Singleton service behavior in Angular - Mini Project: Build & Apply

Singleton Service Behavior in Angular

📖 Scenario: You are building a simple Angular app that tracks a user's favorite color. The app has two components that both show and update the favorite color. You want to use a singleton service so both components share the same color data.

🎯 Goal: Create an Angular singleton service to hold the favorite color. Then build two components that both read and update this color through the service, so changes in one component reflect in the other.

📋 What You'll Learn

Create a singleton service called FavoriteColorService with a color property initialized to 'blue'.

Create a component called ColorDisplayComponent that shows the current favorite color from the service.

Create a component called ColorChangerComponent that updates the favorite color in the service.

Ensure the service is provided in the root injector to maintain singleton behavior.

💡 Why This Matters

🌍 Real World

Singleton services are used in Angular apps to share data or logic across multiple components without duplicating state.

💼 Career

Understanding singleton services is essential for Angular developers to manage shared state and build maintainable apps.

1

Create the singleton service with initial color

Create an Angular service called FavoriteColorService with a public property color set to 'blue'. Use the @Injectable decorator with providedIn: 'root' to make it a singleton.

Angular

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

// Create FavoriteColorService with color property
// Your code here

Hint

Use @Injectable({ providedIn: 'root' }) to make the service singleton.

2

Create ColorDisplayComponent to show the color

Create a component called ColorDisplayComponent that injects FavoriteColorService in its constructor. Display the service's color property in the template using interpolation.

Angular

import { Component } from '@angular/core';
import { FavoriteColorService } from './favorite-color.service';

// Create ColorDisplayComponent that shows favorite color
// Your code here

Hint

Inject the service in the constructor and use {{ favoriteColorService.color }} in the template.

3

Create ColorChangerComponent to update the color

Create a component called ColorChangerComponent that injects FavoriteColorService in its constructor. Add an input box bound to a local variable newColor and a button. When the button is clicked, update the service's color property to newColor.

Angular

import { Component } from '@angular/core';
import { FavoriteColorService } from './favorite-color.service';

// Create ColorChangerComponent with input and button to update color
// Your code here

Hint

Use two-way binding with [(ngModel)] for the input and update the service color on button click.

4

Add components to app template for testing

In the main app component template, add the selectors <app-color-display> and <app-color-changer> so both components appear on the page. This will let you see the shared color and change it.

Angular

Hint

Use the component selectors as HTML tags in the app template.

Practice

1. What does it mean when an Angular service is a singleton?

easy

A. There is only one shared instance of the service across the entire app.

B. A new instance of the service is created for every component.

C. The service can only be used in one component at a time.

D. The service is automatically destroyed after use.

Solution

1. Step 1: Understand singleton concept in Angular

   A singleton service means Angular creates one instance of the service and shares it everywhere.

2. Step 2: Compare options with singleton definition

   Only There is only one shared instance of the service across the entire app. correctly describes a single shared instance across the app.

3. Final Answer:

   There is only one shared instance of the service across the entire app. -> Option A

4. Quick Check:

   Singleton = one shared instance [OK]

Hint: Singleton means one instance shared app-wide [OK]

Common Mistakes:

• Thinking each component gets its own service instance
• Confusing singleton with service destruction
• Believing singleton limits service usage to one component

2. Which is the correct way to make an Angular service a singleton using the @Injectable decorator?

easy

A. @Injectable({ scope: 'singleton' })

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

C. @Injectable({ singleton: true })

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

Solution

1. Step 1: Recall Angular service singleton syntax

   Angular uses providedIn: 'root' to provide a singleton service at the app root level.

2. Step 2: Check each option's validity

   Only @Injectable({ providedIn: 'root' }) uses the correct syntax. Others are invalid or do not exist in Angular.

3. Final Answer:

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

4. Quick Check:

   Singleton service = providedIn: 'root' [OK]

Hint: Use providedIn: 'root' for singleton services [OK]

Common Mistakes:

• Using non-existent properties like singleton or scope
• Confusing component-level providers with root-level
• Omitting the @Injectable decorator

3. Given this Angular service and two components using it:

export class CounterService {
  count = 0;
  increment() { this.count++; }
  getCount() { return this.count; }
}

Both components call increment() once. What will getCount() return when called from either component?

medium

A. 0

B. 1

C. 2

D. Undefined

Solution

1. Step 1: Understand singleton service shared state

   Since the service is singleton, both components share the same count variable.

2. Step 2: Calculate increments from both components

   Each component calls increment() once, so total increments = 2.

3. Final Answer:

   2 -> Option C

4. Quick Check:

   Shared state increments twice = 2 [OK]

Hint: Singleton shares state; increments add up [OK]

Common Mistakes:

• Assuming each component has separate service instance
• Thinking count resets per component
• Ignoring shared service state

4. You created a service with @Injectable() but forgot to add providedIn: 'root'. What problem will occur when multiple components inject this service?

medium

A. Angular will throw a compile-time error.

B. The service will not be injectable anywhere.

C. The service will be singleton by default anyway.

D. Angular will create separate instances for each component, breaking singleton behavior.

Solution

1. Step 1: Understand default service provider behavior

   Without providedIn: 'root', Angular does not provide the service globally.

2. Step 2: Effect on multiple injections

   Each component that provides the service separately will get its own instance, so no singleton.

3. Final Answer:

   Angular will create separate instances for each component, breaking singleton behavior. -> Option D

4. Quick Check:

   Missing providedIn: 'root' = multiple instances [OK]

Hint: No providedIn: 'root' means no singleton [OK]

Common Mistakes:

• Assuming singleton is default without providedIn
• Expecting compile errors for missing providedIn
• Thinking service becomes unusable

5. You want to share user login status across multiple components using a singleton service. Which approach correctly ensures the service is singleton and updates are reflected everywhere?

hard

A. Create service with @Injectable({ providedIn: 'root' }) and use a BehaviorSubject to hold login status.

B. Create service without providedIn and add it to each component's providers array.

C. Create service with @Injectable({ providedIn: 'root' }) but store login status in a local variable inside each component.

D. Create service with @Injectable() and manually instantiate it in each component constructor.

Solution

1. Step 1: Ensure singleton service provision

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

2. Step 2: Use BehaviorSubject for reactive shared state

   BehaviorSubject allows components to subscribe and get updates when login status changes.

3. Step 3: Evaluate other options

   Options B, C, and D break singleton or do not share state properly.

4. Final Answer:

   Create service with @Injectable({ providedIn: 'root' }) and use a BehaviorSubject to hold login status. -> Option A

5. Quick Check:

   Singleton + reactive state = shared updates [OK]

Hint: Use providedIn: 'root' and BehaviorSubject for shared reactive state [OK]

Common Mistakes:

• Adding service to component providers breaking singleton
• Storing shared data only inside components
• Manually creating service instances instead of DI