angular-state-management

name	Angular State Management
description	Master modern Angular state management with Signals, NgRx, and RxJS. Use when setting up global state, managing component stores, choosing between state solutions, or migrating from legacy patterns.
risk	safe
source	self

Angular State Management

Comprehensive guide to modern Angular state management patterns, from Signal-based local state to global stores and server state synchronization.

When to Use This Skill

• Setting up global state management in Angular
• Choosing between Signals, NgRx, or Akita
• Managing component-level stores
• Implementing optimistic updates
• Debugging state-related issues
• Migrating from legacy state patterns

Do Not Use This Skill When

• The task is unrelated to Angular state management
• You need React state management → use react-state-management

---

Core Concepts

State Categories

Type	Description	Solutions
Local State	Component-specific, UI state	Signals, signal()
Shared State	Between related components	Signal services
Global State	App-wide, complex	NgRx, Akita, Elf
Server State	Remote data, caching	NgRx Query, RxAngular
URL State	Route parameters	ActivatedRoute
Form State	Input values, validation	Reactive Forms

Selection Criteria

Small app, simple state → Signal Services
Medium app, moderate state → Component Stores
Large app, complex state → NgRx Store
Heavy server interaction → NgRx Query + Signal Services
Real-time updates → RxAngular + Signals

---

Quick Start: Signal-Based State

Pattern 1: Simple Signal Service

// services/counter.service.ts
import { Injectable, signal, computed } from "@angular/core";

@Injectable({ providedIn: "root" })
export class CounterService {
  // Private writable signals
  private _count = signal(0);

  // Public read-only
  readonly count = this._count.asReadonly();
  readonly doubled = computed(() => this._count() * 2);
  readonly isPositive = computed(() => this._count() > 0);

  increment() {
    this._count.update((v) => v + 1);
  }

  decrement() {
    this._count.update((v) => v - 1);
  }

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

// Usage in component
@Component({
  template: `
    <p>Count: {{ counter.count() }}</p>
    <p>Doubled: {{ counter.doubled() }}</p>
    <button (click)="counter.increment()">+</button>
  `,
})
export class CounterComponent {
  counter = inject(CounterService);
}

Pattern 2: Feature Signal Store

// stores/user.store.ts
import { Injectable, signal, computed, inject } from "@angular/core";
import { HttpClient } from "@angular/common/http";
import { toSignal } from "@angular/core/rxjs-interop";

interface User {
  id: string;
  name: string;
  email: string;
}

interface UserState {
  user: User | null;
  loading: boolean;
  error: string | null;
}

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

  // State signals
  private _user = signal<User | null>(null);
  private _loading = signal(false);
  private _error = signal<string | null>(null);

  // Selectors (read-only computed)
  readonly user = computed(() => this._user());
  readonly loading = computed(() => this._loading());
  readonly error = computed(() => this._error());
  readonly isAuthenticated = computed(() => this._user() !== null);
  readonly displayName = computed(() => this._user()?.name ?? "Guest");

  // Actions
  async loadUser(id: string) {
    this._loading.set(true);
    this._error.set(null);

    try {
      const user = await fetch(`/api/users/${id}`).then((r) => r.json());
      this._user.set(user);
    } catch (e) {
      this._error.set("Failed to load user");
    } finally {
      this._loading.set(false);
    }
  }

  updateUser(updates: Partial<User>) {
    this._user.update((user) => (user ? { ...user, ...updates } : null));
  }

  logout() {
    this._user.set(null);
    this._error.set(null);
  }
}

Pattern 3: SignalStore (NgRx Signals)

// stores/products.store.ts
import {
  signalStore,
  withState,
  withMethods,
  withComputed,
  patchState,
} from "@ngrx/signals";
import { inject } from "@angular/core";
import { ProductService } from "./product.service";

interface ProductState {
  products: Product[];
  loading: boolean;
  filter: string;
}

const initialState: ProductState = {
  products: [],
  loading: false,
  filter: "",
};

export const ProductStore = signalStore(
  { providedIn: "root" },

  withState(initialState),

  withComputed((store) => ({
    filteredProducts: computed(() => {
      const filter = store.filter().toLowerCase();
      return store
        .products()
        .filter((p) => p.name.toLowerCase().includes(filter));
    }),
    totalCount: computed(() => store.products().length),
  })),

  withMethods((store, productService = inject(ProductService)) => ({
    async loadProducts() {
      patchState(store, { loading: true });

      try {
        const products = await productService.getAll();
        patchState(store, { products, loading: false });
      } catch {
        patchState(store, { loading: false });
      }
    },

    setFilter(filter: string) {
      patchState(store, { filter });
    },

    addProduct(product: Product) {
      patchState(store, ({ products }) => ({
        products: [...products, product],
      }));
    },
  })),
);

