Service-based state management in Angular

Introduction

Service-based state management helps keep data shared and updated across parts of your app easily. It avoids repeating data and keeps everything in sync.

You want to share user login status across multiple components.

You need to keep a shopping cart updated in different pages.

You want to manage theme settings that affect many parts of the app.

You want to keep track of form data while moving between steps.

You want to avoid passing data through many component layers.

Syntax

Angular

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

@Injectable({ providedIn: 'root' })
export class StateService {
  private _count = signal(0);

  get count() {
    return this._count();
  }

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

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

Use @Injectable({ providedIn: 'root' }) to make the service available app-wide.

Use Angular signals to hold and update state reactively.

Examples

This service holds a username and lets components get or set it.

Angular

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

@Injectable({ providedIn: 'root' })
export class UserService {
  private _username = signal('');

  get username() {
    return this._username();
  }

  setUsername(name: string) {
    this._username.set(name);
  }
}

This service manages a list of items in a shopping cart.

Angular

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

@Injectable({ providedIn: 'root' })
export class CartService {
  private _items = signal<string[]>([]);

  get items() {
    return this._items();
  }

  addItem(item: string) {
    this._items.update(items => [...items, item]);
  }

  clear() {
    this._items.set([]);
  }
}

Sample Program

This component shows a counter and buttons to change it. It uses the shared StateService to keep the count.

Angular

import { Component, inject } from '@angular/core';
import { StateService } from './state.service';

@Component({
  selector: 'app-counter',
  standalone: true,
  template: `
    <h2>Counter: {{ state.count }}</h2>
    <button (click)="state.increment()">+</button>
    <button (click)="state.decrement()">-</button>
  `
})
export class CounterComponent {
  state = inject(StateService);
}

Important Notes

Services are singletons by default, so all components share the same state instance.

Use Angular signals inside services for reactive updates that components can track.

Inject services using inject() or constructor injection in components.

Summary

Service-based state management shares data across components easily.

Angular signals inside services keep state reactive and simple.

Use @Injectable({ providedIn: 'root' }) to make services app-wide singletons.

Practice

1. What is the main benefit of using a service for state management in Angular?

easy

A. It allows sharing state easily across multiple components.

B. It automatically updates the UI without any coding.

C. It replaces the need for components entirely.

D. It makes the app run faster by skipping change detection.

Solution

1. Step 1: Understand service role in Angular

   Services hold data and logic separate from components.

2. Step 2: Recognize state sharing benefit

   Services can be injected into many components, sharing the same state instance.

3. Final Answer:

   It allows sharing state easily across multiple components. -> Option A

4. Quick Check:

   Service-based state management = shared state [OK]

Hint: Services share data across components easily [OK]

Common Mistakes:

• Thinking services replace components
• Believing services auto-update UI without code
• Assuming services speed up app by skipping detection

2. Which decorator and property make an Angular service a singleton across the app?

easy

A. @NgModule({ providers: [] })

B. @Component({ selector: 'app-root' })

C. @Directive({ selector: '[appService]' })

D. @Injectable({ providedIn: 'root' })

Solution

1. Step 1: Identify Angular service decorator

   @Injectable marks a class as a service for dependency injection.

2. Step 2: Understand providedIn property

   Setting providedIn: 'root' makes the service a singleton app-wide.

3. Final Answer:

   @Injectable({ providedIn: 'root' }) -> Option D

4. Quick Check:

   Singleton service = @Injectable with providedIn root [OK]

Hint: Use @Injectable({ providedIn: 'root' }) for singleton services [OK]

Common Mistakes:

• Confusing @Component with service decorator
• Using @NgModule providers without providedIn
• Mistaking @Directive for service declaration

3. Given this service code, what will the console log after calling increment() twice?

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

@Injectable({ providedIn: 'root' })
export class CounterService {
  count = signal(0);

  increment() {
    this.count.update(c => c + 1);
  }
}

const service = new CounterService();
service.increment();
service.increment();
console.log(service.count());

medium

A. 1

B. 0

C. 2

D. undefined

Solution

1. Step 1: Understand initial signal value

   The signal count starts at 0.

2. Step 2: Apply two increments

   Each increment adds 1, so after two calls, count is 2.

3. Final Answer:

   2 -> Option C

4. Quick Check:

   0 + 1 + 1 = 2 [OK]

Hint: Each update adds 1; two calls add 2 total [OK]

Common Mistakes:

• Forgetting to call the signal as a function to get value
• Assuming count resets after each increment
• Confusing update with set method

4. What is wrong with this Angular service code for state management?

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

@Injectable()
export class DataService {
  data = signal([]);

  addItem(item: string) {
    this.data().push(item);
  }
}

medium

A. The service is missing providedIn: 'root' for singleton scope.

B. The signal value is mutated directly, which breaks reactivity.

C. The addItem method should return the updated array.

D. The signal should be initialized with null, not an empty array.

Solution

1. Step 1: Check signal mutation method

   The code calls this.data() to get the array, then pushes directly.

2. Step 2: Understand signal immutability

   Directly mutating the array breaks Angular's reactivity; must use update() or set() to replace value.

3. Final Answer:

   The signal value is mutated directly, which breaks reactivity. -> Option B

4. Quick Check:

   Mutate signal value immutably to keep reactivity [OK]

Hint: Never mutate signal value directly; use update or set [OK]

Common Mistakes:

• Ignoring providedIn for singleton scope
• Expecting addItem to return value
• Thinking null is better initial value than []

5. You want to share a list of tasks across components using a service with Angular signals. Which approach correctly updates the tasks list immutably when adding a new task?

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

@Injectable({ providedIn: 'root' })
export class TaskService {
  tasks = signal([]);

  addTask(newTask: string) {
    // Which line correctly updates tasks?
  }
}

hard

A. this.tasks.set([...this.tasks(), newTask]);

B. this.tasks = signal([...this.tasks(), newTask]);

C. this.tasks().push(newTask);

D. this.tasks.update(tasks => { tasks.push(newTask); return tasks; });

Solution

1. Step 1: Understand immutable update with signals

   Signals require replacing the value immutably to trigger updates.

2. Step 2: Analyze options for correct update

   this.tasks.set([...this.tasks(), newTask]); uses set() with a new array including the new task, which is correct.

3. Final Answer:

   this.tasks.set([...this.tasks(), newTask]); -> Option A

4. Quick Check:

   Immutable update with set() = correct pattern [OK]

Hint: Use set() with new array copy to update signals immutably [OK]

Common Mistakes:

• Mutating array inside update without returning new array
• Directly pushing to signal value
• Reassigning signal variable instead of updating value