State-driven development of user interface components

Maxim Pavlov — Mon, 07 Sep 2020 10:10:39 +0000

I often see code describing the state of the user interface that needs to be simplified.

Let's see code that outputs a list of users.

  <ng-container *ngIf="isLoading && !error">Loading...</ng-container>
  <ul *ngIf="users && users.length && !error">
    <li *ngFor="let user of users">{{user.name}}</li>
  </ul>
  <ng-container *ngIf="!error && !loading && users && !users.length">Nothing found</ng-container>
  <ng-container *ngIf="!isLoading && error">{{error.message}}</ng-container>

This code is just awful. It is difficult to read and maintain.
I prefer another way. I used to read about the theory of finite-state machines. The state machine has a finite set of states, and it is in one of these states at each moment.

We have four states of the user's list:

Loading
Users loaded
Users were loaded with errors
Users were not founded

Let's describe the state with a discriminated union.

type State =
  | { status: 'loading' }
  | { status: 'success', data: IUser[] }
  | { status: 'failed', error: Error }
  | { status: 'not-founded' }

Let's rewrite view logic.

  <ng-container *ngIf="state.status === 'loading'">Loading...</ng-container>
  <ul *ngIf="state.status === 'success'">
    <li *ngFor="let user of state.data">{{user.name}}</li>
  </ul>
  <ng-container *ngIf="state.status === 'not-found'">Nothing found</ng-container>
  <ng-container *ngIf="state.status === 'failed'">{{state.error.message}}</ng-container>

You can make the state type universal by using generics.

type State<TSuccessData> =
  | { status: 'loading' }
  | { status: 'success', data: TSuccessData }
  | { status: 'failed', error: Error }
  | { status: 'not-founded' }
type UsersListState = State<IUser[]>;

This code is more reading and self-documenting. Presently your IDE gives better hints for you.

Your team will be grateful to you.

P.S. Sorry for my English. It is my first article in English.

DEV Community: Maxim Pavlov

State-driven development of user interface components