DI (Dependency Injection)

Dependency injection (DI) is a design pattern that is used to implement inversion of control (IoC) in software development. The goal of dependency injection is to make code more modular and testable by removing hard-coded dependencies between objects, and instead injecting them at runtime.

In Angular, DI is used to inject services, pipes, directives, and other classes into components and other classes. Angular provides a powerful and flexible mechanism for injecting dependencies through its @Injectable() decorator and the Injector class.

Here’s an example of how to use dependency injection in Angular:

Let’s say you have a service called MyService that you want to inject into a component called MyComponent. First, you would define the MyService class and decorate it with the @Injectable() decorator:

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

@Injectable({
  providedIn: 'root'
})
export class MyService {
  // ...
}

Then, in MyComponent, you would import the MyService class and use the constructor to inject it:

import { Component } from '@angular/core';
import { MyService } from './my.service';

@Component({
  // ...
})
export class MyComponent {
  constructor(private myService: MyService) {
    // Use myService here
  }
}

With this code, when MyComponent is created, Angular will automatically create an instance of MyService and pass it to the constructor. You can then use the myService property within the MyComponent class to access the service’s methods and properties.

Angular allows you to create a hierarchy of injectors, so that you can configure the injector for your entire application, or for individual components. This allows you to create reusable and testable services, pipes and other classes and test them independently, and also allows for more control over the lifetime of the dependencies and also provide different implementations of them.

Also, Angular allows you to define the scope of a service or provider by using the providedIn property of the @Injectable decorator. There are 4 scoping options such as root, module, or component scoping and the default scope is root scope.

Dependency Injection

Advantage of DI :

Dependency injection (DI) has several advantages that can help to make your code more modular, maintainable, and testable:

  1. Loose coupling: One of the key advantages of dependency injection is that it promotes loose coupling between objects. Instead of hard-coding dependencies between objects, you can inject them at runtime. This makes it easier to change or replace one object without affecting the rest of the application.
  2. Easier to test: By using dependency injection, you can easily swap out dependencies for testing. This makes it much easier to write automated tests for your code and helps to ensure that it works correctly.
  3. Easy to update: Using Dependency Injection, you can easily update the implementation of a service or a class. This can be done by updating the implementation in one place and updating the import in all dependent classes.
  4. Reusability: By separating your code into loosely-coupled components and services that can be easily reused in other parts of your application, you can save time and reduce code duplication.
  5. Readability: A class that uses dependency injection is more readable than one that directly instantiates its dependencies. It becomes immediately clear to the reader which dependencies are required by the class, making it easier to understand and maintain.
  6. Scope management: You can also control the scope of a class by using dependency injection. Angular provides several scoping options for classes that are decorated with the @Injectable decorator. This allows you to control the lifetime of the dependencies and also provide different implementations of them.
  7. Testing: Dependency injection makes it easier to write unit tests for your code. By injecting mock dependencies into components during testing, you can isolate components and test them in isolation. This makes it easier to write tests that are reliable and repeatable.

Overall, dependency injection can be a powerful tool for building maintainable and testable code in Angular, and many other frameworks.