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Services and Dependency Injection

In Angular applications, components form the backbone of your application. They control how your application behaves and presents data to the user. However, components can't do it all alone. They often require the help of services to accomplish tasks such as fetching data from a server, logging, and user input validation. In this tutorial, we will explore services and dependency injection in Angular, two concepts that work hand in hand to modularize and organize your code.

What are Services?​

In Angular, a service is a class with a specific purpose. It is designed to perform a specific task or tasks and can be used by any component, directive, or other service. Services are a fundamental part of Angular applications and are used to organize and share code across your application.

Services can be anything from a database API to a simple message service. They are typically used to encapsulate interactions with things like web servers and databases, but they can also be used to encapsulate business logic or computations.

Creating a Service​

To create a service in Angular, you use the @Injectable() decorator. This decorator tells Angular that the class will be used as a service. Here is a simple example of a service.

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

@Injectable({
  providedIn: 'root',
})
export class MessageService {
  messages: string[] = [];

  add(message: string) {
    this.messages.push(message);
  }

  clear() {
    this.messages = [];
  }
}

In this example, we have a MessageService that allows you to add messages to an array and clear all the messages from the array.

What is Dependency Injection?​

Dependency Injection (DI) is a design pattern used in programming to remove hard-coded dependencies and make applications more modular, easier to test and maintain. In Angular, DI is a core concept, and it's heavily used throughout the framework.

Angular uses DI to provide required dependencies to new class instances. To illustrate this, let's create a component that uses our MessageService.

Using a Service in a Component​

To use a service in a component, you need to import the service and add it to the component's constructor. The service becomes a dependency of the component, and Angular will inject it when the component is created. This is done using the constructor of the component.

import { Component } from '@angular/core';
import { MessageService } from './message.service';

@Component({
  selector: 'app-messages',
  templateUrl: './messages.component.html'
})
export class MessagesComponent {
  constructor(public messageService: MessageService) { }
}

In this example, we have a MessagesComponent that depends on MessageService. The MessageService is injected into the MessagesComponent via the constructor. Now, MessageService can be used anywhere within MessagesComponent.

Conclusion​

In this tutorial, we covered the basics of services and dependency injection in Angular. We learned that services are classes that perform specific tasks and can be shared across different parts of your application. We also learned about dependency injection, a design pattern used by Angular to provide dependencies to classes. By using services and dependency injection, you can write more modular, testable, and maintainable code.