Dependency Injection (DI) Fundamentals

Dependency Injection (DI) is a design pattern used to organize and share code across an application by allowing you to "inject" features into different parts. This improves code maintainability, scalability, and testability.

How DI Works in Angular

There are two primary ways code interacts with Angular's DI system:

1. Providing: Making values (objects, functions, primitives) available to the DI system.
2. Injecting: Asking the DI system for those values.

Angular components, directives, and services automatically participate in DI.

Services

A service is the most common way to share data and functionality across an application. It is a TypeScript class decorated with @Injectable().

Creating a Service

Use the providedIn: 'root' option in the @Injectable decorator to make the service a singleton available throughout the entire application. This is the recommended approach for most services.

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

@Injectable({
  providedIn: 'root', // Makes this a singleton available everywhere
})
export class AnalyticsLogger {
  trackEvent(category: string, value: string) {
    console.log('Analytics event logged:', {category, value});
  }
}

Common uses for services include:

• Data clients (API calls)
• State management
• Authentication and authorization
• Logging and error handling
• Utility functions

Injecting Dependencies

Use Angular's inject() function to request dependencies.

The inject() Function

You can use the inject() function to get an instance of a service (or any other provided token).

import {Component, inject} from '@angular/core';
import {Router} from '@angular/router';
import {AnalyticsLogger} from './analytics-logger.service';

@Component({
  selector: 'app-navbar',
  template: `<a href="#" (click)="navigateToDetail($event)">Detail Page</a>`,
})
export class Navbar {
  // Injecting dependencies using class field initializers
  private router = inject(Router);
  private analytics = inject(AnalyticsLogger);

  navigateToDetail(event: Event) {
    event.preventDefault();
    this.analytics.trackEvent('navigation', '/details');
    this.router.navigate(['/details']);
  }
}

Where can inject() be used? (Injection Context)

You can call inject() in an injection context. The most common injection contexts are during the construction of a component, directive, or service.

Valid places to call inject():

1. Class field initializers (Recommended)
2. Constructor body
3. Route guards and resolvers (which are executed in an injection context)
4. Factory functions used in providers

import {Component, Directive, Injectable, inject, ElementRef} from '@angular/core';
import {HttpClient} from '@angular/common/http';

// 1. In a Component (Field Initializer & Constructor)
@Component({
  /*...*/
})
export class Example {
  private service1 = inject(MyService); // Valid field initializer

  private service2: MyService;
  constructor() {
    this.service2 = inject(MyService); // Valid constructor body
  }
}

// 2. In a Directive
@Directive({
  /*...*/
})
export class MyDirective {
  private element = inject(ElementRef); // Valid field initializer
}

// 3. In a Service
@Injectable({providedIn: 'root'})
export class MyService {
  private http = inject(HttpClient); // Valid field initializer
}

// 4. In a Route Guard (Functional)
export const authGuard = () => {
  const auth = inject(AuthService); // Valid route guard
  return auth.isAuthenticated();
};