// Usage
@Component({
  template: `
    <input (input)="store.setFilter($event.target.value)" />
    @if (store.loading()) {
      <app-spinner />
    } @else {
      @for (product of store.filteredProducts(); track product.id) {
        <app-product-card [product]="product" />
      }
    }
  `,
})
export class ProductListComponent {
  store = inject(ProductStore);

  ngOnInit() {
    this.store.loadProducts();
  }
}

---

NgRx Store (Global State)

Setup

// store/app.state.ts
import { ActionReducerMap } from "@ngrx/store";

export interface AppState {
  user: UserState;
  cart: CartState;
}

export const reducers: ActionReducerMap<AppState> = {
  user: userReducer,
  cart: cartReducer,
};

// main.ts
bootstrapApplication(AppComponent, {
  providers: [
    provideStore(reducers),
    provideEffects([UserEffects, CartEffects]),
    provideStoreDevtools({ maxAge: 25 }),
  ],
});

Feature Slice Pattern

// store/user/user.actions.ts
import { createActionGroup, props, emptyProps } from "@ngrx/store";

export const UserActions = createActionGroup({
  source: "User",
  events: {
    "Load User": props<{ userId: string }>(),
    "Load User Success": props<{ user: User }>(),
    "Load User Failure": props<{ error: string }>(),
    "Update User": props<{ updates: Partial<User> }>(),
    Logout: emptyProps(),
  },
});

// store/user/user.reducer.ts
import { createReducer, on } from "@ngrx/store";
import { UserActions } from "./user.actions";

export interface UserState {
  user: User | null;
  loading: boolean;
  error: string | null;
}

const initialState: UserState = {
  user: null,
  loading: false,
  error: null,
};

export const userReducer = createReducer(
  initialState,

  on(UserActions.loadUser, (state) => ({
    ...state,
    loading: true,
    error: null,
  })),

  on(UserActions.loadUserSuccess, (state, { user }) => ({
    ...state,
    user,
    loading: false,
  })),

  on(UserActions.loadUserFailure, (state, { error }) => ({
    ...state,
    loading: false,
    error,
  })),

  on(UserActions.logout, () => initialState),
);

// store/user/user.selectors.ts
import { createFeatureSelector, createSelector } from "@ngrx/store";
import { UserState } from "./user.reducer";

export const selectUserState = createFeatureSelector<UserState>("user");

export const selectUser = createSelector(
  selectUserState,
  (state) => state.user,
);

export const selectUserLoading = createSelector(
  selectUserState,
  (state) => state.loading,
);

export const selectIsAuthenticated = createSelector(
  selectUser,
  (user) => user !== null,
);

// store/user/user.effects.ts
import { Injectable, inject } from "@angular/core";
import { Actions, createEffect, ofType } from "@ngrx/effects";
import { switchMap, map, catchError, of } from "rxjs";

@Injectable()
export class UserEffects {
  private actions$ = inject(Actions);
  private userService = inject(UserService);

  loadUser$ = createEffect(() =>
    this.actions$.pipe(
      ofType(UserActions.loadUser),
      switchMap(({ userId }) =>
        this.userService.getUser(userId).pipe(
          map((user) => UserActions.loadUserSuccess({ user })),
          catchError((error) =>
            of(UserActions.loadUserFailure({ error: error.message })),
          ),
        ),
      ),
    ),
  );
}

Component Usage

@Component({
  template: `
    @if (loading()) {
      <app-spinner />
    } @else if (user(); as user) {
      <h1>Welcome, {{ user.name }}</h1>
      <button (click)="logout()">Logout</button>
    }
  `,
})
export class HeaderComponent {
  private store = inject(Store);

  user = this.store.selectSignal(selectUser);
  loading = this.store.selectSignal(selectUserLoading);

  logout() {
    this.store.dispatch(UserActions.logout());
  }
}

---

RxJS-Based Patterns

Component Store (Local Feature State)

// stores/todo.store.ts
import { Injectable } from "@angular/core";
import { ComponentStore } from "@ngrx/component-store";
import { switchMap, tap, catchError, EMPTY } from "rxjs";

interface TodoState {
  todos: Todo[];
  loading: boolean;
}

@Injectable()
export class TodoStore extends ComponentStore<TodoState> {
  constructor(private todoService: TodoService) {
    super({ todos: [], loading: false });
  }

