Angular Dependency Injection vs Angular Service: What to use when?

Angular is a powerful JavaScript framework that provides a lot of tools to create dynamic and robust applications. One of the most important features of Angular is the Dependency Injection (DI) system that allows developers to easily manage dependencies between components. But what is DI, and when should you use it over Angular services? In this article, we will explore these two concepts and compare them to help you make better decisions when building your Angular applications.

What is Angular Dependency Injection?

Dependency Injection is a design pattern that tackles the issue of managing dependencies between classes in object-oriented programming. The idea is to create objects and their dependencies separately and then “inject” them into other objects that use them. This way, the dependencies can be swapped without changing the code of the class that uses them. This results in a more modular and flexible codebase, which is easier to maintain and test.

Angular Dependency Injection is a DI system that is tightly integrated with the Angular framework. With Angular DI, the framework automatically resolves dependencies and injects them into the class constructors. This means that you don’t have to manually instantiate dependencies or pass them through function parameters.

How to use Angular Dependency Injection

Angular DI relies on the @Injectable decorator. This decorator tells Angular that a class can be injected as a dependency into other classes. For example, suppose we have a service that fetches data from a server. We can create a class that looks like this:

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable } from 'rxjs';

@Injectable()
export class DataService {
  constructor(private http: HttpClient) {}

  getData(): Observable<any> {
    return this.http.get('/api/data');
  }
}

The above class (DataService) uses the HttpClient service to fetch data from the server. Notice that we’ve marked the class with the @Injectable decorator. This tells Angular that this class can be injected as a dependency into other classes.

Now, let’s say we have a component that needs to use this service to display the data on the page. We can create a class that looks like this:

import { Component } from '@angular/core';
import { DataService } from './data.service';

@Component({
  selector: 'app-data',
  template: '{{data | json}}'
})
export class DataComponent {
  data: any[];
  
  constructor(private dataService: DataService) {}

  ngOnInit(): void {
    this.dataService.getData().subscribe(data => this.data = data);
  }
}

The above class (DataComponent) uses the DataService in its constructor to fetch the data and displays it on the page using the Angular template syntax. Notice that we’ve also marked the class with the @Component decorator. This tells Angular that this class is a component and should be registered with Angular as such.

When Angular creates the DataComponent class, it automatically injects an instance of the DataService into its constructor. This is because we’ve marked the DataService with the @Injectable decorator.

What is an Angular Service?

An Angular Service is a class that provides specific functionality to other parts of an Angular application. It can be used for many things, such as data caching, logging, authentication, or any other functionality that needs to be shared between components. Services can also be injected as dependencies into other classes, just like with Angular DI.

The main difference between an Angular Service and an Angular Dependency is that a Service is not automatically injected into a class constructor. Instead, the class that needs to use the Service must explicitly ask for it in its constructor.

How to use an Angular Service

Suppose we have an authentication service that provides a method to check if a user is logged in. We can create a class that looks like this:

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

@Injectable()
export class AuthService {
  isLoggedIn(): boolean {
    return true; // hardcoded for example purposes
  }
}

Notice that we’ve marked this class with the @Injectable decorator. This tells Angular that this class can be injected as a dependency into other classes.

Now, let’s say we have a component that needs to use this service to display a different message depending on whether the user is logged in or not. We can create a class that looks like this:

import { Component } from '@angular/core';
import { AuthService } from './auth.service';

@Component({
  selector: 'app-auth',
  template: '{{message}}'
})
export class AuthComponent {
  message: string;
  
  constructor(private authService: AuthService) {
    if (this.authService.isLoggedIn()) {
      this.message = 'Welcome back!';
    } else {
      this.message = 'Please log in.';
    }
  }
}

Notice that we’ve explicitly asked for the AuthService in the constructor of the AuthComponent. This tells Angular that we want to use this service in this class.

When to use Angular DI vs Angular Services

So, now that we’ve seen how both Angular DI and Angular Services work, when should you use each one? As a general rule of thumb, you should use Angular DI when you have dependencies that are shared between multiple parts of your application. This way, you can inject these dependencies into multiple components or services without having to create multiple instances of the same class.

On the other hand, you should use an Angular Service when you have functionality that needs to be shared between parts of your application but doesn’t have a direct relationship with them. For example, data caching or application logging are tasks that don’t necessarily need to be associated with a specific component or service.

Conclusion

In this article, we have explored the differences between Angular Dependency Injection and Angular Services. We’ve seen how to use both of them and when to use them in different situations. It’s important to remember that both Angular DI and Angular Services are tools that can be used together to create powerful and modular Angular applications. By keeping in mind these concepts and their differences, you will be able to make better decisions when building your next Angular application.