Angular State + RxJS Patterns

Modern Angular era (17+) introduces signals as a first-class reactivity primitive alongside RxJS. Both have a place — signals для component state, RxJS для async streams. Plus optional NgRx variants for complex apps.

Decision tree

Need	Tool
Local component state (count, isOpen)	signal()
Computed from other state	computed()
Side effects on signal change	effect() (sparingly)
Cross-component shared state	@Injectable({ providedIn: 'root' }) service з signals
Server data with caching	TanStack Query Angular OR custom RxJS service з shareReplay
Complex domain state з time-travel debugging	NgRx Store + Effects + Selectors
Lightweight per-component reactive store	NgRx Component Store
Signal-based store API	@ngrx/signals (Angular 17+)
URL state	ActivatedRoute snapshot / queryParams

Detect what's installed; mirror project's choice; never introduce a new state lib without BA approval.

Signals (Angular 17+)

import { signal, computed, effect } from '@angular/core';

count = signal(0);
double = computed(() => this.count() * 2);

constructor() {
  effect(() => {
    console.log('count is now', this.count());
  });
}

increment() {
  this.count.update((c) => c + 1);          // functional update — best for derivations
}

set(value: number) {
  this.count.set(value);                     // direct set
}

reset() {
  this.count.set(0);
}

signal(initial) vs computed(fn) vs effect(fn)

• signal() — writable, returns a function. Read with signal(), write with signal.set() or signal.update().
• computed() — read-only, derived from other signals. Auto-tracks dependencies (calls inside the function are observed). Memoized — recomputes only when deps change.
• effect() — runs after component init and re-runs when read signals change. Use SPARINGLY — most reactivity flows through templates (auto-track via {{ signal() }}).

When NOT to use effect()

• ❌ For derived state — use computed().
• ❌ For state synchronization — usually a sign of bad architecture.
• ❌ For form-control updates — Angular Forms have their own reactivity.
• ✅ For non-Angular DOM access (e.g., third-party charting library that needs imperative updates).
• ✅ For logging / analytics on state changes.

Signal inputs/outputs (Angular 17.1+)

import { input, output, computed } from '@angular/core';

@Component({ ... })
export class UserCardComponent {
  user = input.required<User>();              // required input — InputSignal<User>
  showActions = input(false);                  // optional with default — InputSignal<boolean>
  delete = output<string>();                   // OutputEmitterRef<string>

  isAdmin = computed(() => this.user().role === 'admin');

  onDelete() {
    this.delete.emit(this.user().id);
  }
}

For new components in Angular 17.1+, prefer input() / output() over @Input() / @Output() decorators. Mirror existing project style if all components still use decorators.

Services as state (singleton pattern)

// users.service.ts
import { Injectable, signal, computed, inject } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { firstValueFrom } from 'rxjs';
import type { User } from './user.model';

@Injectable({ providedIn: 'root' })
export class UsersService {
  private http = inject(HttpClient);

  // Private writable signals
  private _users = signal<User[]>([]);
  private _loading = signal(false);
  private _error = signal<string | null>(null);

  // Public readonly signals
  users = this._users.asReadonly();
  loading = this._loading.asReadonly();
  error = this._error.asReadonly();

  // Public computed
  count = computed(() => this._users().length);
  hasError = computed(() => this._error() !== null);

  async loadUsers() {
    this._loading.set(true);
    this._error.set(null);
    try {
      const users = await firstValueFrom(this.http.get<User[]>('/api/users'));
      this._users.set(users);
    } catch (err) {
      this._error.set(err instanceof Error ? err.message : 'Failed to load');
    } finally {
      this._loading.set(false);
    }
  }

  addUser(user: User) {
    this._users.update((users) => [...users, user]);
  }

  removeUser(id: string) {
    this._users.update((users) => users.filter((u) => u.id !== id));
  }
}

asReadonly() exposes a read-only view — consumers can't .set() on it. Internal mutations stay encapsulated.

Component consumers:

@Component({
  template: `
    @if (usersService.loading()) {
      <p>Loading...</p>
    } @else {
      <p>{{ usersService.count() }} users</p>
      <ul>
        @for (user of usersService.users(); track user.id) {
          <li>{{ user.name }}</li>
        }
      </ul>
    }
  `
})
export class UsersListComponent {
  usersService = inject(UsersService);

  ngOnInit() {
    this.usersService.loadUsers();
  }
}

RxJS essentials

Angular still uses RxJS heavily — HttpClient returns Observables, ActivatedRoute params/queryParams are Observables, NgRx Effects are Observables.