  // Selectors
  readonly todos$ = this.select((state) => state.todos);
  readonly loading$ = this.select((state) => state.loading);
  readonly completedCount$ = this.select(
    this.todos$,
    (todos) => todos.filter((t) => t.completed).length,
  );

  // Updaters
  readonly addTodo = this.updater((state, todo: Todo) => ({
    ...state,
    todos: [...state.todos, todo],
  }));

  readonly toggleTodo = this.updater((state, id: string) => ({
    ...state,
    todos: state.todos.map((t) =>
      t.id === id ? { ...t, completed: !t.completed } : t,
    ),
  }));

  // Effects
  readonly loadTodos = this.effect<void>((trigger$) =>
    trigger$.pipe(
      tap(() => this.patchState({ loading: true })),
      switchMap(() =>
        this.todoService.getAll().pipe(
          tap({
            next: (todos) => this.patchState({ todos, loading: false }),
            error: () => this.patchState({ loading: false }),
          }),
          catchError(() => EMPTY),
        ),
      ),
    ),
  );
}

---

Server State with Signals

HTTP + Signals Pattern

// services/api.service.ts
import { Injectable, signal, inject } from "@angular/core";
import { HttpClient } from "@angular/common/http";
import { toSignal } from "@angular/core/rxjs-interop";

interface ApiState<T> {
  data: T | null;
  loading: boolean;
  error: string | null;
}

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

  private _state = signal<ApiState<Product[]>>({
    data: null,
    loading: false,
    error: null,
  });

  readonly products = computed(() => this._state().data ?? []);
  readonly loading = computed(() => this._state().loading);
  readonly error = computed(() => this._state().error);

  async fetchProducts(): Promise<void> {
    this._state.update((s) => ({ ...s, loading: true, error: null }));

    try {
      const data = await firstValueFrom(
        this.http.get<Product[]>("/api/products"),
      );
      this._state.update((s) => ({ ...s, data, loading: false }));
    } catch (e) {
      this._state.update((s) => ({
        ...s,
        loading: false,
        error: "Failed to fetch products",
      }));
    }
  }

  // Optimistic update
  async deleteProduct(id: string): Promise<void> {
    const previousData = this._state().data;

    // Optimistically remove
    this._state.update((s) => ({
      ...s,
      data: s.data?.filter((p) => p.id !== id) ?? null,
    }));

    try {
      await firstValueFrom(this.http.delete(`/api/products/${id}`));
    } catch {
      // Rollback on error
      this._state.update((s) => ({ ...s, data: previousData }));
    }
  }
}

---

Best Practices

Do's

Practice	Why
Use Signals for local state	Simple, reactive, no subscriptions
Use computed() for derived data	Auto-updates, memoized
Colocate state with feature	Easier to maintain
Use NgRx for complex flows	Actions, effects, devtools
Prefer inject() over constructor	Cleaner, works in factories

Don'ts

Anti-Pattern	Instead
Store derived data	Use computed()
Mutate signals directly	Use set() or update()
Over-globalize state	Keep local when possible
Mix RxJS and Signals chaotically	Choose primary, bridge with toSignal/toObservable
Subscribe in components for state	Use template with signals

---

Migration Path

From BehaviorSubject to Signals

// Before: RxJS-based
@Injectable({ providedIn: "root" })
export class OldUserService {
  private userSubject = new BehaviorSubject<User | null>(null);
  user$ = this.userSubject.asObservable();

  setUser(user: User) {
    this.userSubject.next(user);
  }
}

// After: Signal-based
@Injectable({ providedIn: "root" })
export class UserService {
  private _user = signal<User | null>(null);
  readonly user = this._user.asReadonly();

  setUser(user: User) {
    this._user.set(user);
  }
}

Bridging Signals and RxJS

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

// Observable → Signal
@Component({...})
export class ExampleComponent {
  private route = inject(ActivatedRoute);

  // Convert Observable to Signal
  userId = toSignal(
    this.route.params.pipe(map(p => p['id'])),
    { initialValue: '' }
  );
}

// Signal → Observable
export class DataService {
  private filter = signal('');

  // Convert Signal to Observable
  filter$ = toObservable(this.filter);

  filteredData$ = this.filter$.pipe(
    debounceTime(300),
    switchMap(filter => this.http.get(`/api/data?q=${filter}`))
  );
}

---

Resources

• Angular Signals Guide
• NgRx Documentation
• NgRx SignalStore
• RxAngular