successValue }}
</h4>
<h4 *ngIf="remoteData | isFailure" style="color: red">
  {{ remoteData | failureValue }}
</h4>

Library resources

• Github page.
• Some examples with ngrx and plain old services.
• API.
• Pipes