Common operators

import { map, filter, switchMap, mergeMap, exhaustMap, debounceTime, distinctUntilChanged, tap, catchError, finalize, takeUntil, take, shareReplay } from 'rxjs';

this.search$.pipe(
  debounceTime(300),                     // wait 300ms after last input
  distinctUntilChanged(),                // skip duplicates
  switchMap((query) => this.searchService.search(query)),  // cancel prior on new input
  catchError((err) => of([])),           // error fallback
);

Operator	Use
map(fn)	Transform values
filter(fn)	Keep matching values
tap(fn)	Side effect (logging) without changing stream
switchMap(fn)	Cancel previous inner observable on new outer (search, autocomplete)
mergeMap(fn)	Run all inner observables in parallel
concatMap(fn)	Sequential — wait for prior to complete
exhaustMap(fn)	Ignore new outer while inner pending (login, save buttons)
debounceTime(ms)	Wait quiet period before emitting
distinctUntilChanged()	Skip consecutive duplicates
take(n)	Take first n then complete
takeUntil(notifier$)	Stop when notifier emits (manual cleanup pattern)
shareReplay({ bufferSize: 1, refCount: true })	Cache last value for late subscribers (HTTP caching pattern)
catchError(fn)	Recover from errors
finalize(fn)	Cleanup on complete or error

async pipe in templates

@if (users$ | async; as users) {
  @for (user of users; track user.id) {
    <p>{{ user.name }}</p>
  }
} @else {
  <p>Loading...</p>
}

async pipe handles subscribe + unsubscribe automatically. Always prefer over manual subscribe() in component classes.

takeUntilDestroyed() (Angular 16+)

For when you need subscribe() directly:

import { takeUntilDestroyed } from '@angular/core/rxjs-interop';
import { ActivatedRoute } from '@angular/router';
import { inject, Component, OnInit } from '@angular/core';

@Component({ ... })
export class UserDetailComponent implements OnInit {
  private route = inject(ActivatedRoute);

  ngOnInit() {
    this.route.params
      .pipe(takeUntilDestroyed())          // automatic cleanup tied to component
      .subscribe((params) => {
        // ...
      });
  }
}

takeUntilDestroyed() MUST be called in injection context — class field initializer or constructor. If called inside ngOnInit(), you must pass an explicit DestroyRef:

private destroyRef = inject(DestroyRef);
ngOnInit() {
  this.someStream$.pipe(takeUntilDestroyed(this.destroyRef)).subscribe(...);
}

Replaces the older manual pattern:

// ❌ Pre-16 — verbose, easy to forget
private destroy$ = new Subject<void>();
ngOnInit() {
  this.something$.pipe(takeUntil(this.destroy$)).subscribe(...);
}
ngOnDestroy() {
  this.destroy$.next();
  this.destroy$.complete();
}

firstValueFrom / lastValueFrom

For one-shot observables (HTTP), convert to Promise:

import { firstValueFrom } from 'rxjs';

async loadUser(id: string) {
  return await firstValueFrom(this.http.get<User>(`/api/users/${id}`));
}

Replaces deprecated .toPromise().

Bridging signals ↔ RxJS

import { toSignal, toObservable } from '@angular/core/rxjs-interop';

// Observable → Signal
@Injectable({ providedIn: 'root' })
export class UsersService {
  private http = inject(HttpClient);
  users = toSignal(this.http.get<User[]>('/api/users'), { initialValue: [] });
}

// Signal → Observable
const filter = signal('');
const filter$ = toObservable(filter);   // emits whenever filter() changes

filter$.pipe(
  debounceTime(300),
  switchMap((q) => this.search(q)),
).subscribe(...);

toSignal requires either initialValue OR requireSync: true (which throws if observable doesn't emit synchronously). Most HTTP cases need initialValue.

NgRx Store (when @ngrx/store detected)

pnpm add @ngrx/store @ngrx/effects @ngrx/store-devtools @ngrx/entity

Setup (standalone)

// main.ts
bootstrapApplication(AppComponent, {
  providers: [
    provideStore({ users: usersReducer }),
    provideEffects([UsersEffects]),
    provideStoreDevtools({ maxAge: 25, logOnly: !isDevMode() }),
  ],
});

Actions

// users.actions.ts
import { createAction, createActionGroup, props, emptyProps } from '@ngrx/store';

export const UsersActions = createActionGroup({
  source: 'Users',
  events: {
    'Load Users': emptyProps(),
    'Load Users Success': props<{ users: User[] }>(),
    'Load Users Failure': props<{ error: string }>(),
    'Delete User': props<{ id: string }>(),
  },
});

// Dispatch
this.store.dispatch(UsersActions.loadUsers());
this.store.dispatch(UsersActions.deleteUser({ id: '123' }));

Reducer

// users.reducer.ts
import { createReducer, on } from '@ngrx/store';
import { UsersActions } from './users.actions';

export interface UsersState {
  items: User[];
  loading: boolean;
  error: string | null;
}

const initialState: UsersState = { items: [], loading: false, error: null };

export const usersReducer = createReducer(
  initialState,
  on(UsersActions.loadUsers, (state) => ({ ...state, loading: true, error: null })),
  on(UsersActions.loadUsersSuccess, (state, { users }) => ({ ...state, loading: false, items: users })),
  on(UsersActions.loadUsersFailure, (state, { error }) => ({ ...state, loading: false, error })),
  on(UsersActions.deleteUser, (state, { id }) => ({ ...state, items: state.items.filter((u) => u.id !== id) })),
);

