I'm working on an Angular application where I fetch data from an API and display it using my custom state pipe and async pipe. There is a button to refresh the API call. However, if an error occurs, the observable stops emitting values and I can't refresh it anymore.

state.pipe.ts

import { Pipe, PipeTransform } from '@angular/core';
import { Observable, catchError, map, of, startWith } from 'rxjs';

type ObsState<T> = Observable<ObsStateAsync<T>>;

export type ObsStateAsync<T> = { loading: boolean; result?: T; error?: any };
@Pipe({
  name: 'state',
})
export class StatePipe implements PipeTransform {
  transform<T>(val: Observable<T>): ObsState<T> {
    return val.pipe(
      map((value: any) => ({ loading: false, result: value })),
      startWith({ loading: true }),
      catchError((error) => of({ loading: false, error }))
    );
  }
}

@Component({
  selector: 'app-root',
  template: `
    <button (click)="onRefresh()">Refresh</button>
    @let breedState = breeds$ | state | async;

    @if(breedState?.loading){
      Fetching ...
    }

    @if(breedState?.error){
      Error
    }

    @if(breedState?.result){
      @for(breed of breedState?.result;track $index){
        <div> 
          {{ breed.attributes.name }}
        </div>
      }
    }
  `,
  imports: [StatePipe, AsyncPipe],
})
export class App {
  private readonly httpClient = inject(HttpClient);

  refresh = new BehaviorSubject<void>(undefined);

  getBreed$ = this.httpClient
    .get('/api/v2/breeds') // Modify API endpoint to create an error
    .pipe(map((res: any) => res.data));

  breeds$ = this.refresh.asObservable().pipe(switchMap(() => this.getBreed$));

  onRefresh(): void {
    this.refresh.next();
  }
}

And I don't wanna do the catchError(()=>of([])) thing because the error needs to be displayed on the UI.

Stackblitz to my problem

I'm working on an Angular application where I fetch data from an API and display it using my custom state pipe and async pipe. There is a button to refresh the API call. However, if an error occurs, the observable stops emitting values and I can't refresh it anymore.

state.pipe.ts

import { Pipe, PipeTransform } from '@angular/core';
import { Observable, catchError, map, of, startWith } from 'rxjs';

type ObsState<T> = Observable<ObsStateAsync<T>>;

export type ObsStateAsync<T> = { loading: boolean; result?: T; error?: any };
@Pipe({
  name: 'state',
})
export class StatePipe implements PipeTransform {
  transform<T>(val: Observable<T>): ObsState<T> {
    return val.pipe(
      map((value: any) => ({ loading: false, result: value })),
      startWith({ loading: true }),
      catchError((error) => of({ loading: false, error }))
    );
  }
}

@Component({
  selector: 'app-root',
  template: `
    <button (click)="onRefresh()">Refresh</button>
    @let breedState = breeds$ | state | async;

    @if(breedState?.loading){
      Fetching ...
    }

    @if(breedState?.error){
      Error
    }

    @if(breedState?.result){
      @for(breed of breedState?.result;track $index){
        <div> 
          {{ breed.attributes.name }}
        </div>
      }
    }
  `,
  imports: [StatePipe, AsyncPipe],
})
export class App {
  private readonly httpClient = inject(HttpClient);

  refresh = new BehaviorSubject<void>(undefined);

  getBreed$ = this.httpClient
    .get('https://dogapi.dog/api/v2/breeds') // Modify API endpoint to create an error
    .pipe(map((res: any) => res.data));

  breeds$ = this.refresh.asObservable().pipe(switchMap(() => this.getBreed$));

  onRefresh(): void {
    this.refresh.next();
  }
}

And I don't wanna do the catchError(()=>of([])) thing because the error needs to be displayed on the UI.

Stackblitz to my problem

• angular
• rxjs
• angular-pipe

1 Answer 1

You have to place the catchError inside the pipe of the http observable, so I suggest a rxjs reusable pipe operator stateTransform which basically does the same thing, but is inside the pipe of the http.get.

type ObsState<T> = Observable<ObsStateAsync<T>>;

export type ObsStateAsync<T> = { loading: boolean; result?: T; error?: any };

function stateTransform<T>(source$: Observable<T>): ObsState<T> {
  return source$.pipe(
    startWith({ loading: true }),
    map((value: any) => ({ loading: false, result: value })),
    catchError((error) => of({ loading: false, error }))
  );
}

You just need to add this reusable operator below the map transform.

getBreed$ = this.httpClient
  .get<BreedResult>('https://dogapi.dog/api/v2/breedss') //modify api endpoint to create error
  .pipe(
    map((res: BreedResult) => res.data),
    stateTransform
  );

Full Code:

import { Component, inject } from '@angular/core';
import { bootstrapApplication } from '@angular/platform-browser';
import { HttpClient, provideHttpClient } from '@angular/common/http';
import { StatePipe } from './state.pipe';
import { AsyncPipe } from '@angular/common';
import {
  BehaviorSubject,
  map,
  switchMap,
  Observable,
  startWith,
  catchError,
  of,
} from 'rxjs';

export interface BreedResult {
  data: Array<any>;
}

@Component({
  selector: 'app-root',
  template: `
  <button (click)="onRefresh()">Refresh</button>
  @let breedState = breeds$ | async;

  @if(breedState?.loading){
    Fetching ...
  } @else if(breedState?.error){
    Error
  }  @else if(breedState?.result){
    @for(breed of breedState?.result;track $index){
      <div> 
      {{breed.attributes.name}}
    </div>
    }
  }
  `,
  imports: [StatePipe, AsyncPipe],
})
export class App {
  private readonly httpClient = inject(HttpClient);

  refresh = new BehaviorSubject<void>(undefined);

  getBreed$ = this.httpClient
    .get<BreedResult>('https://dogapi.dog/api/v2/breedss') //modify api endpoint to create error
    .pipe(
      map((res: BreedResult) => res.data),
      stateTransform
    );

  breeds$ = this.refresh.asObservable().pipe(switchMap(() => this.getBreed$));

  onRefresh(): void {
    this.refresh.next();
  }
}

bootstrapApplication(App, { providers: [provideHttpClient()] });

type ObsState<T> = Observable<ObsStateAsync<T>>;

export type ObsStateAsync<T> = { loading: boolean; result?: T; error?: any };

function stateTransform<T>(source$: Observable<T>): ObsState<T> {
  return source$.pipe(
    startWith({ loading: true }),
    map((value: any) => ({ loading: false, result: value })),
    catchError((error) => of({ loading: false, error }))
  );
}

Stackblitz Demo