Selectors

// users.selectors.ts
import { createFeatureSelector, createSelector } from '@ngrx/store';
import type { UsersState } from './users.reducer';

export const selectUsersState = createFeatureSelector<UsersState>('users');

export const selectAllUsers = createSelector(
  selectUsersState,
  (state) => state.items
);

export const selectUserCount = createSelector(
  selectAllUsers,
  (users) => users.length
);

export const selectUserById = (id: string) => createSelector(
  selectAllUsers,
  (users) => users.find((u) => u.id === id)
);

Effects

// users.effects.ts
import { Injectable, inject } from '@angular/core';
import { Actions, createEffect, ofType } from '@ngrx/effects';
import { catchError, map, of, switchMap } from 'rxjs';
import { HttpClient } from '@angular/common/http';
import { UsersActions } from './users.actions';

@Injectable()
export class UsersEffects {
  private actions$ = inject(Actions);
  private http = inject(HttpClient);

  loadUsers$ = createEffect(() =>
    this.actions$.pipe(
      ofType(UsersActions.loadUsers),
      switchMap(() =>
        this.http.get<User[]>('/api/users').pipe(
          map((users) => UsersActions.loadUsersSuccess({ users })),
          catchError((err) => of(UsersActions.loadUsersFailure({ error: err.message })))
        )
      )
    )
  );
}

Component usage

@Component({...})
export class UsersListComponent implements OnInit {
  private store = inject(Store);

  users = this.store.selectSignal(selectAllUsers);   // signal-based selector (NgRx 17+)
  count = this.store.selectSignal(selectUserCount);

  ngOnInit() {
    this.store.dispatch(UsersActions.loadUsers());
  }
}

store.selectSignal(selector) returns a Signal<T> — read with users() in template.

For Observable-based: users$ = this.store.select(selectAllUsers) + async pipe.

NgRx Component Store (per-component reactive store)

For feature-local state too small for full Store pattern:

import { ComponentStore } from '@ngrx/component-store';

interface PaginationState {
  page: number;
  pageSize: number;
  total: number;
}

@Injectable()
export class PaginationStore extends ComponentStore<PaginationState> {
  constructor() {
    super({ page: 1, pageSize: 10, total: 0 });
  }

  readonly page$ = this.select((s) => s.page);
  readonly totalPages$ = this.select((s) => Math.ceil(s.total / s.pageSize));

  readonly setPage = this.updater((state, page: number) => ({ ...state, page }));

  readonly loadPage = this.effect((trigger$: Observable<void>) =>
    trigger$.pipe(
      switchMap(() => this.api.fetchPage(this.get().page).pipe(
        tapResponse(
          (data) => this.patchState({ total: data.total }),
          (err) => console.error(err)
        )
      ))
    )
  );
}

// In component
providers: [PaginationStore]

Provided per-component (not providedIn: 'root') — unique instance.

NgRx Signals (newer, 2024+)

@ngrx/signals provides a signal-based store API. Less verbose than full Store + Effects:

import { signalStore, withState, withMethods, withComputed } from '@ngrx/signals';
import { computed, inject } from '@angular/core';
import { HttpClient } from '@angular/common/http';

export const UsersStore = signalStore(
  { providedIn: 'root' },
  withState<{ items: User[]; loading: boolean }>({ items: [], loading: false }),
  withComputed((state) => ({
    count: computed(() => state.items().length),
  })),
  withMethods((state, http = inject(HttpClient)) => ({
    async loadUsers() {
      patchState(state, { loading: true });
      const items = await firstValueFrom(http.get<User[]>('/api/users'));
      patchState(state, { items, loading: false });
    },
  }))
);

// Use
const store = inject(UsersStore);
store.count();         // signal
store.loadUsers();     // method

Drop-in for many feature-state cases. Less boilerplate than full NgRx Store.

Anti-patterns

• ❌ Subscribing to observables without unsubscription strategy — leaks memory.
• ❌ Using effect() for derived state — use computed().
• ❌ Reading this.someService.value() inside effect() then calling this.someService.update() from same effect — circular update warning.
• ❌ Calling signal without parens (*ngIf="user" instead of *ngIf="user()") — common bug, doesn't unwrap.
• ❌ Putting all state in NgRx Store when service+signals would do.
• ❌ Subscribing to NgRx Store.select() then mutating result — selectors return immutable references.
• ❌ Forgetting provideStore / provideEffects in standalone bootstrap — runtime errors.
• ❌ Mixing observable + signal mental models in one feature without toSignal/toObservable bridges.
• ❌ Using BehaviorSubject exposed publicly without .asObservable() — consumers can .next().
• ❌ Manual Subject<void> cleanup pattern in Angular 16+ — use takeUntilDestroyed().